Astronomical Returns

The Other Side of the Clean Room

2022-12-30T23:41:00.004-05:00

Fight On Trojans! I never thought I'd ever say those words

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This month marks 3 years at SpaceX for me, which is a pretty neat milestone! It feels like just yesterday I was turning in my badge at Evercore and setting off for sunny SoCal - so long New York, hello Mars! And while my day-to-day is far from routine, I certainly feel I've found my rhythm on the Finance team: I've covered the financial reporting of 6 different divisions, I've calculated the cost of Falcon, Dragon, Starship, and Raptor, I've witnessed 119 Falcon launches (including 30 astronauts), and I've visited every major SpaceX site except Cape Canaveral. Not bad for just 36 months!

And yet, as if Elon & Co. weren't keeping me busy enough, next month I'm taking on a new challenge: grad school! The admissions officers at USC's Viterbi School of Engineering must've looked kindly upon my non-STEM bachelors, I bet I'll be the only finance major in the Industrial & Systems Engineering master's program. Hopefully I can resurrect whatever math I still remember from high school and college, I certainly haven't needed advanced calculus to book accounting journal entries at work!

I'll admit it feels strange mentally preparing myself for school again. "First day of school", "cramming for finals", and "beating the curve" were all phrases I sincerely hoped I'd never have to hear again. And it's all the more surreal given this degree is totally optional; unlike my previous 16 years of education, these next ~3 are purely of my own volition. Or as my own parents pointed out, "You're sure you want to do this right? Just remember, Kevin won't be there to save you if the classes are hard!"

(Ever the whiz kid, my best friend and college roommate always had an explanation handy whenever I was struggling with math or computer science homework)

Me turning on my TI-84 for the first time in 5 years to make sure it still works before starting grad school

All kidding aside, I really am very proud to be getting an engineering master's. While business school and investment banking have served me incredibly well, it's no secret I regret not also studying something more technical. I've lost count how of many times I've Googled "engineering master's with business undergrad", only to convince myself I was either too busy or too unqualified. But I'm almost 27 now, and I know this is likely my last good window of opportunity where I'll have the freedom to take undertake something like this. So unless I want to be in school in my 30s, it's now or never

Me representing UT Austin at USC's annual stock pitch competition, 2016 and 2017. Who knew I'd come back one day

When I first joined SpaceX, one of my worries was that my role would be considered secondary the engineers, since I don't actually design or build the rocket. Three years later, I can confidently say that's not the case; in fact, it's often the engineers who are eager to learn more about the finance side of the rockets! And yet, I know there's so much at SpaceX that I'll never be qualified to do. On the first floor of the main building in Hawthorne, I pass by the huge cleanroom where the Dragon spacecraft goes through final integration. I've been a lot of places at SpaceX, but I've never been in there before - engineers and technicians only, all in their bunny suits hard at work. Just once, I'd love to stand on the other side of that cleanroom and see the hardware up close. Just once - give me a smock, let me tighten one screw… you'd never suspect I'm just the finance guy! Alas, master's degree or not, I don't know if I'll ever get there. But hey, a guy can dream

In the Astronomical Returns post I wrote when I first got the job at SpaceX, I said that I'm more than content with my sidekick role on the Finance team, and I'll defer to the actual rocket scientists to figure out how to get us to Mars. Three years later, that still holds true - I love my job on the Finance team, and I know that even after I graduate from USC Engineering, I still won't be the one designing or building the rockets. But we've all got a part to play; as long as I learn something in grad school that helps me bring Mars a little bit closer for the rest of us, then it will all have been worth it in the end

Space Shuttle's Legacy of Leaky Hydrogen

2022-10-16T03:54:00.002-04:00

"To understand hydrogen is to understand all of physics" - Victor Frederick Weisskopf

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For most people who don't follow the daily news of the space industry, they typically hear about what NASA has been up to only at climactic moments, like when a new telescope is deployed or a spacecraft reaches its destination. So when the Space Launch System (SLS) suddenly appeared on the launchpad at Cape Canaveral, ready to blast off to the moon, I bet most Americans hadn't even heard of the new megarocket until then. No doubt they were disappointed when a hydrogen leak scrubbed the launch, but that disappointment was all the worse for space fans like me who've been following the development of SLS since its inception and have been patiently waiting more than a decade for it to launch!

Artemis 1 after scrubbing its Aug. 29 launch attempt

Still, I can't say I was surprised Artemis was delayed by leaky hydrogen, given that the rocket was cobbled together from legacy Space Shuttle architecture designed nearly 50 years ago. Per this ArsTechnica article, they tabulated that Shuttle scrubbed on average nearly once every launch attempt, with hydrogen leaks frequently being the culprit. Not only was it a tremendous headache for the launch operations team, but it was also incredibly expensive! Here are two notable instances I found across Shuttle's 30-year history where hydrogen really derailed the mission:

STS-35

In 1990, the Shuttle program appeared to be back on its feet following the Challenger disaster; Discovery had just lofted the brand new Hubble Space Telescope, and NASA was hoping to break their record of most launches in year. Columbia was slated for May to fly the ASTRO-1 suite of ultraviolet astrophysics experiments on STS-35. However, launch was scrubbed after propellant fill caused excessive buildups of hydrogen around both the quick disconnect assembly of the external tank, and the umbilical assembly of the orbiter aft section. They attempted a tanking test a few days later to isolate the leak, but it became clear the leak could not be fixed right there on the pad, so Columbia was rolled back to the Vehicle Assembly Building for repairs (sounds just like SLS). They swapped out the suspect umbilical assembly (taking the replacement from Endeavour, which was still under construction at the time), and rolled back to the pad in early August

Locations of the excessive hydrogen buildup:
External tank quick disconnect (A) and orbiter aft section umbilical (B)

Alas, the problem persisted around the orbiter aft end! The umbilical assembly had already been replaced, so NASA determined there must be another independent leak in the aft end. They tried replacing the hydrogen recirculation pumps, as well as a Teflon seal on main engine 3, but nothing could stop the leak. At a loss, they rolled Columbia back to the VAB again (passing Atlantis on the way, in the image below) while Shuttle Program Manager and former astronaut Bob Crippen assembled a "tiger team" to solve the problem once and for all. Over the next several months, a huge team of engineers pored over an intricate fault tree of all of Columbia's hydrogen fluid systems, which ultimately led them to the main engines themselves. Apparently, after Columbia's previous mission, STS-32, the main engines had needed to be completely disassembled and reassembled for maintenance, and when they did that, tiny glass beads had contaminated the hydrogen disconnect hardware. Having now found the true problem, the team painstakingly leak checked every component on the engines, and finally after yet another full tanking test, they declared Columbia leak-free. After all that effort, STS-35, originally scheduled to launch in May, had a flawless liftoff in December 1990. It was one of the most delayed launches of the Space Shuttle Program

Iconic image of two shuttles passing in the night:
Columbia (left) returns back to the VAB for repairs on STS-35, while Atlantis (right) crawls towards the pad for STS-38. August 9, 1990

STS-93

In 1999, NASA deployed the Chandra X-ray Observatory on STS-93. Coincidentally on Columbia again, this mission suffered a particularly scary hydrogen leak because it actually happened on ascent. During ignition, a gold pin on the right engine came loose and struck the engine's inner nozzle, severing three cooling tubes containing hydrogen. Additionally, an electrical short from bad wiring on an exposed screw disabled both the center engine and right engine's digital controller. Fortunately, the backup controllers kicked in and saved the crew from potential disaster, since a two-engine shutdown would've required a very risky abort sequence. However, because the controllers recognized the decrease in thrust due to the leaky hydrogen, they tried to compensate by opening the oxidizer valves more than normal to get more propellant into the engines. Not only did this force the engines to burn hotter than normal, but all three engines cut out early because the external tank ran out of oxygen sooner than it should have. Columbia and her crew still managed to get to orbit and execute the mission, albeit at 15ft/s less than expected

You can actually see the damage to the right engine. It appears as a bright streak in the nozzle's inner wall,
causing a distortion in the blue shock diamond of the exhaust plume

So given the history of problems, why use hydrogen at all for SLS, why not use an easier fuel like RP-1? Well there's two reasons, one grounded in engineering, one grounded in... politics. The first reason, and the more legitimate one, is that all things equal, hydrogen has the highest specific impulse of any rocket fuel. Said differently, it's the most efficient fuel, meaning that molecule for molecule, it's more energetic than burning methane or kerosene if you're able to wrangle with it. Of course, some of that efficiency is offset by the fact that hydrogen is super light, so it's hard to pack a lot of it into a tank. But the high specific impulse is one of the reasons why Shuttle opted for hydrogen

The other reason is simply that Congress mandated SLS adapt Shuttle architecture to make use of the leftover hardware. But the idea that we could take all the finicky and expensive systems of Shuttle, alter them somewhat and stack them into SLS, and expect to see cost savings, was certainly silly and overly optimistic in retrospect. No doubt the contractors that supplied NASA in the Shuttle days were all too eager to keep the same infrastructure going for Artemis. So we'll see how many more attempts it takes to get SLS off the ground. Hopefully the hydrogen cooperates and stays in the tank!

Hill Spheres: Where the MoonMoons Are

2022-08-16T02:39:00.014-04:00

"We are, many of us, a planet orbiting somebody's sun, unconscious of a lonely moon, orbiting our planet" - Robert Breault

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In case Astronomical Returns hadn't made it obvious already, 4-year-old Hans was little space nerd who tried to memorize every conceivable fact about each of the 9 planets (yes, there were 9 planets until I turned ten). Naturally I knew exactly how many moons each planet had, and it always struck me as odd that Mercury and Venus didn't have any, when even little ol' Pluto had a cosmic companion. Of course, orbital mechanics meant nothing to me back then, I had no idea why that was the case; I assumed it was just coincidence, as if Mercury and Venus were the two weirdos that nobody wanted to take to the school dance.

Poor Venus | From Reddit r/planetball

If you were to take an educated guess, you might surmise it has to do with the two planets' proximity to the Sun. But let's table that for now, we'll come back to it...

In orbital mechanics, the Hill Sphere of an astronomical body (named after American astronomer George William Hill) refers to its sphere of gravitational dominance, particularly in the face of perturbations (i.e. gravitational distortions) from the presence of a larger body. For example, Earth (the smaller body) orbits the Sun (a larger body): if you were to try put a satellite into low Earth orbit, the satellite should have no problem because the Earth's gravity is dominant there. But if you try put it into a super high Earth orbit, the satellite's orbit would be very unstable because of constant perturbations from the Sun; eventually, the satellite would break free from Earth's orbit and orbit the Sun instead. In that case, the satellite was trying to orbit outside of Earth's Hill Sphere, and was really in the Sun's!

The red circle represent's Earth's Hill Sphere. Anything within it is inside of Earth's gravitational sphere of influence.
Hence why the Moon orbits Earth and not the Sun

So how do we know how far Earth's Hill Sphere extends? The radius of any Hill Sphere can be approximated by the formula:

$r_H \approx a(1-e) \sqrt[3] \frac{m}{3M}$ (see derivation here)

Whereby

$a$: semi-major axis of the orbiting body (Earth)
$e$: eccentricity of the orbiting body (Earth)
$m$: mass of the orbiting body (Earth)
$M$: mass of the central body (Sun)

Because Earth's orbit is basically circular (e = 0), the 1 - e term falls away from the formula (circular orbits are most favorable for orbital stability). If you crunch the numbers, you'll get that Earth's Hill Sphere extends 1.5M km away. For context, the Moon's orbit is 384,000 km away, proving that the Moon is comfortably within Earth's gravitational influence and won't get pulled away by the Sun into a heliocentric orbit.

And interestingly, if you think of Hill Spheres in the context of Kepler's Third Law of Planetary Motion (which relates orbital period to the semi-major axis of the orbit), that means that the longest possible orbital period of an Earth satellite is seven months. If you tried to have a satellite orbit more slowly, you'd have to raise its orbit beyond Earth's Hill Sphere, and it would eventually end up orbiting the Sun!

$T = 2\pi \sqrt \frac{a^3}{GM}$

$G$: gravitational constant
$M$: mass of the central body (Earth in this case)

Back to the question about Mercury and Venus' lack of moons - the above table applies the same formula to determine the Hill Spheres of each of the 8 planets. Immediately you'll see Mercury would have a tough (but not impossible) time holding onto a moon, as its Hill Sphere is less than 1/8th the size of Earth's. Venus' chances were somewhat better with a Hill Sphere about 2/3rds of Earth's, but alas it never got anything. Our moon came about after primordial Earth suffered a planetary-scale collision with another object, so there was a ton of debris within Earth's Hill Sphere that easily coalesced into a moon. As for Mars, well, I guess Mars was just luckier than Venus; their Hill Spheres are about the same, but Mars happened to get Phobos and Deimos.

It's clear the size of a body's Hill Sphere is a factor of both its distance from the larger body (semi-major axis) and its mass. The question is, which matters more? The answer is the distance, and this is apparent in the formula: you'll notice the ratio of masses (i.e. the Earth's mass vs the Sun's mass) is under the cube root function, so it has much less of an impact on the Hill Sphere than the semi-major axis. For this reason, Pluto's Hill Sphere is 4x bigger than Earth's. Its distance from the Sun (the perturbing body) matters way more than its mass. As it turns out, Neptune has the biggest Hill Sphere in the solar system because it has the best combination of mass and distance from the Sun.

Neptune's enormous Hill Sphere allows for some really wacky satellites.
Its most distant moon, Neso, orbits about 130x further than our moon orbits Earth, and it takes over 26 years to orbit Neptune

Lastly, there's one more interesting hypothetical question I came across in researching Hill Spheres: could an astronaut have been able to orbit a Space Shuttle placed in low Earth orbit? The Space Shuttle weighed about 104 tons and orbited 300 km above the Earth, so if you plug those assumptions into the formula, you'd get that its Hill Sphere in LEO would be a measly 120 cm, way smaller than the shuttle's own volume! In order to compress the 104 tons into that 120 cm sphere, the Space Shuttle would have needed the density of a ball of solid lead!

I liked this thought experiment because it allows us to relate Hill Spheres to satellite densities. Turns out it's not so easy to create a moonmoon!

With 3 Reaction Wheels, the Universe is Ours

2022-06-22T03:19:00.001-04:00

"The more torque I can come up with, the better" - Tim Lincecum

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My formal training in physics may be limited, but I still remember learning about the law of conservation of angular momentum way back in high school. Now don't worry, it's not that hard! If you're familiar with conservation of linear momentum:

p = mv
(momentum = mass * velocity)

Well then angular momentum is basically the same thing, it's given by the formula:

L = Iω
(angular momentum = moment of inertia * angular speed)

Yikes, weird variables! But I promise they're equivalent - moment of inertia for angular momentum is akin to mass for linear momentum, the only difference being that moment of inertia is a factor of both the mass and radius of rotation of the spinning body. The example they always gave in school was that of a spinning figure skater: she starts of with her body extended and spinning slowly, but as she draws her limbs in (reducing her radius of rotation, and therefore her moment of inertia), her spinning increases because angular speed must go up to conserve angular momentum!

The equivalency extends to changes in linear / angular momentum too. If you want to change the linear momentum of a moving object, you can apply a force (F). If you want to change the angular momentum of a spinning object, you can apply a torque (τ). And just as Newton's 3rd law states that for every action there is an equal and opposite reaction, that same principle applies to torque!

Now I learned the concept of torque through a way cooler example than a silly figure skater in a textbook: younger Hans spent a lot of his childhood watching nerdy TV documentaries, and the Discovery Channel used to have a really awesome show called Weaponology that explained the history and development of modern weaponry. In the episode about the attack helicopter, early helicopter prototypes suffered from an annoying problem: the act of generating lift by spinning the rotor blades one direction (adding torque) created an equal and opposite reaction... meaning the body of the helicopter would spin in the opposite direction, obviously destabilizing the vehicle! The solution: add a single, small tail rotor that rotates perpendicular to the main rotor to cancel out the torque!

I'm honestly astonished I still remember this episode, it came out 15 years ago!
It's actually on YouTube, skip to 5:57 for where they discuss the torque problem

Great, now that we're past the long intro: what does this have to do with space? There's a mechanism commonly used on spacecraft called a reaction wheel. It applies the law of conservation of angular momentum - the same one that was a nuisance for the helicopter - to maintain very precise attitude control in orbit. The spacecraft provides an electric power source (usually from solar panels) that spins the reaction wheel. As it speeds up, the law of conservation of angular momentum causes the spacecraft to counterrotate. With three reaction wheels - one for each axis of motion - you can point a spacecraft anywhere and maintain its orientation... no heavy thrusters and no fuel required! Quite useful for a space telescope that needs to point very steadily at a star lightyears away

This is a sweet demonstration of how reaction wheels maintain the cube's orientation, even as the surface is tilted

So reaction wheels are pretty sweet, but what are their drawbacks? First, reaction wheels are only able to rotate the spacecraft around its center of gravity, they're not capable of translational force. So if you have to actually move your spacecraft from A to B, then you need an actual thruster

Second, reaction wheels are susceptible to failure; in fact, many high profile spacecraft ended their missions after multiple reaction wheels failed. Two of Hubble reaction wheels have failed before, though fortunately they were replaced during the various crewed servicing missions. Most famously, the Kepler exoplanet-hunting telescope launched in 2009, but after just 4 years, two of the four reaction wheels had already failed! One of the four was there for redundancy, but with the loss of the second wheel, Kepler could no longer maintain 3-axis control. Mission planners were able to compensate using an ingenious method that balanced Kepler against pressure from the solar wind, but the telescope's operations were still hampered and Kepler needed to occasionally burn its onboard propellant to maintain orientation. When the propellant ran out in 2018, Kepler had to be retired

Just like how you can balance a pencil on your fingertip, the solar wind became the third reaction wheel on Kepler!

Finally, the last drawback of reaction wheels is that they can become "saturated". The torques from the reaction wheels are not the only torques acting upon the spacecraft. Over time, the spacecraft must also overcome external torques like atmospheric drag, gravitational perturbations, or even tiny vents or leaks on the spacecraft. Let's say a spacecraft has been in operation for 5 years already, and its x-axis reaction wheel is currently spinning at 5,999 RPM to maintain orientation, but it has a design limit of 6,000 RPM. This means that over the past 5 years, the spacecraft has already transferred a lot of torque to this reaction wheel in order to maintain attitude control. At this point, if the onboard computer determines that the spacecraft needs to correct for even more torque in the x-direction, the only option is for the spacecraft to use some other mechanism - like burning propellant with its thrusters - to "null out" the stored angular momentum. It can't just slow down the reaction wheel, because the act of slowing down the wheel is applying a torque, and that torque will apply a counter-torque that will destabilize the spacecraft

All it takes is three wheels to point anywhere in the Universe!

To learn more about reaction wheel saturation, this Quora article explains it well

Also, to learn more about what caused the reaction wheel failures on Kepler and various other spacecraft, check out this article! The tl;dr is that charged particles from the solar wind can induce a voltage across the tiny ball bearings in the reaction wheels. These little electrical discharges can create minuscule particles that introduce excessive friction in the reaction wheel, causing them to fail. What a brutal way to lose a multi-million dollar spacecraft!

The Missing Zap and Force Field On Venus

2022-04-23T18:21:00.004-04:00

"Reading gives us someplace to go when we have to stay where we are" - Mason Cooley

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The older I get, the more I've come to relish moments of unexpected childhood nostalgia ("Oh shut up Hans, you're only 26!"). When I was a kid, my mom would bring me and my sisters to the local branch of the San Antonio library in the summertime to make us read. Although my sisters were the more avid readers - they were the ones always taking home the top Accelerated Reader prizes in school each year - I had my little niche at the public library: the shelf with the books about the solar system! While they were reading Harry Potter and Twilight and Eragon, their dorky little brother was off in some corner memorizing the atmospheric composition of Neptune.

Well a few months ago I visited the Los Angeles Central Library for the first time, and of course I dragged my long-suffering girlfriend to the astronomy section to tell her all about the solar system! Patient and wonderful as she may be, eventually even she tired of my planetary perseverations and abandoned me to my own devices in the library. So I sat there cross legged alone on the floor like a child, reading a book about Venus, and there I learned about TWO unsolved mysteries about our sister planet: Venus is unexpectedly missing both the presence of lightning and a detectable magnetic field!

Doesn't matter if I'm 6 or 26, or if it's my local library in San Antonio (left)
or the flagship branch in Los Angeles (right), as long as there's space books, I'm all for it!

Lightning has been confirmed on other planets with thick atmospheres like Jupiter and Saturn, so planetary scientists have long expected Venus to be the same. Yet multiple spacecraft from many different countries have carefully studied Venus, and still the evidence of lightning, visual or otherwise, has been circumstantial at best. The early Soviet Venera landers detected curious pings on its sensors before the hellish atmosphere crushed the probes; the American Pioneer Venus Orbiter noticed some energetic bursts, but the Cassini spacecraft came up empty when it passed Venus on its way to Saturn a decade later; the European Venus Express heard "whistler" radio waves that can be generated by lightning on Earth, but also by many other atmospheric instabilities seen both on our planet and on others. Now our best bet is the Japanese Akatsuki probe, the only spacecraft actively orbiting Venus right now. So what has Akatsuki seen?

Unlike the dazzling display that Jupiter treats us to, Venus' lighting has been far more reclusive, assuming it exists at all!

Akatsuki has observed a single atmospheric flash of light over Venus. Just one. Does this count as lightning? Well it's hard to say; we'd expect lightning to occur in bunches, so maybe it was just a cosmic ray happening to strike the atmosphere, or a meteor burning up, or even a one-off instrumentation glitch. All of those seem pretty unlikely, so unfortunately it looks like we'll have to just keep watching.

But if it were lightning, what could've caused it? Let's back up and consider what lightning is in the first place. Lightning occurs when an electrical charge builds up in the atmosphere and causes a spark, just like when you accidentally shock yourself with static electricity. On Earth this occurs when water vapor in clouds circulates due to convection; the water near the top of the cloud freezes into ice crystals that bump into each other and generate a positive charge, while negatively charged particles sink. When the charges get big enough, they generate a giant spark - lightning! But Venus' clouds are different. They're made of sulfuric acid and are pretty good electrical conductors, so it's harder for these large positive and negative charges to build up. Furthermore, it's unclear whether the same convective cycles exist in Venus' clouds in the first place.

There is one other potential trigger for lightning on Venus though: volcanoes. Though we haven't observed one directly due to Venus' opaque atmosphere, we're pretty confident that these eruptions are occurring, and the enormous release of ashy plumes could generate the static electricity necessary to spark Venus' atmosphere! While no one knows for sure yet, it's an interesting question to think about because the energy release from lightning can break apart and recombine molecules in ways that would be otherwise improbable. The presence of lightning over the primordial soup of ancient Earth likely contributed to the development of life.

Venus is scary enough, maybe an intense lightning storm like this would simply be overkill!

So now the other mystery: what happened to Venus' magnetic field? Every instrument we've ever pointed at Venus has unequivocally confirmed that it has no magnetic field to speak of. How could this be, when Venus and Earth are roughly the same size and density and made of the same rocky materials? Shouldn't Venus also have a molten metal interior, whose convective motions create the necessary internal dynamo? And has Venus always lacked it, or did the magnetic field fade away at some point in the past?

There's no solid consensus yet, though many theories exist. For a long time the most common explanation was that Venus' rotation is too slow to promote the internal dynamo, but this doesn't really hold up to scrutiny since the interaction of heat from the core countered by the cooling of plate tectonics on the surface is more important to driving convection. More recently, an alternate theory has arisen that perhaps Earth is the anomaly! Maybe planets of Earth and Venus' size normally shouldn't have strong magnetic fields, but Earth's impact with a Mars-sized object 4.5 billion years ago (the same collision that formed the Moon) literally shook our planet to its core and jumbled up the stratified layers of the Earth's interior. The theory argues that it's this de-homogenization that has helped promote the convective motion that powers our magnetic field to this day!

Our understanding of Venus' interior would be hugely advanced if we could build a long-duration lander that could survive the hellish surface

This begs the question then, how has Venus' atmosphere not been completely stripped away? While it's true Venus doesn't have an intrinsic magnetic field, it still has an induced magnetic field caused by ionizing solar winds striking the thick upper atmosphere, just like Earth's magnetosphere. Furthermore, Venus' atmosphere is mostly carbon dioxide, a heavier molecule than the oxygen and nitrogen in our atmosphere, so it takes much more energy to dislodge Venus' atmosphere. Those thick, mysterious clouds aren't going anywhere anytime soon!

So Why Does Ivan Power the Atlas V, Anyway?

2022-03-19T13:55:00.003-04:00

"After analyzing the sanctions against our space agency, I suggest to the USA to bring their astronauts to the International Space Station using a trampoline" - Dmitry Rogozin, 2014

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It's funny how things come full circle sometimes. When Russia first annexed Crimea in 2014, I was a freshman at UT Austin starting to get involved in one of the stock-trading student orgs in the business school. Congress had just mandated that the ULA phase out the import of Russian rocket engines - our fund happened to own shares of Aerojet Rocketdyne, which at the time was vying to supply ULA with a domestically manufactured replacement, and one of the upperclassmen was giving a really neat presentation on the Russian RD-180 engine still used on the Atlas V today. Back then, I wasn't nearly as familiar with the aerospace industry. Eight years later, with me now working at SpaceX, a full-blown invasion of Ukraine underway, and the Russians refusing to sell any more rocket engines, the question still remains: why on Earth does the Atlas V use a Russian engine?

Check it out! Here's the cover page of the portfolio review I wrote on Aerojet Rocketdyne for my college student investing org, back in 2016.
Alas, ULA didn't pick Aerojet Rocketdyne's AR1 engine, they went with Blue Origin's BE-4 instead

Believe it or not, to answer this question we have to go back all the way to the heyday of the Space Race, when both the Americans and the Soviets were investing a ton of R&D into creating the best possible rocket engine. In the 1950s, the Air Force had commissioned the development of the enormous F-1 Rocketdyne engine, only to realize a few years later that it had absolutely no use for an engine that huge. Fortunately, NASA had just been created around that time, and they picked up Rocketdyne's tab for the F-1's development because they knew they'd need it. The F-1 was the perfect engine to power the first stage of the Saturn V: it was big enough to get Saturn V off the pad, but also relatively simple, using a gas-generator cycle and burning kerosene (RP-1). For the first stage, thrust was the more important factor; then for the second and third stages where efficiency mattered more, NASA leveraged prior work done by Lockheed to develop the hydrogen-burning J-2 engine, since hydrogen has better specific impulse than RP-1. In short: powerful kerosene-burning engines for the first stage + efficient hydrogen-burning engines for the second and third stage

5 kerosene-burning F-1 engines (left) powered the Saturn V first stage
5 hydrogen-burning J-2 engines (right) powered the second stage, and 1 powered the third stage

So what did the Soviets pursue? They opted to stay away from hydrogen, because working with hydrogen as your fuel is an absolute pain: super low density, insanely cold, highly flammable and explosive, embrittles metal, leaks easily, you name it. So left using kerosene for the upper stages, they needed a way to improve the efficiency of their engines to compensate for the lower specific impulse that kerosene provides versus hydrogen. To that end, the Soviets did something the Americans thought impossible - they developed a staged combustion cycle engine, the NK-15, for their N1 moon rocket (the Soviet counterpart to the Saturn V). What does that mean?

Rocket engines need a turbopump to draw propellant into the combustion chamber, and to power that turbopump, some propellant has to be diverted and combusted on the side to generate exhaust. In a gas-generator engine like the F-1, the exhaust that powers the turbopump is then dumped overboard, essentially wasting that propellant. A staged combustion cycle engine like the NK-15 is way more complex, but it manages to recycle the exhaust from the turbopump and redirect it back into the combustion chamber, so that no propellant gets wasted. Pretty neat!

So now the Soviets have a revolutionary staged combustion cycle engine that burns kerosene and is highly efficient. But it's showing up pretty late in the race to the moon, and it's not very big (only 1/5th the thrust of the F-1), so the Soviets need to stick a ton of them (30) on the first stage of the N1 rocket. Having lots of engines on a stage is fine in theory if your quality assurance is good (SpaceX's Super Heavy plans to have 33 Raptor engines), but unfortunately the Soviets proved better at design than manufacturing; all 4 launches of the N1 rocket suffered anomalies in flight and exploded

Lots of NK-15 engines at the base of the N1 rocket! The diagram on the right shows the propellant flow of a staged combustion cycle engine. Where I've put the green circle that says "HERE" shows the recycled exhaust from the turbopump being redirected back into the combustion chamber

Now jump forward a few years: the Space Race is over, no one's going to the moon anymore, which means most of the payloads being launched now are only going to Earth orbit. With less thrust required, there's no longer any need for a supermassive rocket engine or a boatload of engines strapped together; a medium-sized, highly-efficient engine makes for a perfect choice for a first stage booster. The Soviet Union is now the global leader in staged combustion, kerosene-burning engines, and new engines like the NK-33 and the RD-180 pop up in the succeeding decades (if you want to learn more about the lineage of Soviet rocket engines, this video by The Everyday Astronaut is the best source)

But then something unexpected happened - the Soviet Union collapses! Flush with our victory in the Cold War and convinced of the inevitability of global democracy, yet fearful of Russian rocket scientists and nuclear experts dispersing and lending their expertise to unsavory third parties, the US effectively pumps money into Russia's space program through various partnerships (like the ISS) to prop it up. The hope in the 1990s was that if Russia can be free and democratic, then surely we can do business with them!

Maiden launch of the Atlas III with its RD-180 in 2000, the first American rocket to launch using a Russian engine

It was around this time that General Dynamics (and later Lockheed Martin after acquiring GD's space division) was developing the next iteration of its storied Atlas line of rockets, the Atlas III. Rather than trying to develop a new engine internally, which was sure to be an exorbitant effort, it was just so much easier to buy reliable, high-performance Russian engines right off the shelf, now that they're available. A joint venture was formed between Russia's NPO Energomash and Pratt & Whitney, whereby Pratt & Whitney received the license to manufacture the RD-180 for Lockheed Martin. From there, for reasons that continue to escape me to this day (as this Quora user aptly put it), the US government approved the partnership, and the Atlas III got the RD-180. Today's Atlas V still uses it

An Antares rocket being transported, with its Russian engines and Ukrainian first stage

Finally, it's worth noting that the Atlas V isn't the only American rocket with a Russian engine. While Atlas V and its RD-180 engine is more flown and more noteworthy, Northrop Grumman's Antares rocket is even more bizarre. Not only does it use a Russian engine, either the NK-33 or the RD-181 depending on the variant, but Northrop Grumman also subcontracts the construction of the entire first stage core to the Ukrainian Yuzhmash state-owned factory! Obviously Russia and Ukraine are at war with each other now, and there were unconfirmed reports that Yuzhmash had been damaged or destroyed in the fighting, so I have no idea what will become of the Antares rocket

One thing's for sure though, domestic manufacturing of high-performance rocket engines is a matter of national security. Thank goodness SpaceX has Merlin and Raptor!

To Mars In 3 Days With The Epstein Drive!

2022-02-05T14:05:00.007-05:00

“A hundred and fifty years before, when the parochial disagreements between Earth and Mars had been on the verge of war, the Belt had been a far horizon of tremendous mineral wealth beyond viable economic reach, and the outer planets had been beyond even the most unrealistic corporate dream. Then Solomon Epstein had built his little modified fusion drive, popped it on the back of his three-man yacht, and turned it on. With a good scope, you could still see his ship going at a marginal percentage of the speed of light, heading out into the big empty. The best, longest funeral in the history of mankind. Fortunately, he’d left the plans on his home computer. The Epstein Drive hadn’t given humanity the stars, but it had delivered the planets.”

- James S.A. Corey, Leviathan Wakes

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I gotta admit, I'm usually more of a movie fan than an ardent TV show follower. TV shows are just so much commitment, you have to keep up with each season year after year, all the while remembering all the characters and subplots going on. But when it comes to space, one series has stood above the rest for me, and that's The Expanse! The first season came out when I was a sophomore in college - I remember watching each episode in my dorm at UT Austin, feeling devastated when Syfy cancelled it after season 3, relieved when Amazon Video saved it for season 4, and now melancholy that the sixth and final season has just wrapped up. So I'd like to dedicate an Astronomical Returns post to the engine that makes the whole show possible: the futuristic Epstein drive!

The Expanse is set in a world centuries from now where humans have colonized the solar system. Earth and Mars have become independent military rivals locked in a Cold War, and both are dependent on the resources and labor of people living in the Asteroid Belt and Outer Planets. This expansion was made possible with the invention of the Epstein drive, a modified fusion rocket that uses magnetic coils to drastically accelerate reaction mass. The super-efficient Epstein drive allows spacecraft to burn for the entire length of the voyage, constantly accelerating to the halfway point before flipping around and gradually decelerating. Contrast that to our real-world propulsion systems, which expend almost all of their propellant just getting off-planet before slowly coasting to their destination!

Fun fact: when the creators of The Expanse were asked how exactly Epstein drive worked, they responded tongue-in-cheek: "It runs on efficiency"

The Epstein Drive in action on one of the spacecraft in The Expanse

According to The Expanse's Wiki page, the Epstein drive on the Rocinante (the main characters' spacecraft) generates 6.3 million Newtons of thrust, has a specific impulse of almost 2 million seconds, and produces an exhaust velocity of 19 million m/s (6.8% the speed of light)! To ground those numbers for you:

The F1 engine on the Saturn V first stage generated 6.7 million Newtons of thrust, but it only burned for 2.5 minutes before it was totally out of fuel; imagine accelerating with the thrust of the Saturn V engine... but basically indefinitely!
The combustion of hydrogen and oxygen, like what the Space Shuttle burned, has a measly specific impulse of 450 seconds. And yet, this is about the theoretical maximum a chemical rocket can provide
And for a good real-world rocket engine, exhaust is ejected at a few thousand meters per second

The Raptor engine on the test stand - in my *unbiased* opinion, the very best in rocket propulsion that humanity currently has!

Now of course, like all good science fiction, the Epstein drive has basis in real technology. I mentioned that 1) it's a fusion rocket, and 2) it uses magnetic coils to accelerate its reaction mass

Every rocket that has ever flown to space used a chemical engine. But that doesn't mean nuclear rockets aren't possible! In fact, way back in the 1960s, NASA successfully test fired various nuclear thermal rocket engines, which used nuclear fission from uranium to superheat liquid hydrogen and blast it out of an engine nozzle! But tragically, this promising technology got shelved when the Apollo Program ended and NASA's funding was cut. Going one step further, a rocket engine based on nuclear fusion would take a pellet of some fuel, flash-compress it with some advanced mechanism (perhaps converging lasers? Look up inertial confinement fusion) so that the atoms fuse into a superheated plasma that can be ejected out of a nozzle
The superheated plasma is then accelerated further with electromagnetic power. Using electricity to accelerate plasma and generate thrust is not fiction - that's exactly how ion drives work, like what powered the Dawn spacecraft or SpaceX's Starlink satellites!

We are thus not limited by our conceptual imaginations, but by the constraints of heat and power. Nuclear fusion produces an insane amount of heat - way more than we can contain in any sort of energy efficient manner - and in the context of a rocket engine, a fusion reactor and the accompanying heat sink it would require would end up being unacceptably heavy. And while our ion drives are quite efficient, they only produce a tiny amount of thrust because we simply can't generate the power density necessary to accelerate the enormous amount of reaction mass being ejected by the Epstein drive

If you magically combine a working fusion reactor to the highly experimental VASIMR plasma engine that the Ad Astra Rocket Company is working on, that's about as close as we can currently get to the Epstein drive

But say one day that we resolve these technical challenges and build a working Epstein drive in real life. What would that mean for interplanetary space travel? If we could produce an engine that generates a perpetual 1G of acceleration, we could get to the moon in 3 hours, to Mars in 3.5 days, to Jupiter in 6.5 days, and all the way to Neptune in an astonishing 16 days (all using the flip and decelerate maneuver at the halfway point)!

A screenshot of the Epstein drive schematics from the show

Alas, for now the Epstein drive remains solely a figment of our imagination. I just hope that one day in the future when we do invent it (or something similar), the inventor doesn't meet an untimely demise like Solomon Epstein did in the Expanse! He discovered his eponymous rocket engine by accident, tinkering around with his space yacht in Martian orbit when suddenly the engine unleashed an unbelievable amount of acceleration, trapping him in his spacecraft until the G forces killed him. The flashback scene in the show that depicts this event is fantastic, you can watch it on YouTube here

How the Universe Synthesized JWST's Beryllium

2022-01-15T04:45:00.003-05:00

"If you wish to make a ~~apple pie~~ [next-generation space telescope] from scratch, you must first invent the universe"
- Carl Sagan

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So we were undoubtedly all thrilled by the long-awaited James Webb Space Telescope's flawless Christmas morning launch. As a hardcore space fan, when I heard that liftoff from French Guiana would occur around 6am my time, I dutifully set my alarm for the early morning countdown. Unfortunately, the timing was such that I'd gotten my COVID-19 vaccine booster the day before; I hadn't thought much of it since my first two shots produced minimal side-effects, but man alive did I feel horrendous by the time that 6am alarm went off! As JWST took flight, the nausea and full-body soreness combined with my sleep deprivation had me almost ready to hurl. But in the end, both boosters - Ariane 5 and Moderna - were more than worth it! (heh heh see what I did there??)

The Ariane 5 carrying JWST, moments after liftoff

My unfortunate timing notwithstanding, I've actually had this article idea in mind for months now, ever since NASA Earth Observatory published this great article about where the beryllium for those 18 super-shiny gold hexagonal mirrors on JWST came from. As it turns out, the vast majority of the world's beryllium comes from Spor Mountain in Utah, locked away in minerals that formed 25 million years ago from the lava of a volcanic eruption. NASA has long used beryllium from Spor Mountain for its missions; for JWST, beryllium was perfect because it's 1/3 lighter than aluminum and 6 times stiffer than steel, even under extreme temperatures, so the mirrors can withstand any collisions with micrometeoroids

From mine to mirror, the beryllium in JWST's mirrors has undergone quite a transformation!

But I wanted to go one level deeper: how did all the beryllium at Spor Mountain get there in the first place, or any of the beryllium on Earth for that matter? During my elective astronomy classes in college, I really enjoyed the unit on nucleosynthesis (how all the various elements get produced in the Universe), and I remembered that beryllium is a pretty strange edge case…

If you were to take a look at the periodic table of elements and make a wild guess as to the relative abundance of each element in the Universe, you'd probably end up with a result that has lighter elements being way more abundant than those weird heavy elements like bismuth or molybdenum. And you'd basically be right, especially if you knew that hydrogen and helium were the two elements largely produced by default from the Big Bang, and that stars are almost entirely made up of! Looking at the figure below plotting relative abundance in the Universe as a function of atomic number, it's a pretty clear trend… EXCEPT for three elements: lithium, beryllium, and boron. Why is that??

Let's back it up a bit. I mentioned just now that hydrogen and helium were produced by default from the Big Bang. We also know that stars release their energy through nuclear fusion, so shouldn't that solve our problem? Once the Universe cooled enough for matter to coalesce, why couldn't stars just start fusing hydrogen and helium into lithium, beryllium, and boron?

The problem is that if you take the available isotopes of hydrogen and helium (H-1, H-2, He-3, He-4) and slam them together, none of the isotopes of lithium, beryllium, or boron that you'd get are stable. For example, if you fuse two He-4 nuclei together, you'd get beryllium-8, an incredibly unstable isotope that decays almost immediately! Instead, the first stable isotope heavier than helium that stars can produce through nuclear fusion is carbon-12, and that only happens through an incredible reaction called the triple-alpha process, where three He-4 nuclei (aka alpha particles) collide at the same time! From there, once carbon is formed, the nuclear fusion in stars can continue producing the heavier and heavier elements we see on the periodic table (as least up through iron; after that, more exotic processes like supernovae and white dwarf/neutron star collisions are needed). But the point is that stellar nuclear fusion skips lithium, beryllium, and boron. What gives?

Diagram of the triple-alpha process. In the moment the Be-8 nucleus is formed, another He-4 nucleus
better come along and slam into it immediately! Otherwise it'll decay right back to He-4

Clearly there's a different process that produces lithium, beryllium, and boron, and it's called cosmic ray spallation, which is basically a fancy way of saying "nature's atom smashers." Think of it this way; here on Earth we humans have built particle accelerators to smash exotic large atoms into interesting smaller ones. The Universe has its own particle accelerators, things like pulsars, supernovae, and black holes that are constantly firing cosmic rays in all directions. Sometimes, these energetic particles will strike a larger atom and cause it to break apart. And sometimes, the smaller piece that breaks off happens to be a stable isotope of lithium, beryllium, or boron! If this sounds like an unlikely scenario, you're absolutely right, which is exactly why these three elements are relatively uncommon in the cosmos! They're the only elements not made from stars or from the Big Bang itself

Simple diagram of boron spallation forming lithium and helium

So the next time you see a picture of JWST's magnificent hexagonal mirrors, know that the beryllium in those mirrors is a rare delicacy of the Universe, and carry a little bit of pride knowing that we humans have taken these very special atoms and constructed them into something truly worthy of their value

The Angry Alligator on Gemini 9A

2021-12-24T19:50:00.003-05:00

"The answer is not throwing away the fairings or even trying to catch them.
The best way is to never get rid of them in the first place" - Peter Beck

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These days we've got so many rocket launch startups all striving to bring their launch vehicles to fruition, but sometimes it feels as though they've all roughly converged to the same general design. Obviously I know each has their nuances, like Relativity's 3D printing and Astra's barebones architecture, but at least visually they all look more or less the same: two stage configuration, RP-1/LOX engines, conical fairing, etc. But once in a while, a new rendering pops up that makes me do a double take, like, "Oh man, they're really gonna try build that thing, that's wild!"

So when I saw the fairing design on Rocket Lab's upcoming Neutron rocket, you can bet I was pretty excited! Unlike a typical rocket where the upper stage is stacked on top of the booster, Neutron is planning to encapsulate the ENTIRE second stage within the surrounding booster. And rather than jettisoning the nose cone after passing through the atmosphere, the unique four-petal fairing will simply hinge open to release the second stage. Already affectionately dubbed "the hungry hippo", this design will hopefully eliminate the cost of recovering and refurbishing the fairing separately from the first stage

I see the hippo, but honestly what it reminds me of is the Piranha Plant from Super Mario!

And yet, while Rocket Lab's design is daring and innovative, as soon as I saw the rendering I knew I'd seen something similar from an old NASA mission. Sure enough, I found it: the Augmented Target Docking Adapter on Gemini 9A!

By 1966, the buildup to the moon landings was well underway; Project Mercury had proven human beings could successfully operate in orbit, and now NASA needed Gemini to demonstrate longer duration spaceflights and more complex maneuvers like rendezvous and docking. But Gemini 9A was beset with problems right from the start; the original crew was supposed to be Elliot See and Charles Bassett, but just four months before the mission, both men were killed in a plane crash. The two astronauts were flying from Texas to inspect the Gemini spacecraft being constructed by McDonnell Aircraft in St. Louis, but poor visibility due to rain and fog combined with pilot error on the part of See caused them to miss the runway and crash their T-38 Talon into the side of one of the McDonnell buildings, killing them instantly. Despite their tragic demise, the plane crash didn't damage the spacecraft or cause any other delays to the mission, so NASA pressed forward and moved the backup crew of Thomas Stafford and Eugene Cernan to the prime crew

Prime crew in front, backup crew in rear

The initial objective of Gemini 9 was to dock with the Agena Target Vehicle (ATV) that would be launched separately ahead of the crew. Gemini 8 had done something similar, but that mission was aborted promptly after docking when a stuck thruster sent Neil Armstrong and David Scott spiraling out of control, nearly killing them. Unfortunately, any hope of Gemini 9 improving upon 8 was quickly dashed when the Atlas rocket launching 9's ATV malfunctioned, sending the spacecraft crashing back down to Earth. NASA quickly scrambled and launched the backup Augmented Target Docking Adapter (ADTA), then replanned Stafford and Cernan's mission to dock with the ADTA instead, renaming the mission from Gemini 9 to Gemini 9A. Once the ADTA was safely in orbit, Stafford and Cernan then blasted off into orbit on their Titan II rocket and rendezvoused with the ADTA

A fantastic illustration of the original Agena Target Vehicle (ATV) above and the replacement Augmented Target Docking Adapter (ADTV) below

As Stafford and Cernan drew closer to the ADTA, they saw to their dismay that the fairing halves had failed to separate, preventing them from docking. With the two conical pieces jammed ajar, Stafford commented, "it looks like an angry alligator out here rotating around." It was apparent that the fairing's explosive bolts had fired, but there were loose electrical wires tied to two steel bands that were still holding the fairing halves together. The astronauts brainstormed ways to resolve the problem with Mission Control, but every solution they came up with was deemed way too risky: if they sent Cernan out on an EVA to cut the wires, they were afraid the sudden release of tension would whip out dangerous wiring and sharp edges that could easily puncture his spacesuit. Stafford even asked if they could ram the ADTA with the Gemini capsule to try force the fairings to open, but Mission Control ruled it out, fearing they would damage their spacecraft

The reason for the faulty wiring was quickly discovered: multiple miscommunications and last-minute design changes to both the ADTA and the Atlas rocket resulted in the fairings being installed by a crew of McDonnell technicians completely unfamiliar with the design, rather than the Douglas and Lockheed crew as originally intended. And even though there had been a Douglas engineer on-site contracted by NASA to certify the installation, the McDonnell crew refused to let him inspect their work despite the protests of both NASA and Douglas

The jammed fairings on the ADTA, as seen from Stafford and Cernan's Gemini capsule

With the ADTA docking abandoned, Stafford and Cernan focused on the other mission objective: the Air Force had designed the Astronaut Maneuvering Unit (AMU), which was essentially a jetpack that Cernan was supposed to test out during an EVA. Cernan began preparing for his spacewalk, but after pressurizing his suit, "the suit took on a life of its own" and became so stiff that it moved like "a rusty suit of armor." As he struggled to climb outside towards the back of the spacecraft, he quickly became exhausted as his heartrate skyrocketed to 180 bpm, so much so that the flight surgeon was afraid he would pass out

At this point everyone knew it would've been incredibly risky to proceed to with the AMU test. Not only was Cernan exhausted, but his spacesuit's cooling system had been completely overwhelmed, and as Cernan overheated his suit visor fogged up entirely. Stafford ordered him to abandon the test and get back inside, and after Cernan miraculously managed to get back to the cockpit area blind, Stafford literally had to grab onto his legs and help cram him and his overpressurized spacesuit back into the hatch

Cernan in the middle of his incredibly challenging spacewalk with the AMU, which you can see on the right during pre-flight testing

Fortunately for the tired crew, finishing the EVA was the last of their travails as their reentry was smooth and precise. Post-flight medical examination found Cernan had lost an astonishing 13 pounds during the spaceflight, and when they sent his spacesuit back to Houston, there was "about a pound or a pound and a half of water in each boot" worth of sweat!

While not quite successful, NASA still learned many valuable lessons from Gemini 9A. Later Gemini missions were able to dock far more successfully, and the Apollo spacesuit switched to a more robust water-cooled design rather than Gemini's air-cooled spacesuits, which proved extremely effective for the astronauts that walked on the moon!

The SRB Gets a Makeover

2021-11-30T02:18:00.006-05:00

"There was a lot of light and a lot of rumbling and vibration... and then after about two minutes, when the solid rocket boosters separated, the ride got a lot smoother" - Ellen Ochoa

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I was pondering this question the other day: what's the longest you've ever waited for something to happen? As an avid golfer who spent his whole childhood watching Tiger Woods on TV, I think the 11-year wait (2008-2019) I endured for Tiger to complete his comeback and win his 15th major definitely takes the cake. But like sports, the world of aerospace demands extreme patience, and we space fans are nothing if not patient - I feel like I've been waiting forever for NASA's Space Launch System to launch! I first heard of SLS back in 2013, when my family visited Kennedy Space Center. By then the Space Shuttle had been gone for two years, and NASA was promising that its replacement would be ready by 2017. "Four more years? How in the world am I supposed to wait another four years to see this thing fly??"

Of course we all know now that the 2017 launch date proved impossibly optimistic. But now almost nine years after my family vacation, at long last SLS is fully stacked at the Cape and getting ready for its Artemis 1 mission! In eager anticipation, I've been trying to get up to speed with all the components of SLS, and the upgrades to the solid rocket boosters caught my eye, particularly because my knowledge of SRBs is more sparse (since SpaceX doesn't use solid propellants)

The fully stacked Artemis 1 Space Launch System

Like much of SLS's architecture, the two solid rocket boosters attached to the core stage were congressionally mandated to make use of legacy Space Shuttle hardware. The original Space Shuttle SRBs were made by Thiokol, which was later acquired by Orbital ATK, which in turn was acquired by Northrop Grumman in 2018, meaning NG is in charge of the SLS SRBs. The shuttle SRBs by Thiokol had the distinction of being the first-ever solid propellant rocket to be used on a human-rated launch vehicle; supplying over 3 million pounds of thrust each, they were the most powerful SRBs ever designed and provided 85% of the Space Shuttle's thrust at liftoff, with the rest coming from the Space Shuttle's liquid hydrogen RS-25 engines. To start us off, here are a few features that are the same between the original Space Shuttle and new SLS SRBs:

Overall structure: Nose cone and forward skirt (which contains avionics), multiple center segments containing propellant, aft segment containing the nozzle and thrust vector control
Propellant: PBAN-APCP. This intimidating looking acronym is shorthand for a unique blend of ammonium perchlorate (which serves as the oxidizer), aluminum powder (fuel), polybutadiene acrylonitrile (fuel, also acts as a binder), iron oxide as a catalyst, and epoxy
Grain geometry: 11-sided star, which provided high thrust right after ignition but throttled down as the shuttle passed max-Q
Ignition system: Pyrotechnic devices using NASA standard detonators, which can only be fired after a manual safety pin has been removed prior to launch, and the flight computer confirms the orbiter's RS-25 main engines have achieved proper thrust and there are no other problems
Segment cases: D6AC high-strength low-allow steel
Burn time: Just over 2 minutes

An exploded diagram of the Space Shuttle SRB

Fast forward a few decades, and the souped up Artemis SRBs now stand vertical at Kennedy Space Center! It's kind of neat that even after Northrop Grumman took over Thiokol, they've maintained the same booster manufacturing facility in Promontory, Utah. The completed SRB segments are still shipped via rail from Utah all the way to Florida! And yet today's SRBs can take advantage of modern improvements - here are the primary upgrades that have been made to the SRBs flying on SLS:

Additional propellant: The original shuttle SRBs had four main segments full of propellant, but these upgraded boosters now have five in order to provide SLS 20% greater thrust and 24% greater total impulse
New insulation: The new SRBs got rid of the old asbestos-laden insulation, which not only eliminates the hazardous material, but also provides cost and weight savings
Expendable configuration: Unlike the shuttle's SRBs which were intended to be reused, the entire SLS stack is single-use, including the SRBs. If that sounds more like a downgrade to you than an improvement, well... yeah I'm with you there. But I'll spare you my rant, the big-wigs in Congress are set on SLS's expendable design, so that ship has sailed
Other upgrades: Modernized avionics, new nozzle design, improved non-destructive evaluation techniques during testing

They may not be cheap, but they're certainly huge

Finally, a few months ago NASA Spaceflight released a really great article which goes in-depth into the future design changes that Northrop Grumman is working on for next generation of SRBs. The upcoming Artemis 1 is launching on a Block 1 SLS, but NASA intends to develop a Block 2 variant down the road that will make use of a further-enhanced SRB known as BOLE (Booster Obsolescence and Life Extension). Here's a summary of the future-state improvements planned on BOLE:

New propellant formula: Still uses ammonium perchlorate as the oxidizer (the APCP of the original formula), but replaces PBAN fuel with a different solid called hydroxyl-terminated polybutadiene (HTPB) which can handle higher strain levels and can be more densely packed. HTPB is already used on some existing solid rocket boosters, like the Indian PSLV
Composite cases: Replaces the current steel casings used since shuttle. Composite technology has advanced significantly over the past few decades and will translate to 30% weight savings
Thrust vector control: Existing hydrazine powered TVC is highly toxic and will be replaced with a new battery-powered system, leveraging the work Northrop Grumman was doing on its proposed OmegA rocket
Better mating to SLS: The shuttle-derived SRBs always required some reverse engineering to adapt them to the new SLS architecture. BOLE has attach points that are better suited to the SLS booster, as well as a revamped separation system
Other structural improvements: Combined nose cone and frustrum, optimized forward and aft skirts, redesigned nozzle

Pretty neat CAD imaging released by Northrop Grumman showing the redesigned attach points to SLS

If you want to read the whole article, you can check it out here!

The Short Game Guru's X-Ray Explorer

2021-10-17T00:56:00.001-04:00

"Half of golf is fun; the other half is putting"

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One of the interesting quirks of being on the SpaceX Finance team is that because I'm not an engineer, obviously no one expects me to have a in-depth technical understanding of all the components we build. Yet at the same time I've found such knowledge is absolutely indispensable, even in my supporting role, so I end up spending a lot of my time on the side reading up on SpaceX hardware, which I don't mind at all since I enjoy playing the role of armchair expert! Of course self-teaching is really hard, learning is way easier when you have smart friends who are able to explain everything, and to that end I'm super lucky: my roommate is a former Merlin integration engineer, and another friend of ours in our building has done tons of design work on Starship and Raptor, so the two of them have gotten me up to speed on SpaceX propulsion systems! It pays to have friends in high places, hopefully they haven't gotten sick of my incessant questions yet.

Fortunately this past weekend I was able to partially return the favor by teaching them how to play golf! I've played for a really long time, ever since I was four, so it was a lot of fun taking them to the driving range and being the teacher instead of the student for once. Yet even after more than 20 years, I'm still discovering new things about the game, and a few months ago I learned a really cool fact - it turns out Dave Pelz, one of the most well-known golf instructors and the putting coach for many famous PGA tour pros, studied physics in college and spent the first 14 years of his career at NASA's Goddard Space Flight Center studying planetary atmospheres!

Phil Mickelson is unquestionably Pelz's most famous pupil, and if you follow golf at all, you know Phil's short game is second to none

Dave Pelz majored in physics at Indiana University, and while he was an extremely accomplished golfer himself, attending on a four-year scholarship, he didn't quite make the cut for the PGA Tour (he and Jack Nicklaus were contemporaries, and he lost to Nicklaus on 22 occasions! Though that's not so bad when you consider that Nicklaus is arguably the GOAT). So after graduating in 1960, he joined NASA Goddard and began studying the atmospheres of Earth, Venus, and Mars. Back then, planetary science was still in its infancy; the first ever interplanetary probe, Mariner 2, wouldn't reach Venus until two years later, so Pelz was joining at an incredibly exciting time! During his tenure, he rose to the position of senior scientist and was in charge of multiple satellite payloads, primarily for the Explorers Program. In his own words:

"Doing space research was the dream. At the time, the Soviet Union had just put up Sputnik, and NASA was new, exciting, and doing things that had never been done before... I got to work with brilliant people - the best minds in the world"

Pelz (fifth from the right) with the Goddard team

The Explorers Program is the US's oldest satellite program - recall that right after the Soviet's launched Sputnik, the US responded by launching a satellite of our own in 1958... called Explorer 1. In the decades since then, Explorers has sent over 90 missions to space related to geophysics, heliophysics, and astrophysics on a wide variety of launch vehicles, and in collaboration with multiple institutions and countries. It was surprisingly tough to research exactly which missions Pelz contributed the most to, but given that he was at NASA from 1961 to 1975, I figured I could just time fence the Explorer missions that launched in that 14 year period. The one that caught my eye the most was one focused on x-ray astronomy called Uhuru, launched in 1970

Explorers missions are categorized into three classes: Medium, Small, and University-level missions

Uhuru stood out to me for three reasons: 1) it was the first satellite entirely dedicated to x-ray astrophysics, 2) it launched on a rocket I'd never heard of before, and 3) it launched from a spaceport I'd never heard of before! First, because extraterrestrial x-rays are blocked by Earth's atmosphere, it was not until the mid-20th century that we confirmed their existence using instruments lofted on V2 rockets. While various other sounding rockets and high-altitude balloons studying cosmic x-rays preceded Uhuru, launching a dedicated orbital satellite allowed astronomers to build a comprehensive map of all x-ray sources in the sky. During its three year mission, it made many astonishing discoveries, such as the first x-ray binaries (binary stars with significant x-ray emissions) and detailed observation of Cygnus X-1, the first ever confirmed black hole!

Second, Uhuru launched on a Scout rocket, the first launch vehicle to reach orbit using only solid propellant. Solid fuel tends to have lower performance than liquid, and even with its four-stage design it could still only lift payloads of a few hundred kilograms, but the hardware was more-or-less off the shelf from existing ICBM designs and therefore easy to design

And finally, Uhuru launched from the San Marco platform of the Broglio Space Center, a spaceport owned by Italy off the coast of Kenya. Now I don't know about you, but I had no clue there were orbital launch facilities of any kind in Africa, so it was really neat to learn about this facility and how the US launched a few satellites from there in partnership with the Italians. The name Uhuru was chosen in recognition of the Kenyans, the word means freedom in Swahili. Unfortunately the site has been inactive since 1988, but just seeing a rocket stacked on an ocean platform is a cool precursor to what Starship could look like, launching from the sea one day

The Scout rocket, the Uhuru satellite, and the Broglio Space Center

And as for Pelz, well things turned out quite nicely for him since he left NASA Goddard in 1975. He was ranked one of the top 25 most influential golf instructors of the century, holds numerous golf patents, and coached many of his students to major championship victories. But he certainly remembers the good times he had at NASA - in 2006, he reunited with nine scientists from his former team, and after reminiscing with them he said,

"I am probably one of the few people who has ever left a job they loved as much as I loved working at Goddard... I've had an incredible life. I feel like I'm living the dream, and it has continued even after I left NASA"

Your City Still Looks Very Beautiful From Space

2021-09-11T03:54:00.000-04:00

"No day shall erase you from the memory of time"

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The purpose of Astronomical Returns has always been to make space more relatable to everyday people, so as we commemorate the 20th anniversary of the September 11th attacks, I thought it'd be fitting to share my own recollection of that dark day in American history, as well as that of the lone American astronaut not on Earth during 9/11, Frank Culbertson. I still find it hard to believe that two decades have passed. At this point there are basically two groups of people: those who experienced 9/11 and remember it vividly, and those who were born afterwards and only learned about it in history books. But there's a slim sliver of the population, people like me around 25 years old, who at the time were just barely old enough to recognize that something terrible was unfolding, but were simply too young to fully comprehend. All I have are fuzzy memories of my kindergarten teacher suddenly leaving the classroom one morning, then coming back with a small box-shaped TV and turning on the news to see smoke and flames

So for me and my contemporaries, I feel like over the course of our entire childhoods we had to retroactively piece together the significance of what we experienced that day as toddlers, the older we got and the more we learned about it. The number of lives lost, the history of American military power and Middle Eastern conflicts, the motivations of those who would commit such atrocities. Of course by the time I graduated college I'd long since gotten up to speed, but when I moved to New York to start work, I felt a deeper sense of the tragedy after visiting the 9/11 Museum at Ground Zero and seeing all the exhibits. By far the most moving one for me was the video taken by Frank Culbertson from the ISS, with the smoke from the Twin Towers clearly visible over Manhattan, and the words of comfort he offered to all Americans suffering on the ground below

As seen from orbit: smoke rising from the burning World Trade Center, the morning of September 11, 2001

Culbertson was on the ISS as part of Expedition 3, just the third long-term crew to inhabit the newly constructed space station (for reference, the current ISS crew is Expedition 65). This was Culberton's third spaceflight, having previously flown on STS-38 and STS-51, but unlike those short missions, he would be spending 4 months in space with his Russian crewmates, Vladimir Dezhurov and Mikhail Tyurin. One month into that mission, on the morning of September 11th, he was chatting with the flight surgeon in Mission Control, Steve Hart, who broke the news of the attacks to him with the now-famous statement, "Frank, we're not having a very good day down here on Earth"

Culbertson realized they happened to be passing over the southeastern part of Canada and would be coming up on the eastern seaboard soon, so he and his crewmates grabbed their video cameras and headed to the window that would give them the best view of lower Manhattan. As they passed overhead, Culbertson offered these words as he witnessed the destruction from 250 miles above the Earth:

"I hope that the people who are responsible are caught and brought to justice as soon as possible. But first, our prayers and condolences to all involved. And I just wanted the folks to know that their city still looks very beautiful from space"

Frank Culbertson (center), Mikhail Tyurin (left), and Vladimir Dezhurov (right) on Expedition 3

9/11 would bring personal tragedy to Culbertson as well. After passing over New York, their orbital trajectory took them over Washington DC, where he described a sort of haze over the city that he couldn't identify the source of. Only later did he learn that the Pentagon had been struck, and that the pilot of American Airlines Flight 77 had been Charles Burlingame, his close friend and fellow classmate from the US Naval Academy. To honor his fallen friend and all the others killed in the terrorist attacks, Culbertson played "taps" from the trumpet he had brought on board the space station

A fitting tribute from orbit. The full video from NASA showing 9/11 from ISS can be found here

To close, I'd like to share an excerpt from the emails Culbertson wrote in the days after 9/11, sharing his reflections on the tragedy he had witnessed, and how isolated he felt being the only American off-world while his country suffered below. It's publicly available on the NASA website, so I encourage you to read it in its entirety. It's really beautifully written

It's difficult to describe how it feels to be the only American complete off the planet at a time such as this. The feeling that I should be there with all of you, dealing with this, helping in some way, is overwhelming. I know we are on the threshold (or beyond) of a terrible shift in the history of the world. Many things will never be the same again after September 11, 2001. Not just for the thousands and thousands of people directly affected by these horrendous acts of terrorism, but probably for all of us. We will find ourselves feeling differently about dozens of things, including probably space exploration, unfortunately.

It's horrible to see smoke pouring from wounds in your own country from such a fantastic vantage point. The dichotomy of being on a spacecraft dedicated to improving life on the earth and watching life being destroyed by such willful, terrible acts is jolting to the psyche, no matter who you are. And the knowledge that everything will be different than when we launched by the time we land is a little disconcerting. I have confidence in our country and in our leadership that we will do everything possible to better defend her and our families, and to bring justice for what has been done. I have confidence that the good people at NASA will do everything necessary to continue our mission safely and return us safely at the right time. And I miss all of you very much. I can't be there with you in person, and we have a long way to go to complete our mission, but be certain that my heart is with you, and know you are in my prayers

It's A Hard Knock Life For Lunar Rocks

2021-08-06T03:47:00.001-04:00

"It wouldn't look very good if we went to the moon and didn't have something to do when we got there" - Max Faget

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Over the past two and a half years, I've been dedicating a full week of posts on the Astronomical Returns Instagram to each of the Apollo missions as they hit their 50-year anniversaries. Since this past week was Apollo 15's 50th birthday, lunar geology instantly came to mind!

All of the Apollo lunar landings were of course enormous technological achievements, but the more I've learned about them, the more it surprises me how much of the heavier geological science was only done on the later missions. Each of the crewed Apollo missions was given a letter designation to specify the nature of the mission: Apollo 11 was a G-type mission, meaning it's primary goal was simply to get boots on the moon, and not much else. Indeed, Neil and Buzz only spent two and a half hours on the surface, barely enough time to scoop some rocks, deploy the flag, chat with Nixon, and head back inside. Apollos 12-14 were H type missions, meaning their mission objectives were more ambitious and involved more precise landings and scientific instruments. Yet even with 2 EVAs, without the Lunar Rover they could only explore as far as they could walk and could only carry back 30-40 kgs of moon rocks. The task of really diving into the geology of the moon would fall on Apollo 15, the first of the J missions

Apollo 15 Lunar Module Pilot Jim Irwin with the LRV in the mountains of Hadley-Apennine

The Apollo astronauts were largely pilots, not scientists; training time leading up to a mission was a precious commodity that needed to be optimized, and most of them preferred to focus on flight objectives rather than learning about rocks. To bolster interest, NASA brought on geologists Lee Silver and Farouk El-Baz, and lucky for them Commander David Scott quickly took to the geology lessons and wanted to make sure he and crew members Jim Irwin and Al Worden gathered as much scientific data as possible. Fortunately, all three had been the backup crew of Apollo 12, so they were already familiar with the CSM/LM and could focus more of their training on science. Soon, the astronauts were making regular visits to Arizona and New Mexico, where Silver would accompany Scott and Irwin on long expeditions training them how to identify various rocks and minerals, while Worden would fly overhead in an airplane learning how to call out geographic features from above. Worden also spent significant time with El-Baz, studying lunar maps and practicing how he'd describe features on the moon while in lunar orbit to relay the most accurate information possible to geologists back on Earth. And after NASA had narrowed down the final landing site to two choices, Hadley-Apennine or Marius Crater, Scott pushed heavily for Hadley-Apennine due to its greater variety of geological features, and ultimately got his way

Irwin (left) and Scott (right) examining a rock formation at Cococino Point, Arizona

Once on the moon, Scott and Irwin got the chance to flex their newfound geology knowledge. In general there are two broad types of moon rocks: basalt, an igneous rock formed from the lava flow of ancient lunar volcanos, and breccia, a sedimentary rock formed from the remnants of smashed meteors and other minerals that got glassed and fused together (both basalt and breccia exist on Earth too). But beyond that, there are tons of subtleties in the regolith that reveal the history of the moon. From looking at just a single grain of sand, rounded edges would suggest a past where water flowed freely and moved the sediment around. With no liquid water to speak of, and no atmosphere to blow the dust around, the particles the moon remain jagged and hazardous to spacesuits and machinery. Furthermore, the surface of the moon varies widely in its age. The lowlands visited by the previous Apollo missions were relatively younger at 3 billions years old, and there are still yet-unexplored regions that may be as young as 1 billion years old. But Apollo 15 was looking for the other end of the spectrum in the highlands of Hadley-Apennine: a specific mineral called anorthosite, thought to be over 4 billion years old and come from the primordial lunar crust. When Scott and Irwin found their precious anorthosite sample, later dubbed the Genesis Rock, you can literally hear the excitement in their voices:

Scott: "Guess what we just found? Guess what we just found! I think we found what we came here for!... I think we might have ourselves something close to anorthosite, because it's all crystalline. What a beaut!"

Irwin: "That is really a beauty"

Basalt (left) and breccia (right), both from the moon. Contrast the basalt's porous structure, often seen in igneous rocks, with the breccia's speckled hodgepodge of minerals often seen in sedimentary rocks

So what was the big deal about anorthosite anyway? Well stepping back for a moment - even as recently as the mid-20th century, before the Apollo moon landings, there was still a ton of uncertainty about the origins of the moon. Some scientists speculated that perhaps the moon was some wandering body that got captured by Earth's gravity. But others believed in the Giant Impact Hypothesis, which suggests that 4.5 billion years ago, the young Earth collided with a Mars-sized planet (dubbed Theia) which blasted a ton of material into Earth's orbit that gradually coalesced into our nearest cosmic neighbor

For the highlands on the moon to be covered in anorthosite, there had to have been a time in the moon's history where its entire surface was completely molten. That way, this light, white igneous rock would float to the surface while denser materials would sink into the magma and down to the core. If the moon had indeed been formed out of a violent, planetary-scale collision, that would explain how it remained molten for much of its early history, and how all this anorthosite came to be. So Apollo 15's Genesis Rock provided crucial evidence for the Giant Impact Hypothesis and developed our modern understanding of the moon's origins

Other evidence for the Giant Impact Hypothesis: the moon's core is proportionally really small, relative to other solar system bodies. During the Earth-Theia collision, Earth would've absorbed most of Theia's core, leaving little leftover for the moon

Lastly, one other interesting fact about the lunar samples returned from the Apollo missions is that the vast majority of them remain untouched. While over the years NASA has carefully doled out samples to labs across the country and around the world, about 80% of humanity's moon rocks are still locked away in airtight vaults meant to preserve them for as long as possible, so that future scientists with new questions and more advanced technology will still have samples to work with. As much as I commend NASA for their foresight, part of me is sad they were right in predicting that 50 years after Apollo, moon rocks would still be an incredibly precious scientific commodity. Maybe in another 50 years, once the Artemis Program establishes a moon base and commercial lunar expeditions are a regular occurrence, lunar soil will finally be cheap as dirt

Lunar samples still in their storage vaults at Johnson Space Center

500 Years of Magellanic Magnificence

2021-07-19T03:53:00.003-04:00

"The Antarctic pole has no star of the fate of the Arctic pole, but we see many stars congregated together, which are like two nebulae, a little separated from each other" - Antonio Pigafetta (1520)

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Compared to most adults, I feel like I have particularly clear memories of my childhood days and the things I learned in school. In elementary, my teachers periodically had these "units" where we'd spend weeks going into great detail about an interesting topic. The space unit in 2nd grade was obviously my favorite, but I also distinctly recall a unit on Australia in 3rd grade, where we learned all about the Great Barrier Reef, Aborigines, kangaroos, and whatever the heck Vegemite is! Or in 4th grade, my teacher taught us about the Renaissance, and how da Vinci sketched out the first helicopter centuries ahead of his time, or how the Medici family's vast banking empire dominated Italy for generations (and whose pioneering double-entry accounting system follows, or should I say, haunts me to this very day!). In 5th grade, we had a unit on the Age of Exploration, and each of us was assigned a famous explorer to give a presentation on: I got Sir Francis Drake, and I was a little disappointed that my guy wasn't the first to circumnavigate the globe; that title belongs to Ferdinand Magellan!

Magellan was actually killed in the Philippines (where my parents are from) in 1521;
curiously, my dad told me that the guy who killed him, Lapulapu, is regarded as something of a national hero in modern times!

The Magellan voyage has long been lauded as a crowning achievement of human endeavor, so naturally Magellan's name lives on in modern astronomy and spaceflight, the grand voyages of our time. But while the most notable example is the Magellan probe, launched by NASA to Venus in 1989, I thought it'd be neat to talk about two prominent astronomical features that also bear his name, especially as we mark the 500-year anniversary of Magellan's journey (1519-1522)

The Small and Large Magellanic Clouds are two dwarf galaxies gravitationally bound to the Milky Way, about 200,000 and 160,000 light years away from us, respectively. They're easily visible features in the night sky and have been known by indigenous people in South America and Africa since antiquity, and by Arab astronomers as early as the 9th century. But because they're only visible from the Southern Hemisphere, they went completely undiscovered by Europeans until Magellan's time

The Small and Large Magellanic Clouds, visible over the Paranal Observatory in Chile

Curiously, despite their obvious prominence, there's relatively little mention of the Magellanic Clouds from navigational writings of that time. As Western sailors voyaged further and further south, they encountered new star fields that they quickly needed to master, and their number one priority was to find a star in the Southern Hemisphere that would readily tell them their latitude, just like Polaris does in the Northern Hemisphere. Because the Magellanic Clouds are too diffuse to serve any navigational purpose, they were mostly ignored by contemporary navigators. In fact, there were probably plenty of Europeans who saw the Magellanic Clouds before Magellan did - Portuguese explorer Alvise de Ca'da Mosto saw the Southern Cross in 1455, and though he didn't mention any fuzzy blotches, it's possible he could've seen the Magellanic Clouds. For sure the Italian explorers Amerigo Vespucci and Andrea Corsali saw it before Magellan, because both wrote brief mentions of the clouds in 1501 and 1515

The main reason the two dwarf galaxies ended up being associated with the Magellan Expedition is because of Magellan's chronicler, Antonio Pigafetta. He was neither an astronomer nor a navigator, so he simply wrote in colorful detail about whatever caught his eye! And because the Magellan Expedition soon became famous, much of his writings survived the course of history, and over time sailors colloquially started referring to them as the Magellanic Clouds

Two pages from Pigafetta's manuscript, showing exquisite illustrations and handwriting

Today we know much more about the Magellanic Clouds than Magellan or his contemporaries ever could've imagined. The Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) are separated by about 75,000 light years, and the LMC has a diameter of 14,000 light years (roughly double the SMC). Compared to the Milky Way, the LMC is estimated to have about 1/10th the mass, and the Clouds have somewhat younger stars and a higher proportion of hydrogen and helium than heavier elements

More interestingly (in my opinion), astronomers have been trying to better describe the motion of the Magellanic Clouds relative to the Milky Way. Based on recent observations by Hubble, they appear to be much closer to us now than they have been in the past and may be moving too fast to be long-term companions of the Milky Way. In fact, we might even be witnessing the beginnings of a galactic merger with the Magellanic Clouds, which would take billions of years and would precede or coincide with the Milky Way-Andromeda collision that we know will happen in 4.5 billion years! On time scales that long, the Magellan Expedition feels like it happened just a blink of an eye ago!

Supernova SN1987A within the Large Magellanic Cloud (the bright one in the center).
It was the brightest supernova ever observed since the invention of the telescope

To learn more, check out this paper by Michel Dennefeld, titled "A History of the Magellanic Clouds and the European Exploration of the Southern Hemisphere"

Roche Limit: The Radius of Disintegration

2021-06-20T21:24:00.014-04:00

"Nature does not hurry, yet everything is accomplished" - Lao Tzu

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I think most guys can recall memorable father-son activities they did growing up - for me it was primarily golf, but another one I distinctly remember is watching old-school Chinese kung fu movies with my dad. Most of them were exactly as you'd expect: awful, complete with ketchup blood, goofy one-liners, dreadful acting. Of course it was fun nonetheless, and some of them were halfway decent! (check out Snake in the Eagle's Shadow and Drunken Master: two classics by a young Jackie Chan)

But to this day, I still don't really like watching Chinese-made action movies: the modern ones are chock-full of heavy-handed CGI, not-so-subtle propaganda, and more bad acting (Wolf Warriors is a perfect example of a film that was crazy popular in China, but that I found abysmal). So when a good friend of mine recommended I watch The Wandering Planet, I was pretty skeptical and only agreed after I learned the movie is based off a short story by Cixin Liu, whose Three Body Problem trilogy is one of the best sci-fi stories I've ever read. The movie was pretty mediocre, but it did introduce me to a neat concept in orbital mechanics: the Roche limit!

哎呀 - AIYAAAAAAAAAA!!!!!!
Ladies and gentlemen, I present to you the greatest death scene in the history of cinema

The basic premise of The Wandering Planet is that the sun is dying, so humanity builds enormous planet-moving thrusters to eject Earth from the solar system and find a new host star. But a malfunction en route causes Earth to be captured by Jupiter's gravity, leading to a desperate effort to restart the engines before Earth falls within Jupiter's Roche limit and gets ripped apart by tidal forces**. The Roche limit is a real phenomenon; calculated in 1848 by French mathematician Edouard Roche, it's the distance from a central body within which a satellite would disintegrate as tidal forces overcome the satellite's gravitational self-attraction. Said differently, matter inside the Roche limit cannot coalesce into moons, but will instead break apart and possibly form rings. Now, the Roche limit is different for every central body-satellite combination. The question is, what factors does it depend on?

Tidal force**: the gravitational effect that stretches a satellite in the direction of the central body, because the parts of the satellite closer to the central body are more strongly attracted than the parts that are farther away. If this disparity is greater than the gravitational self-attraction that holds the satellite together, it can rip the satellite apart

Diagram of tidal forces, showing the satellite being stretched in the direction of the central body

Visualizing the Roche limit

The answer is that it mostly comes down to two factors: the ratio between the densities of the central body and the satellite, and the rigidity of the satellite. For the densities, although determining these values is challenging in practice (we've got pretty good values for the densities of the Earth and the moon, but what about some random unexplored body like Neptune's moon Thalassa?), mathematically it's a simple input. It's the rigidity of the satellite that makes determining the Roche limit extremely difficult.

Consider the simplest possible scenario: a perfectly rigid spherical satellite (meaning it will maintain its shape exactly, right up until the moment it breaks apart). In this case, the formula for calculating the Roche limit is given as:

$d = R_M (2 \frac{\rho_M}{\rho_m})^\frac{1}{3}$

Whereby

$d$: Roche limit $R_M$: radius of the central body $\rho_M$: density of the central body $\rho_m$: density of the satellite

Now consider the opposite extreme: a perfectly fluid spherical satellite (imagine an enormous spherical water droplet orbiting a planet). In this case, the formula for calculating the Roche limit is given as:

$d \approx 2.44 R_M (\frac{\rho_M}{\rho_m})^\frac{1}{3}$

You'll notice the approximate sign for the fluid formula: even in this already heavily simplified example, the Roche limit can't be determined by an exact algebraic formula (see the Wikipedia page for an in-depth derivation of the Roche limit formulas)

Great animation from BBC's The Planets showing an icy moon disintegrating as it passes the Roche limit

So if these two formulas can only describe our highly-contrived scenarios (perfectly rigid / perfectly fluid) and ignore other factors like the satellite's rotation or tensile strength, what good are they? Well, since actual satellites would fall somewhere in between being perfectly rigid and perfectly fluid, we can use them to estimate what the upper and lower bounds of the Roche limit would be! Taking the familiar example of the Earth and the moon, knowing that Earth's radius is 6,378km, Earth's mean density is 5,513kg/m^3, and the moon's mean density is 3,346kg/m^3, we can determine that the Earth-moon Roche limit is 9,492km based on the rigid formula and 18,381km based on the fluid formula, and that the actual Roche limit likely lies somewhere in between. Given that the moon orbits at about 384,399km (21x further than the more conservative fluid Roche limit formula), we can feel pretty confident that the moon isn't going to disintegrate anytime soon!

Other selected examples of Roche limits for various central bodies-satellites

To see more detailed and accurate Roche limit computations, definitely check out the Wikipedia!

The Lost Decade of Planetary Science

2021-05-26T02:35:00.000-04:00

"I think the Space Shuttle is worth one billion dollars a launch. I think that it is worth two billion dollars for what it does.

I think the Shuttle is worth it for the work it does." - Pete Conrad

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One of the many reasons I enjoy writing about space so much on Astronomical Returns is that there's an incredibly diverse array of space-related subfields, so I never run out of topics to explore. Since I work at SpaceX's Finance department, space finance has become my favorite and most unique specialty, but way back when I was a kid, one subject piqued my interest above all else: planetary science! The reason for this was quite simple - it's the easiest space topic for a 5 year old to grasp. Rocket science is too hard, theoretical physics and cosmology are too abstract, space history is too esoteric, etc. But learning about the solar system came naturally to me; after all, any kid with unfettered imagination and half a brain can admire the incredible photos we have of the planets, moons, and asteroids!

I suppose it's my good fortune then that I was born in the 1990s and grew up in the 2000s, a period of relative resurgence for NASA's planetary science programs. Had I grown up two decades prior, perhaps my interests would've been shaped differently, because the 1980s were unequivocally the lost decade of planetary science. In fact, between 1978 and 1989, the US didn't launch a single planetary mission. What happened??

Pathfinder and Sojourner hold a special place in my heart because they launched in 1996, the year I was born.
Perhaps my niece Isabelle, born in the year 2020, will grow up to have a special affinity for Perseverance and Ingenuity.

When the Space Race began, Congress and NASA were laser focused on putting a man on the moon, but fortunately they still had the foresight to recognize that rapid developments in rocketry would also enable robotic expeditions across the solar system, and they allocated funding accordingly. With generous grants, NASA was able to attract scientists from other fields like geology and chemistry and offered them the chance to launch their instruments to space. Perhaps most crucially, the Jet Propulsion Laboratory, which had been founded through Caltech back in 1936, was transferred from the Army to NASA when NASA was established in 1958. Almost immediately, the scientific community at JPL proved themselves to be the springboard of US planetary exploration: the Mariner probes built at JPL in the 1960s and 70s became the first spacecraft to fly by Venus, Mars, and Mercury, followed soon after by the Viking landers which reached the surface of Mars, as well as the Voyager probes that flew by Uranus and Neptune and have since left our solar system! The Golden Age of planetary science seemed at hand!

The entrance to JPL in 1957, just before NASA was created

But as the 1970s progressed and the Apollo Program ended, it became clear that NASA's budget was facing major cutbacks, and once Ronald Reagan was swept into office in 1981, small government and fiscal discipline became the rallying cry of the new administration. NASA had three priorities at the start of the 1980s: 1) complete the Space Shuttle, 2) maintain a solar system exploration program, and 3) conduct advanced aeronautical and other scientific research. Despite adamant protests from NASA officials, Congress' draconian budget proposal would have left NASA with no choice but to shut down one of these three programs. Axing the Space Shuttle was a non-starter even in the face of rapidly mounting costs due to its importance for both human spaceflight and national security, so solar system exploration was left to compete with other scientific fields like astronomy and astrophysics. All of a sudden, proposals like the Hubble Space Telescope, the Galileo spacecraft to Jupiter, a flyby of Halley's comet, and a Venusian radar-mapping mission were all on the chopping block, and NASA was seriously contemplating divesting itself entirely from JPL. Consider this excerpt from NASA Administrator James Beggs:

"It is our judgment that in terms of scientific priority [planetary exploration] ranks below space astronomy and astrophysics... it is ultimately better for future planetary exploration to concentrate on developing the Shuttle capabilities rather than attempt to run a "subcritical" planetary program given the current financial restrictions we face. Of course, elimination of the planetary exploration program would make the Jet Propulsion Laboratory in California surplus to our needs"

Many scientists at NASA fought tooth and nail for a mission to Halley's Comet but ultimately couldn't get funding.
Instead, the European Space Agency sent the Giotto spacecraft in 1986

With the moribund state of planetary exploration becoming undeniably apparent, a herculean effort on behalf of both the scientific community and grassroots space fans arose to find powerful allies who could save the program. The newly founded Planetary Society saw its ranks grow immensely as members began reaching out to their Congressmen. Caltech in particular was outspoken in their support, given that they manage JPL. In one letter, Caltech trustee Arnold Beckman wrote to a senior Reagan Administration official that the budget cuts threatened to "create total chaos and rapid disintegration of a 5,000 person, \$400 million Southern California enterprise" and would be a complete calamity for Reagan and the Republican Party, especially in the President's own home state. All the political pressure gradually bore fruit, as Senate Majority Leader Howard Baker wrote President Reagan asking to restore $87 million in funding for the Galileo mission in the FY 1983 budget. Planetary science was still hanging by a thread, but it had a lifeline

Though it didn't launch until 1989 (due to delays from the Challenger disaster), Galileo successfully revived US planetary exploration

Seizing their chance to save their field, planetary scientists formed a NASA-sanctioned Solar System Exploration Committee (SSEC) and held numerous meetings to hammer out a planetary exploration program that could endure in the new political environment. During the 1960s and 70s, there was no real long-range plan for approving and funding planetary missions; instead, big flagship projects were proposed and considered ad hoc, and though missions like Viking and Voyager proved to be wildly successful, their high price tags left little funding to spare. Without no regular cadence, planetary science would always be subject to wild swings in interest and funding. Thus, the SSEC proposed the following structure: future missions would be characterized into three funding tiers: Pioneer-class (\$100 - \$150 million), Mariner-class (\$300 - \$500 million), and Viking-class (over \$1 billion), with much greater emphasis on the more cost-effective Pioneer-class missions to make budget approvals easier. To this day, this three-tiered mission categorization still exists, albeit with different monikers. And more recently, NASA and the National Academies of Sciences began publishing a decadal planetary science survey to compile the scientific community's top priorities for solar system exploration in the coming decade, so as to better evaluate future mission proposals. With structure in place, and with NASA's funding pressures somewhat alleviated once the Space Shuttle got off the ground (even in spite of the Challenger disaster), planetary science survived its lost decade and has grown into the modern program we know today

Lastly, I'm a finance guy, which means you know I like charts. These line charts by Jason Callahan at the Planetary Society, showing planetary science missions in the '60s, '70s, and '80s, really depict just how near death planetary science was in the '80s

And if you want to learn more about how planetary science nearly met its demise in the 1980s, I highly recommend this essay here by John M. Logsdon

Oh So Many Ways to Weld A Rocket

2021-05-02T21:34:00.002-04:00

"Welders were created because engineers need heroes too"

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I once saw a meme poking fun at two books titled "What They Teach You at Harvard Business School" and "What They Don't Teach You at Harvard Business School". The joke was that by definition, these two books combined must contain the complete sum of all human knowledge! Well if that's the case, then the chapter on welding techniques must be in the latter book, because as someone who studied finance in undergrad, I can personally attest that they do NOT teach you welding in business school...

When I transitioned from Wall Street to SpaceX, I knew that the sort of finance I did for investment banking is pretty different from corporate finance at an actual company, so I'm constantly trying to learn more and more about the operations the actual engineers and technicians on the floor are performing. As it turns out, apparently there's a bazillion different ways to weld metal together, each with its unique pros and cons for building a rocket. So I thought it'd be neat to dive into a few examples!

The practice of welding dates back several millennia to the Bronze and Iron Ages, yet for most of human history, if you wanted to join two pieces of metal together, pretty much your only option was forge welding, whereby a blacksmith heated the metal in a furnace until it approached its melting point, then manually hammered the red-hot pieces together into the desired shape. This process works fine if you're building a pretty simple tool like a sword or a plow, but as the Industrial Age began, modern factories needed more efficient and consistent methods of joining metal

The first real advancement in welding was the invention of arc welding in the late 1800s, which uses an electric current generated between a metal stick (the electrode) and the base metal to provide the heat necessary to melt the material. Arc welding, in its numerous variants and techniques, was crucial behind the production efforts of all the countries involved in the World Wars, and even by the onset of the Space Race, the Saturn V and much of the other hardware needed to get to the moon was still being constructed by skilled workers manually joining the components of the F-1 engines and propellant tanks with arc welding. No such thing as computer-aided design (CAD) back then, just expert craftsmen with unmatched dexterity!

Perfect arc welds done on the Saturn V F-1 engine's gimbal mount (bottom left) and turbopump exhaust manifold

While conventional arc welding is simple and inexpensive, it inevitably comes with its drawbacks, mainly that certain materials like high-strength steel, titanium, and aluminum alloys may be susceptible to embrittlement due to reactions with the surrounding air. To protect the weld from ambient nitrogen and oxygen, welders began using inert shielding gases in the first half of the 20th century. Many techniques were developed, but one known as tungsten gas arc welding (GTAW), also known as tungsten inert gas (TIG) welding, is used particularly often in aerospace

Northrop was the first to perfect GTAW in 1941. It uses a torch containing both a non-consumable tungsten electrode and a constant source of shielding gas (often helium or argon) to simultaneously heat the metal enough to create the weld and protect the weld from the surrounding atmosphere. I found this pretty neat article about how GTAW was crucial to repairing a cracked hydrogen fuel line on Space Shuttle Atlantis that was discovered just before its flight on STS-112 in 2002. Because the cracks were minuscule (some weren't even visible to the naked eye and were only discovered with borescopes, X-rays, and ultrasonic scans), it took extreme coordination by the welders to ensure all the cracks were successfully penetrated without excessive heat deformation to the surrounding metal

A diagram of Gas Tungsten Arc Welding, and liftoff of STS-112 in October 2002

Of course, welding technology has continued to advance in more recent decades, with newer techniques employing more exotic methods of melting and joining metal while minimizing defects as much as possible. Rather than relying on electricity or combustion to generate heat, energy beam welding uses either lasers or high-velocity electrons to generate the heat needed to make extremely precise, high-penetration welds with minimal distortion to the surrounding metal. While these high-tech processes are fast and automated, they're unfortunately quite costly. Electron beam welding is also challenging because it can only be performed in a vacuum environment, otherwise air molecules would completely disrupt the beam of electrons! To that end, while any factory here on Earth must first invest in a vacuum chamber to take advantage of electron beam welding, in a cool technology demonstration, the Soviets used a handheld electron beam gun to make welding repairs to the Salyut 7 space station during a spacewalk in 1984!

Soviet cosmonaut Svetlana Savitskaya, the first woman to ever walk in space, using her electron beam gun to perform maintenance on Salyut 7

Finally, there's an entire class of solid-state welding methods that completely sidesteps the process of melting metals together and instead joins them via some other highly energetic process, such as intense vibrations or pressure. One relatively recent technique called friction stir welding is very popular nowadays for manufacturing the propellant tanks on rockets, due to its ability to rapidly make high strength welds across very large workpieces. Rather than melting the metal plates, a rapidly rotating tool generates just enough heat through friction to soften the metal, then mechanical pressure is applied to join the two "plasticized" pieces together, much like molding clay. There's an insightful article by Ars Technica that discusses the enormous friction stir weld tools that had to be installed at NASA's Michoud Assembly Facility in Louisiana for the Space Launch System

A liquid oxygen tank test article being constructed for SLS at Michoud using friction stir welding

Any other welding techniques you know of that are commonly used in aerospace? The only joining I do at SpaceX is the INNER JOIN of a SQL query (sorry, nerdy finance joke!), so would love to hear your thoughts!

A Lunar Landing Of My Own

2021-04-18T01:05:00.001-04:00

We are going to the Moon. This time, to stay

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My very first day of college, I wrote a letter to my future self, sealed away in an envelope and only to be read again in four years when it came time for me to graduate. It wasn't my idea, it's actually a tradition of the Business Honors Program at UT Austin, so all my classmates did the exact same thing. I thought it was pretty goofy at the time, and I certainly didn't dwell on it much during my four years of undergrad. But graduation came along in the blink of an eye, and when I held that envelope in my hand again, I realized I'd completely forgotten what I'd written four years ago! I figured it was probably super cringeworthy - who knows what kind of crap 18-year-old Hans jotted down?! Fortunately, I was pleasantly surprised. Naive as I may have been, my younger self had some nice words of wisdom

This article will be a similar time capsule for me, because it captures a moment of euphoric exhilaration that I desperately wish to preserve. I knew SpaceX was in the running for the Artemis Human Landing System contract, and I knew NASA was going to make the announcement soon, but somehow the news still hit me out of the blue. There I was, sitting at my desk Friday morning when I overheard one of the other Finance managers:

"We just won it! We won it all! Starship is going the Moon!!"

Word spread quickly, by lunchtime it was trending all over social media. By the time NASA made the official announcement that afternoon, even my mom had heard the news! And when Kathy Leuders, NASA's head of human spaceflight, declared that SpaceX had been selected, everyone around me started applauding and cheering!

Like every other space fan alive today, I was inspired by the Apollo moon landings. I spent my entire childhood drawing outlandish crayon rocket ships, hoping that one day, I might live to witness one of my crazy vehicles touch down on that silvery wafer in the night sky. But of course, even by the time I was born in 1996, the last footprints on the moon had already been left untouched for 24 years. The sad, simple truth is that those legendary moon landings, the foundation of my career and my calling, well... they were never really mine to begin with. And unless you're at least in your 50s, they aren't yours to claim either. The moon landings were the achievements of our parents and grandparents; for my generation, a generation confined to Low Earth Orbit, all they are is an unfulfilled legacy, images and videos of a bygone era when humanity was briefly a species of two worlds

As kids, we don't think much of our parents' careers. Children go to school and adults go to work, that's just the way it is. Only now, having myself been in the workforce for a little while, have I come to appreciate what a profound statement it is for someone to say they've dedicated their entire life to a cause they believed in. It's been only three years since I graduated college; I've spent a year and a half on Wall Street, almost a year and a half at SpaceX, and already I catch myself dreaming about how wonderful retirement must be, or what I wouldn't give to at least have an extra week of vacation per year. But then I remember that my dad has been going into the same office in the San Antonio Medical Center, treating patients day in and day out for the past 25 years (and that's after an entire decade of medical training!), with no plans to quit anytime soon, and suddenly I feel silly for ever having complained. Three decades from now, I wonder if my future kids will say the same thing about me. "My dad is crazy - he's spent the last thirty years going into the same office, sitting at the same desk, building Excel spreadsheets, answering tedious phone calls, sending countless emails, all because he wanted a lunar landing that he could claim as his own. A lifetime of work, just so someone else could put a flag on Mars"

Maybe I am crazy. But if I am, that's probably something I'll only realize in retrospect. I say this article is a time capsule for me, because I eagerly look forward to rereading it the day Starship lands astronauts on the moon. I've already seen ten thousand times over the videos of everyone at SpaceX going absolutely nuts when Falcon Heavy first landed. I was there when Dragon lifted the first astronauts from US soil in nearly a decade - I stood right next to the CFO as the entire building erupted in cheers. And I can only imagine how it'll feel for me to be on the floor in front of SpaceX mission control and witness the Artemis III crew take their first steps on the moon, knowing that my efforts made a tiny contribution to the spacecraft that got them there. Only then will I finally have a lunar landing of my own

Martian Color Calibration with Mastcam-Z

2021-04-11T21:20:00.002-04:00

"Are we alone? We came here to look for signs of life, and to collect samples of Mars for study on Earth.

To those who follow, we wish a safe journey and the joy of discovery"

- Inscription on the Perseverance rover

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I've been writing Astronomical Returns for more than two years now, and I still have visions of grandeur that one day I'll go viral, just like The Everyday Astronaut on YouTube or @BocaChicaGal on Twitter. Of course, the secret to success on social media is good photography, and the secret to good photography is, you guessed it... good lighting! My sister certainly has this skill down, her insanely popular food and dog pages owe their following to the great lengths she goes through to get the best lighting for her pictures: waking up at the crack of dawn for the perfect sunrise shot, expensive flash stands for a subtle white glow, and hours and hours of photo editing

Now ever since Perseverance landed on Mars last month, we've all been treated to a barrage of breathtaking HD photos from the Red Planet. Just like on Earth, lighting on Mars varies greatly depending on the time of day, the season of the year, and the presence of dust or other atmospheric phenomenon. But unfortunately for Percy, or any other robotic photographer, the rover doesn't have the luxury of a human eye on-site, which means there's no way to adjust for the constantly changing lighting conditions. All the rover can do is point and shoot, and leave post-processing to the scientists receiving the data back on Earth. So how do we solve this problem?

First full-color image from Perseverance, taking February 19, 2021

The solution is to bring along a little calibration target, a physical color palette whose brightness and hue is pre-measured on Earth before launch. Then whenever Perseverance's two Mastcam-Z scientific cameras snap an image of the Red Planet, we can also tell it to take a picture of the calibration targets. By comparing the live image of the calibration target against the predetermined standards, the Perseverance camera team will know exactly how to edit the images to reflect the actual lighting conditions on Mars!

Perseverance carries two calibration targets in different positions on the rover so that at least one of them isn't blocked by shadow. The primary target is a funny looking contraption: just 4 inches wide and weighing only 118 grams, it consists of 4 concentric grayscale rings that are used primarily to determine proper exposure. Outside of those rings, there are 8 colored dots used to calibrate image coloring, and in the center there's a small post called a gnomon that functions like a sundial to intentionally cast a shadow on the calibration target. Finally, the calibration target has heavy duty samarium-cobalt magnets mounted underneath it to repel Martian dust that would cover it up and render it useless

"Two Worlds, One Beginning" - A closer look at the calibration target in an actual image from Mars

The concept of carrying a color calibration target is not new; every mission NASA has sent to the Martian surface carried one, and even the manned Apollo missions brought along some calibration targets as they were exploring and photographing the lunar geology. One image from Apollo 17 was particularly memorable when the astronauts discovered surprisingly orange soil after digging just a few inches beneath the dull gray surface - you can see the color calibration target in the back left, which corroborates that the soil was indeed orange and not just the result of weird lighting or poor photo editing. Of course, the astronauts' own eyes were proof enough, as lunar module pilot and geologist Harrison Schmitt excitedly declared to mission control, "There is orange soil! It's all over! Orange!!"

As imaged by Apollo 17. The orange soil was the result of ancient volcanic processes when the Moon was still geologically active

More recently, when China's Chang'e 4 became the first spacecraft to land on the far side of the moon in 2019, it caught a lot of people by surprise when the images it returned depicted the surface as a deep reddish-orange. Has the far side always been so colorful, and we've just never seen it up close and personal? No, the real explanation is that Chang'e 4 didn't include a color calibration target, since the primary objective of the Yutu-2 rover it was carrying was navigation and exploration, not scientific photography. Getting the color exactly right wasn't a priority, so the Chinese simply released the unedited photo. By subsequently adjusting the RGB balance to weight green and blue more heavily, space fans online provided their best approximation of what the true color on the far side of the moon would have looked like

Raw photo on the left, as released by the Chinese space agency, vs approximate true color on the right

To learn more, check out the Planetary Society's detailed article on Perseverance's Mastcam-Z calibration targets here!

The Rock Star's Dusty Astronomy Thesis

2021-03-21T02:52:00.006-04:00

"Astronomy's much more fun when you're not an astronomer" - Brian May

I wholeheartedly agree, that's why I studied finance

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I'm only 25 years old, so undoubtedly there's a ton of great music from the last century that I wasn't around for and won't ever fully appreciate. But of all the classic rock bands, I'd definitely say Queen is my favorite: Bohemian Rhapsody is still a kick-ass karaoke song, 46 years after its original release. Plus, Queen's lead guitarist Brian May happens to also hold a Ph.D. in astrophysics, which is all the more reason for me to like them! Just recently, I watched the Bohemian Rhapsody movie about Freddie Mercury and Queen, and it dawned on me I actually didn't know what exactly May wrote his thesis on, so I thought it'd be fun to try jump into it and provide a synopsis

At 73, Brian May is still making music! In 2019 he released a single called New Horizons after the spacecraft to Pluto

As an undergraduate, May had studied math and physics at Imperial College London and remained there from 1970 to 1974 to pursue a doctorate in astrophysics, but he decided to put his Ph.D. on hold after Queen's meteoric rise to fame. What he was studying was the zodiacal light, a faint glow visible in the night sky that can be seen just after twilight in the spring or just before dawn in the fall. This phenomenon has been observed and recorded for centuries; the Islamic prophet Muhammad referred to it as the "false dawn" and described how to distinguish it to avoid mistiming the 5 daily prayers in Muslim tradition. In modern times, we know this glow is caused by sunlight being scattered by the enormous, diffuse cloud of interplanetary dust that extends far beyond Earth's orbit. Researchers had analyzed the spectroscopy of the zodiacal light and realized that it had the exact same absorption spectrum as the sun, meaning it must be caused by reflections off solid objects, rather than a gas which would alter the sunlight's observed spectrum. Yet even after the Pioneer 10 spacecraft confirmed this in 1972, there remained another question: where did all this dust come from in the first place?

Beautiful image of the zodiacal light, which is glowing even brighter than the Milky Way off to the right

That's where May comes in - by 2006, there still hasn't been much research done on the nature of the interplanetary dust cloud, so May picks up his dusty, unfinished thesis (pun intended!) from over 30 years ago and reregisters for his Ph.D. His paper, titled "A Survey of Radial Velocities in the Zodiacal Dust Cloud", depicted the overall motions of this dust cloud relative to Earth's orbit by examining the tiny redshifts/blueshifts in the cloud's absorption spectrum over repeated observations. Now unlike detecting redshift in a single star or galaxy rapidly expanding away from us, May's tasks was not as simple as examining the absorption spectrum from a single grain of dust: every observation is a sample of a bazillion particles, all moving haphazardly throughout the solar system. So what May was measuring was a change in the statistical distribution of the spectral lines from the dust cloud, and to do this he needed a device called a Fabry-Perot Interferometer

Some figures from May's actual thesis - bottom left illustrates the point about statistical distributions of redshifts and blueshifts

Similar to how a microscope allows a scientist to closely examine a small sample, a Fabry-Perot Interferometer allows an astronomer to zoom in on an extremely precise wavelength range of the electromagnetic spectrum for closer study. It lets light inside the device where it is partially reflected and partially transmitted between two closely spaced mirrors, then focused back onto a screen to create an interference pattern. Armed with his interferometer and a coelostat (basically a motorized mirror that tracks a fixed portion of the sky over time as it rotates overhead), May spent a bunch of time at Teide Observatory in Tenerife, Spain, making repeated observations of the zodiacal light to collect data on their absorption spectra. After crunching the numbers, he found the patterns were consistent with a flat interplanetary dust cloud that orbited the sun in the same direction as the Earth. This is a crucial finding because it suggests the dust cloud is generated by the asteroid belt, which rotates in the same direction as the rest of the solar system, as opposed to comet tails (which come into the solar system equally in prograde and retrograde orbits) or interstellar dust (which would come in at an angle completely different from the solar system)

Great animation of a Fabry-Perot Interferometer | Credit: YouTube

Two other points worth noting: first, May's conclusion that the zodiacal dust cloud originates from the asteroid belt is supported by observations made by the Infrared Astronomical Satellite (IRAS) in 1983, which revealed that the dust cloud had an internal structure of concentric disks, many of which lined up with known asteroid clusters. And second, why did astronomers believe that the zodiacal dust cloud had to "originate" from some source in the first place? Couldn't it have just been leftovers from the formation of the solar system? The reason is that if it weren't being constantly replenished, such a diffuse cloud of tiny particles would be washed out of the solar system within a few hundred thousand to a few million years by two forces: 1) solar wind would easily push the dust cloud off into interstellar space, and 2) another phenomenon known as Poynting-Robertson drag, whereby solar radiation reduces the angular momentum of an orbiting dust particle, causing it to fall into the sun. So the fact there is a dust cloud within the vicinity of the Earth, billions of years after the formation of the solar system, means it had to be produced relatively recently

The zodiacal dust cloud, as illustrated in May's thesis

There's a cool Wikipedia page titled "List of Celebrities With Advanced Degrees", and I saw that Dexter Holland, the lead singer for the band The Offspring (another band I like) has a Ph.D. in molecular biology, which is pretty neat! So, if Brian May can get his Ph.D. in astrophysics at the age of 60, then that means I have 35 more years to supplement my bachelors in finance with a degree in aerospace engineering!

To learn more about his thesis on the zodiacal dust cloud, check out Scott Manley's fantastic video here

Acquire and Assemble a Spacebound String-of-Pearls

2021-03-07T00:35:00.001-05:00

"How do you make money? Spinoffs, split-ups, liquidations, mergers and acquisitions" - Mario Gabelli

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With each day, the time I spent working on Wall Street recedes further and further into the rear view mirror. Of course, I absolutely love my role on the SpaceX Finance team, but the sort of analysis I do now is very different from the finance I did in investment banking: I haven't read an SEC filing, built a DCF (discounted cash flow) model, or calculated WACC (weighted average cost of capital) in over a year! But in keeping up with the aerospace industry, I still get really excited whenever I read about IPOs or M&A deals where I can draw from my experience in banking - it makes all those all-nighters I pulled a little more worth it...

Recently I stumbled across a fascinating company named Redwire. They're an innovative provider of a whole suite of space infrastructure solutions, everything from payload adapters to in-space manufacturing. I saw they'd just acquired Deployable Space Systems, a company that makes deployable solar arrays, but what really surprised me was that this acquisition was their 7th purchase in the past year, when the company was first bought out by AE Industrial Partners, a private equity firm that specializes in the aerospace, defense, and government services markets. Immediately I could tell that AE's strategy was to build Redwire into a diversified platform via roll-up acquisitions, known in business jargon as assembling a "string-of-pearls". So what does this mean, and how does it work?

A Roll Out Solar Array built by Deployable Space Systems being tested on the ISS in 2017

Back when I was a student at the University of Texas, I once gave a lecture to the freshmen of my investing org on why companies pursue M&A. The ultimate goal is quite simple: create a combined company that's worth more than the sum of its parts due to the synergies of the partnership. Of course in practice the results are often mixed: Disney/Pixar did it well, as did Facebook/Instagram. AOL/Time Warner, well... that's a different story (it was a total disaster). The thing about these well-publicized mega-deals is that they're risky, since the acquiring company is probably writing a huge check to buy out a well-established asset at a hefty premium, and then there's the challenge of integrating such a large purchase into the existing company

A great summary of M&A rationales from Breaking Into Wall Street. My old modules from back when I was training for investment banking

Now apart from the few dozen mega-deals that make it to the front page of the Wall Street Journal, in reality tens of thousands of smaller acquisitions like Redwire buying Deployable Space Systems happen every year. And while less flashy, there are certain advantages to pursuing these smaller "tuck-in" acquisitions, namely that they're often cheaper (not just on an absolute basis, but relative to earnings) and easier to integrate. In the case of Redwire, AE Industrial Partners takes it one step further - rather than just opportunistically acquiring good companies as they come along, Redwire's fundamental corporate strategy is to consolidate the disparate players in the space manufacturing industry in a quick acquisition spree. Assembling a string-of-pearls works best in a fragmented sector with many small players, and space manufacturing fits the bill perfectly with the explosion of startups all trying to take advantage of the increased access to orbit we've seen in the past decade. The hope is that a consolidated technology platform with economies of scale and stronger capital backing will be worth more than a bunch of startups all struggling to compete and stay afloat

Timeline of Redwire acquisitions

Of course, attempting to build a string-of-pearls is not without its risks. To succeed, the company must quickly identify multiple well-run and cheaply-priced targets, otherwise the premiums paid on so many acquisitions will eat into the return on investment. And post-acquisition, the management team needs to be adept enough to run the various and potentially dissimilar business segments. Otherwise, a poorly executed rollup won't leave you with a string-of-pearls, just a hodgepodge of misfit assets all bundled together. The assets that AE Industrial Partners has cobbled together in Redwire certainly look solid, so I'm excited to see how the company develops! Hope you enjoyed learning about corporate acquisition strategy!

Chien-Shiung Wu and the Demise of Parity

2021-02-16T02:02:00.007-05:00

"It is shameful that there are so few women in science... There is a misconception
in America that women scientists are all dowdy spinsters" - Chien-Shiung Wu

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This actually wasn't the Astronomical Returns article I was planning to write this week, but recently there's been a spike in terrible hate crimes against Asian Americans across the US amid the coronavirus pandemic, so I decided this would be the perfect story to share, especially since I myself am Asian American. In the realm of cosmology and theoretical physics, icons like Albert Einstein and Stephen Hawking are household names, and those of us who can remember our high school physics classes may recognize others like Heisenberg, Schrodinger, and Planck. But few people have heard of Chinese-American experimental physicist Chien-Shiung Wu, whose discovery of parity violation was a major contribution to the development of the Standard Model of Particle Physics. So who was Dr. Wu, and what does parity violation even mean?

Chien-Shiung Wu in the 1940s, shortly before joining the Manhattan Project

Chien-Shiung Wu was born in Jiangsu Province, China in 1912, and attended various institutions across China, including the National Central University in Nanjing and Zhejiang University in Hangzhou, initially studying math but later transferring to physics. While doing graduate level research, her supervisor encouraged her to apply for a doctorate abroad at the University of Michigan like he did, but although she was accepted, she was extremely put off by the fact that women weren't even allowed to use the front entrance at Michigan, so she attended UC Berkeley instead. Her thesis there laid the foundation of her expertise in beta decay (the emission of an energetic electron or positron from an atomic nucleus) - she studied the radioactive isotope phosphorus-32 for potential use as a radioactive tracer for cancer treatment, as well as the radioactive isotopes of xenon produced by uranium fission

Wu and her husband moved to the East Coast following her PhD, and with the outbreak of WWII and the rush to build the atomic bomb, Wu's experience in nuclear physics was very much in demand. She joined the Manhattan Project's Substitute Alloy Materials Laboratories at Columbia and was instrumental in developing a method of uranium-235 enrichment by gaseous diffusion. And after the war ended, she remained at Columbia continuing her research in beta decay

Chien-Shiung Wu in 1963 at Columbia University

So now we've come to this question about parity. At the time, physicists operated under the assumption of "the law of conservation of parity", which essentially states that the laws of the Universe are the same regardless of any sense of "direction". In other words, if you were to take a perfect mirror image of the world, it would be indistinguishable from the Universe that we actually live in. Now in the context of particle physics, the assumption of parity means that subatomic particles should behave exactly the same as their mirror image counterparts. Wu was aware of this assumption, and she knew that of the 4 fundamental forces of nature, parity had already been proven for electromagnetic interactions and strong nuclear interactions (gravity is negligible at the subatomic level). However, parity had not yet been proven for the weak nuclear force, the one that mediates radioactive decay, so in 1956 she assembled a team including fellow Chinese-American physicists Tsung-Dao Lee and Chen Ning Yang to test it out

For electromagnetism and the strong nuclear force, particles behave exactly the same as their mirrors. What about the weak nuclear force?

The Wu Experiment, as it later became known, took a cryogenic sample of Cobalt-60, a radioactive isotope that undergoes beta decay, and used an extremely uniform magnetic field from the National Bureau of Standards to align the spins of all the atomic nuclei in the same direction. If the law of conservation of parity holds true for the weak nuclear force, then the byproducts of the radioactive decay should be emitted equally in the "up" and "down" directions, regardless of whether the atoms in the sample are spun "clockwise" or "counterclockwise". Instead, they found that beta particles tended to fly out in the direction opposite the atom's spin! Thus, it would be possible to distinguish between the cobalt sample and its mirror image, breaking the law of conservation of parity!

"Nature, it seems, is a semi-ambidextrous southpaw"

Wu's discovery has major implications in the field of cosmology; to this day, one of the greatest unsolved mysteries in science is the question of why there's more matter than antimatter in the Universe. Known formally as baryon asymmetry, the idea is that the Big Bang theoretically should've produced equal amounts of matter and antimatter, which would've instantly annihilated and left the Universe a matterless sea of radiation. The fact that matter seems to have won the primordial battle for dominance implies that the laws of physics must've acted differently on the two, tipping the scales in favor of matter. Physicists have defined these circumstances (known as the Sakharov conditions), and one of them is a violation of charge-parity symmetry**. So while we don't have a solution to baryon asymmetry yet, we know Wu's discovery will play a huge role in the ultimate explanation!

** Charge-parity symmetry essentially combines Wu's notion of parity (mirror images) with the notion of charge (particle vs antiparticle). From Wikipedia: "CP-symmetry states that the laws of physics should be the same if a particle is interchanged with its antiparticle (C symmetry) while its spatial coordinates are inverted ("mirror" or P symmetry)". CP-violation was discovered in 1964, just a few years after Wu's P-violation. And if you've read Stephen Hawking's A Brief History of Time, you'll recall he discusses the acronym CPT-symmetry in the Universe: charge, parity, and time

Illustrating CP-symmetry, and it's potential impact on the matter-antimatter distribution of the early universe | Credit: Antimatter Matters

Finally, it's worth noting that although Wu's fellow researchers Lee and Yang won the 1957 Nobel Prize for their theoretical work on parity, Wu's experimental design was snubbed and she wasn't included in the award (though Lee and Yang did give her credit in their acceptance speech, and Wu subsequently won the Wolf Prize in 1978). Also, in the decades after Wu first left China to begin her doctoral studies, she became separated from her family back home, who endured enormous suffering in the wake of WWII, the Communist takeover following the Chinese Civil War, and the ensuing chaos of the Cultural Revolution. Wu initially found it very difficult to travel, since her passport had been issued by the now exiled Kuomintang government. Later on she became a US citizen, but the State Department had imposed restrictions on Americans traveling to Communist countries. Not until Nixon's groundbreaking 1972 visit to China, when Chinese-American relations began to improve, was Wu able to return to China, but by then her parents, uncle, and two brothers had all perished

Dr. Chien-Shiung Wu's contributions to experimental physics and cosmology were enormous, and although she passed away in 1997, she remains a pioneer and an icon for Asian Americans

That Looked Like a Slice to me, Al

2021-01-30T04:10:00.003-05:00

"If I hit it right, it's a slice. If I hit it left, it's a hook. If I hit it straight, it's a miracle!"

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This is my 100th post on Astronomical Returns! I've been writing for almost two years now, and it's been incredible to see how much the little space blog I started in the windowless backroom of a Wall Street investment bank has grown. To celebrate this milestone, I thought it'd be neat to rewrite my very first article, which was about Alan Shepard playing golf on the moon! And it's perfect because this week marks the 50th anniversary of the launch of Apollo 14!

One year in at Evercore vs one year in at SpaceX! Different job, different coast, 100 articles later!

Even among the pantheon of American legends, Alan Shepard’s resume dwarfs most others’ accomplishments: the commander of the 3^rd lunar landing was already the first American in space, flying on Mercury-Redstone 3 ten years prior, and before that he was a WWII Navy veteran who would eventually attain the rank of rear admiral. But in the world of golf, Shepard holds an achievement that not even the all-time greats like Tiger Woods or Jack Nicklaus could ever hope to accomplish: the first man to hit golf balls on the moon! On February 6, 1971, millions of people around the world tuned into their televisions and watched Alan Shepard turn the dusty lunar landscape into a driving range

After Shepard chunks and shanks the first two shots:
Edgar Mitchell (Lunar Module Pilot): "Hey, you got more dirt than ball!"
Fred Haise (Mission Control): "That looked like a slice to me Al!"
Commander Shepard's next shot is a little better: "Here we go, straight as a die; one more"
Upon nailing the fourth shot: "Miles and miles and miles!!"

Contrary to what some people thought, Shepard did not have to smuggle the golf balls and sawed-off 6-iron into the Lunar Module. Each of the moonwalking astronauts were allowed to bring a few personal items to the surface (for example, Buzz Aldrin, a devout Christian, brought communion bread and wine). Shepard had actually conceived of the idea as a cool way to demonstrate lunar gravity after comedian Bob Hope visited NASA in Houston and brought a golf club with him to the facility. Still, it took quite a bit of convincing - Bob Gilruth, the director of the Johnson Space Center, was worried it would be a distraction and make the agency look goofy on what was supposed to be a grand scientific voyage. But Shepard assured Gilruth that he would pay for all the golf equipment himself (no cost to the taxpayer!) and that he would only bring it out at the very end of the mission if everything else had gone smoothly

Alan Shepard (left) and Bob Gilruth (right) in Mission Control together, taken during Apollo 10

To his credit, Shepard put a lot of effort into preparing for the lunar links; the last thing he wanted to do was make a fool of himself in front of millions of people while standing on the moon! Once he had NASA's approval, Shepard had some technicians at NASA help him attach the iron to the end of a collapsible aluminum and Teflon lunar sampling tool. Then, with club in hand, he brought his 200lb lunar EVA spacesuit to a nearby golf course and started practicing one-armed swings out of a bunker wearing the bulky garment! Frankly, if I had the chance of the lifetime to hit golf balls on the moon, I'd probably do the exact same thing

Shepard's famous Wilson Dyna-Power 6-iron moon club, now on display at the USGA Museum in New Jersey

Now golfers love to exaggerate about how far they can hit the ball; did Shepard make good on his remark about crushing it "miles and miles and miles"? In reality, he says his fourth and best shot on the moon flew about 200 yards, which is still pretty darn solid given he swung one-handed wearing the bulky Apollo A7L spacesuit! But apparently if you do the math, an unencumbered swing by a skilled golfer, which would probably fly just under 300 yards on Earth, could go as far as 2.5 miles on the Moon! And even better, despite Fred Haise's jokingly derisive comment about Shepard's shot looking like a slice, it's actually impossible to hook or slice a golf ball in the airless environment of the moon, since a ball's curve stems from the imbalance in air pressure caused by the direction of the ball's spin. For golfers like me who struggle to keep the ball in the fairway, that sounds like an absolute godsend. As if I needed another reason to want to go to the moon!

Shepard with his moon club, many years after Apollo 14

I've played golf for 20 years now (since I was 4!), which is why I've always loved this story about Apollo 14. And while I can hit the ball about 275 yards, in 50 years when I'm old and decrepit and can barely get it past the ladies' tee on Earth, maybe I'll hop in a spacecraft and take one last rip at 300 yards on the surface of the Moon!

TPC San Antonio, Christmas 2019

Propellant Feed Systems - Pump or Pressurize?

2021-01-10T22:16:00.002-05:00

"Success in space demands perfection... unless [the engines] operate flawlessly first,
none of the other systems will get a chance to perform" - Wernher von Braun

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As someone who works at SpaceX as a financial analyst, not an engineer, I don't exactly need to know advanced rocket science to do my job. But it's still super helpful for me to understand the major components of a rocket, especially from an accounting and cost perspective. Over the past year I've been trying to expand my knowledge beyond just the top level assemblies (engines, first stage, second stage, payload) and go one level deeper (for example: thrust chamber assembly, gas generator, composite overwrapped pressure vessel, etc). To that end, my SpaceX engineer roommate sent me a PDF of the book Design of Liquid-Propellant Rocket Engines, which is apparently like the Bible for aerospace engineers. In my opinion, he's either vastly overestimated the extent of my self-taught math and engineering knowledge, or he's intentionally trying to send me down a rabbit hole of rocket science torture!

I don't think anything I learned for my finance degree is going to help me out with this one :(

Nonetheless, I dutifully started reading through the more approachable sections, and I came across an interesting topic I haven't discussed before: the selection of the optimal propellant feed system. Most of my rocket science posts on Astronomical Returns have focused on the propellant itself, but having the perfect fuel and oxidizer is pointless if you can't get it out of the tanks and into the combustion chamber! Propellant feed systems tend to fall into one of two categories: those that use pressurized gas, and those that use turbopumps. There's no simple rule for choosing between the two, as mission requirements vary drastically and materials science is always advancing, but in general pressurized gas has been used for simpler, lower impulse propulsion systems while turbopumps have been used on larger vehicle applications

The Saturn V burned 40,000lbs of fuel per second. Obviously, gravity alone isn't going to get all that into the combustion chamber!

Starting with pressurized gas: a separate, smaller tank storing a gas at high pressure (about 3,000 - 5,000 psi) is connected to the main fuel and oxidizer tanks inside the rocket, with a start valve and a regulator to ensure the gas always flows out at the proper rate. Technically any gas could be used, but modern applications all use helium due to its low molecular weight, low boiling point, and non-reactivity. As propellant is consumed and the pressure in the tanks starts to drop, the helium is pumped in to pressurize the vacuum and force the propellants down out of the tanks and into the combustion chambers for the engines to burn. At its simplest, the cold, pressurized helium can be pumped directly into the tanks, but better performance can be achieved by first running the helium around the combustion chamber, where fuel is already being burned, as a heat exchanger to heat up and expand the helium as much as possible, maximizing the volume and reducing the mass of helium needed to be carried

Notable past and present pressure-fed propulsion systems include the Apollo Command Module main engine, Lunar Module ascent/descent engines, the Space Shuttle reaction control systems and orbital maneuvering systems, the AJ10 used in the Delta II second stage, and the SpaceX SuperDraco engines

Two excellent diagrams showing the flow of helium pressurizing the propellant tanks as they're being depleted

While pressure-fed systems are quite simple and reliable, the helium is another consumable that adds weight to the rocket, especially since the helium tank needs to be very thick to contain the high pressure. Given practical limits on tank pressure, larger propulsion systems (i.e. virtually all 1st stage engines) must use turbopumps to generate the pressure necessary to pump propellant into the combustion chamber. The advantage here is that only low inlet pressures are required since it's the turbopump's job to raise the pressure of the propellants, avoiding the need for heavy, thick-skinned tanks. How turbopumps do that is actually quite interesting (and new to me; prior to researching this post, it was one of those things that just "magically" happened) - an impeller (the spinning wheel) is made to rotate, imparting centrifugal force to the fluid that translates into increased velocity and pressure via conservation of angular momentum. This motion generates suction that draws in more propellant from the tanks, while high pressure propellant can now be discharged into the combustion chamber

The propellant is drawn down from the tanks via suction, spun around in the impeller, and discharged to the combustion chamber

It's worth noting, turbopump assemblies still need some pressurized helium, both to spin up the turbopump at ignition sequence start as well as to fill the ullage (empty space) in the propellant tanks to prevent them from collapsing as they're depleted (think what happens to an empty plastic bottle if you try to suck the air out). Additionally, the intricate plumbing adds to the engineering complexity of the rocket: the power source of the turbopump is a separate discussion in and of itself! (Long story short: some of the fuel and oxidizer is burned separately from the combustion chamber, and the hot gases from that reaction power the turbine. See here). But when big engines are consuming vast quantities of propellant, turbopumps are the way to go!

Turbopump cutaway of the RL10, used on the Delta IV and SLS upper stages

Zero-G Immunology and ISS Vaccine Research

2020-12-28T16:26:00.003-05:00

"Who owns the patent on this vaccine?"

"Well, the people, I would say. There is no patent. Could you patent the Sun?"

- Dr. Jonas Salk, inventor of the polio vaccine

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With Pfizer and Moderna's COVID-19 vaccine finally on the way, what better occasion than now to explore how long-duration spaceflights have contributed to the field of immunology! As the son of a doctor and a former healthcare-sector investment banker, space medicine is one of my favorite topics to write about, especially since much of the research conducted on the ISS is related to human physiology. Besides the obvious risk of launching on a rocket going 20x faster than the speed of sound, astronauts must contend with the far more nebulous detriments that microgravity wreaks upon the human body: bone loss, muscle degeneration, visual impairment, radiation exposure, etc. Unsurprisingly, the immune system suffers as well; astronauts' immunity becomes suppressed during long stays off-planet, putting them at risk of illness that can derail a mission

Poor guy - Apollo 7 Commander Wally Schirra looks like he's about to sneeze right this instant!

Getting sick in space is not unprecedented. The crew of Apollo 7 all infamously caught colds almost immediately after reaching orbit, leading to horrible sinus congestion that couldn't drain without gravity and increasingly irritable moods towards Mission Control. In fact, their nasal discomfort was so intolerable that the crew adamantly refused to wear their helmets during reentry! But only more recently have observations on the ISS dug deeper into how microgravity dampens the immune system. Two studies published in the Journal of Interferon & Cytokine Research analyzed blood plasma taken by various ISS crew members to measure concentrations of various cytokines, proteins that stimulate immune responses and signal other cells towards sites of inflammation or infection. What researchers found was astronauts' immune systems were often "confused" - cytokine activity sometimes caused overreactions (possibly the culprit behind increased allergies reported by some astronauts), but at other times was depressed, leaving the astronauts susceptible to infection. And they concluded that the stresses of spaceflight, such as radiation, isolation, altered sleep schedules, all likely contributed to the immune imbalance

Cytokines can be characterized many ways, but a simple classification is by method of transmission: autocrine, paracrine, and endocrine

But if astronauts quarantine before flight, and if the ISS is presumably a sterile environment, how do astronauts get sick in the first place? Stowaway germs not withstanding, hidden viruses lurk within all our bodies, even though we aren't constantly sick because our immune systems keep them in check. However, studies have shown that these dormant viruses sometimes reactivated once in microgravity, capitalizing on the astronauts' weakened immune systems. The astronauts took repeated samples of their saliva, blood, and urine, and researchers found increased secretion of cortisol and adrenaline, stress hormones known to affect the immune system. And while most of the astronauts were asymptomatic (only 6 reported any symptoms at all), this "viral shedding" was still readily observable, as four of the eight known human herpes viruses were detected in their bodies. Knowing that viral resurgence will pose a much greater risk on long-duration spaceflights to the Moon and Mars, NASA has begun testing rapid viral detection systems and immune boosters to be used both by astronauts on future deep space missions, as well as immunocompromised patients here on Earth

Astronauts Alexander Gerst (Germany) and Serena Auñón-Chancellor (USA) performing a blood draw on the Human Research Facility of the ISS

Of course, the best preventative measure against disease is a vaccine, and although the ISS wasn't directly involved in the development of the COVID-19 vaccine, over the past two decades much research has been devoted to the behavior of both microbes and human immune cells in weightlessness to create more effective vaccines. Salmonella and MRSA bacteria samples were selected as the pathfinder trials, both of which are common pathogens that have shown increased resistance to antibiotics. Scientists observed various behaviors in the samples directly related to virulence, such as increased growth rates, antibiotic resistance, and genetic alterations. One hypothesis is that in the absence of gravity, the bacterial cells are deprived of normal forces like buoyancy and sedimentation that help drive nutrient absorption and waste excretion, forcing genetic expressions that ultimately make the pathogens hardier and more virulent. By pinpointing factors that contribute to pathogenic proliferation, scientists can design vaccines that allow the immune system to better inhibit infection

Astronaut Sandra Magnus handling a salmonella experiment on STS-135, the final Space Shuttle mission

Personally I can't wait to get the COVID vaccine, though as the only member of my family not in the healthcare field, I'm not on any kind of priority list so it'll be a while. But when I do, it'll be cool to know that research on the ISS play a small part in our advancements in immunology