Universalb.CoM

What if The Moon is Gone

2011-01-15T11:55:00.000-08:00

When the Moon is gone

If the time of Earth’s existence was condensed into a 24-hour clock, the moon formation event occurred just 10 minutes after the Earth was born. The Earth formed 4.56 billion years ago, and the Moon formed about 30 million years later. At that time, the Earth was a magma ocean. An impactor about the size of Mars struck the Earth at an oblique angle, and removed some of the magmatic mantle. This mantle was put in orbit around the Earth, together with some of the debris from the impactor itself, and this material eventually formed the Moon.

The tidal effect of a body increases as a cube of the distance, so the effect of the Moon’s tidal forcing on the Earth was extremely high at this time, to the point that the early magma ocean was affected. This provided some additional energy to the heating from radioactive elements present, but after the radiogenic heating decayed, the Moon still was a source of heating that may have had some geological effect, keeping the Earth’s magma hot and perhaps forcing additional convection in the Earth’s mantle.

After the Earth started to cool, the first crust started to float on top of the magma. During this period the Earth was subjected to increased meteor bombardment. The bombardment had been very intense at the beginning of the solar system and then had started to decline, but about 500 million years after the birth of the Earth, or about 2 hours and 40 minutes into our clock of 24 hours, there was a burst of impactors. This lasted for about hundred million years, and we call this “the late heavy bombardment.” Many of the large basins on the Moon are evidence of this late heavy bombardment period. In this way, the Moon is a history book for the inner solar system and the Earth. We have studied these basins with the SMART-1 mission.

When some of these impactors hit the Earth, the explosion caused rocks and dirt from Earth to shoot up and away from our planet. Some of that projected material flew all over the solar system, and some of it landed on the Moon. There could be a few hundred kilograms of Earth material per square kilometer of the Moon’s surface, buried under a few meters of lunar soil. It would be interesting to retrieve those rocks and bring back samples of the early Earth. Almost nothing from this time period has survived on the Earth because of tectonic recycling of the crust plates or because of atmospheric weathering. We would try to detect some organics within those rocks, and that could tell us about the history of organic chemistry on Earth. Some of these rocks could even have preserved fossils of life. Such rocks could help us look further back into the fossil record, which now stops at 3.5 billion years ago. This way, we could possibly learn about the emergence of life on Earth.

By exploring the Moon, we also can get clues on how the Earth has evolved. We can study processes on the Moon that have also shaped the Earth, like volcanism and tectonics. Because the Moon is smaller than the Earth, the Moon’s radiogenic heating dissipated much faster. After about one billion years, the interior of the Moon didn’t evolve much, and surface changes mostly were due to impacts. There was a brief period of magmatic activity from the subsurface -- a few plumes of magma made their way up to the surface and filled newly formed impact basins with basalt, creating what we call the Maria. This happened up to about 2 billion years ago. Because the Moon offers different conditions than the Earth, we can better understand how physical processes work generally by studying a larger range of parameters than just the Earth’s.

The crust of the Earth is also affected. The Moon’s tidal forcing causes significant heating and dissipation of energy to take place. Part of this energy is heating the Earth, and part of it is dissipated by forcing the Moon to recede from the Earth over time. There are people who propose that the tidal effect of the Moon may have helped trigger the convection on the Earth that led to the multi-plate tectonics. The other planets don’t have the same tectonic cycle. For most of them, the crust is like a lid that doesn’t move much horizontally, and the magma and heat are blocked by this lid on the surface. The Earth instead has rolling convective motion that drags the crust, and then the crust plunges back down into the mantle and gets recycled.

There are some very subtle effects of the Moon in the climate and the oceans. One pattern that has been found recently is related to the Pacific Ocean’s El Niño phenomenon. You have a cold undersea current coming from the Antarctic sea, and that creates the Humboldt stream which keeps the sea around the South American coast near Peru and Chile quite cold. Because of this, there are fewer clouds and less precipitation there. Sometimes this current drifts away from the coast, and then you have much more cloud formation and a period of very bad weather over South America. Satellites have monitored this stream over the Pacific Ocean and they have found some streams which were not known before. They can connect some of these streams with how the Moon’s tidal effect influences the mixing of the deep ocean. There was a French-American mission called TOPEX/Poseidon that accurately measured the altitude of the sea and detected a little stream a few centimeters high. That doesn’t seem like a lot, but over the whole area of the Pacific Ocean it represents a huge amount of water transferred from one place to another.

The Moon has been a stabilizing factor for the axis of rotation of the Earth. If you look at Mars, for instance, that planet has wobbled quite dramatically on its axis over time due to the gravitational influence of all the other planets in the solar system. Because of this obliquity change, the ice that is now at the poles on Mars would sometimes drift to the equator. But the Earth’s moon has helped stabilize our planet so that its axis of rotation stays in the same direction. For this reason, we had much less climatic change than if the Earth had been alone. And this has changed the way life evolved on Earth, allowing for the emergence of more complex multi-cellular organisms compared to a planet where drastic climatic change would allow only small, robust organisms to survive.

The Moon has influenced biology in other ways as well. For species living near the coast, the tide is an important factor. When you look at the shorelines, you can recognize different layers of organisms that have adapted to the salt water conditions based on the ebb and flow of the tide.

The eyesight of many mammals is sensitive to moonlight. The level of adaptation of night vision would be very different without the Moon. Many of these species have evolved in such a way that their night vision could work in even partial lunar illumination, because that’s when they are most active. But they can be more subjected to predators, too, so there is a balance between your ability to see and your ability not to be seen. The Moon has completely changed evolution in that aspect.

In a recent workshop called “Earth-Moon Relationships,” psychologists discussed the relation between the lunar phases and several aspects of life. There was a very interesting correlation, not with the birth of children, but with the time of conception. Perhaps that is due to some social or sentimental value of the Moon. We tend to forget the impact the Moon has on our lives because we use electric lights, but for most of our history we had to adapt our behavior to the lunar phases.

Finally, the Moon had a key role in the emergence of science, and in our understanding of our place in the universe. We saw the repetition of the phenomena of lunar phases, and we observed solar and lunar eclipses. These were big challenges to our understanding of nature, and a few astronomers were put to death because they weren’t able to predict the eclipses. This challenged us to develop accurate predictions for the motion of the sun and the motion of the Moon.

Studying the Moon helped us determine distances in the solar system and the size of celestial objects. By studying lunar phases, for example, people were able to determine how far the Moon is from the Earth, the size of the Earth, and our distance from the sun. More recently, the Moon was the terrain where the space race took place between two political systems, allowing for great technical and scientific achievements. The Moon has inspired humankind to learn how to travel to space, and to bring life beyond Earth’s cradle.

Neptune

2011-01-14T03:55:00.000-08:00

Neptune is the eighth planet from the Sun and the fourth largest (by diameter). Neptune is smaller in diameter but larger in mass than Uranus.

        orbit:    4,504,000,000 km (30.06 AU) from Sun
        diameter: 49,532 km (equatorial)
        mass:     1.0247e26 kg


In Roman mythology Neptune (Greek: Poseidon) was the god of the Sea.

After the discovery of Uranus, it was noticed that its orbit was not as it should be in accordance with Newton's laws. It was therefore predicted that another more distant planet must be perturbing Uranus' orbit. Neptune was first observed by Galle and d'Arrest on 1846 Sept 23 very near to the locations independently predicted by Adams and Le Verrier from calculations based on the observed positions of Jupiter, Saturn and Uranus. An international dispute arose between the English and French (though not, apparently between Adams and Le Verrier personally) over priority and the right to name the new planet; they are now jointly credited with Neptune's discovery. Subsequent observations have shown that the orbits calculated by Adams and Le Verrier diverge from Neptune's actual orbit fairly quickly. Had the search for the planet taken place a few years earlier or later it would not have been found anywhere near the predicted location.


More than two centuries earlier, in 1613, Galileo observed Neptune when it happened to be very near Jupiter, but he thought it was just a star. On two successive nights he actually noticed that it moved slightly with respect to another nearby star. But on the subsequent nights it was out of his field of view. Had he seen it on the previous few nights Neptune's motion would have been obvious to him. But, alas, cloudy skies prevented obsevations on those few critical days.


Neptune has been visited by only one spacecraft, Voyager 2 on Aug 25 1989. Much of we know about Neptune comes from this single encounter. But fortunately, recent ground-based and HST observations have added a great deal, too.


Because Pluto's orbit is so eccentric, it sometimes crosses the orbit of Neptune making Neptune the most distant planet from the Sun for a few years.


Neptune's composition is probably similar to Uranus': various "ices" and rock with about 15% hydrogen and a little helium. Like Uranus, but unlike Jupiter and Saturn, it may not have a distinct internal layering but rather to be more or less uniform in composition. But there is most likely a small core (about the mass of the Earth) of rocky material. Its atmosphere is mostly hydrogen and helium with a small amount of methane.


Neptune's blue color is largely the result of absorption of red light by methane in the atmosphere but there is some additional as-yet-unidentified chromophore which gives the clouds their rich blue tint.


Like a typical gas planet, Neptune has rapid winds confined to bands of latitude and large storms or vortices. Neptune's winds are the fastest in the solar system, reaching 2000 km/hour.


Like Jupiter and Saturn, Neptune has an internal heat source -- it radiates more than twice as much energy as it receives from the Sun.


At the time of the Voyager encounter, Neptune's most prominent feature was the Great Dark Spot (left) in the southern hemisphere. It was about half the size as Jupiter's Great Red Spot (about the same diameter as Earth). Neptune's winds blew the Great Dark Spot westward at 300 meters/second (700 mph). Voyager 2 also saw a smaller dark spot in the southern hemisphere and a small irregular white cloud that zips around Neptune every 16 hours or so now known as "The Scooter" (right). It may be a plume rising from lower in the atmosphere but its true nature remains a mystery.


However, HST observations of Neptune (left) in 1994 show that the Great Dark Spot has disappeared! It has either simply dissipated or is currently being masked by other aspects of the atmosphere. A few months later HST discovered a new dark spot in Neptune's northern hemisphere. This indicates that Neptune's atmosphere changes rapidly, perhaps due to slight changes in the temperature differences between the tops and bottoms of the clouds.


Neptune also has rings. Earth-based observations showed only faint arcs instead of complete rings, but Voyager 2's images showed them to be complete rings with bright clumps. One of the rings appears to have a curious twisted structure (right).


Like Uranus and Jupiter, Neptune's rings are very dark but their composition is unknown.


Neptune's rings have been given names: the outermost is Adams (which contains three prominent arcs now named Liberty, Equality and Fraternity), next is an unnamed ring co-orbital with Galatea, then Leverrier (whose outer extensions are called Lassell and Arago), and finally the faint but broad Galle.


Neptune's magnetic field is, like Uranus', oddly oriented and probably generated by motions of conductive material (probably water) in its middle layers.


Neptune can be seen with binoculars (if you know exactly where to look) but a large telescope is needed to see anything other than a tiny disk. There are several Web sites that show the current position of Neptune (and the other planets) in the sky, but much more detailed charts will be required to actually find it. Such charts can be created with a planetarium program.



Neptune's Satellites

Neptune has 13 known moons; 7 small named ones and Triton plus four discovered in 2002 and one discovered in 2003.           Distance  Radius    Mass
Satellite  (000 km)   (km)     (kg)   Discoverer   Date
---------  --------  ------  -------  ----------  -----
Naiad            48      29      ?    Voyager 2    1989
Thalassa         50      40      ?    Voyager 2    1989
Despina          53      74      ?    Voyager 2    1989
Galatea          62      79      ?    Voyager 2    1989
Larissa          74      96      ?    Voyager 2    1989
Proteus         118     209      ?    Voyager 2    1989
Triton          355    1350  2.14e22  Lassell      1846
Nereid         5509     170      ?    Kuiper       1949
Halimede      15728      61      ?                 2002 
Sao           22422      40      ?                 2002 
Laomedeia     23571      40      ?                 2002 
Psamathe      46695      38      ?                 2003 
Neso          48387      60      ?                 2002 





Neptune's Rings

         Distance   Width
Ring       (km)      (km)     aka
-------  --------   -----   -------
Diffuse    41900       15   1989N3R, Galle
Inner      53200       15   1989N2R, LeVerrier
Plateau    53200     5800   1989N4R, Lassell, Arago
Main       62930     < 50   1989N1R, Adams

(distance is from Neptune's center to the ring's inner edge)





More about Neptune and its satellites





more Neptune images
from NSSDC
Changing seasons on Neptune
Neptune's Ring System
Voyager Neptune Science Summary from JPL
Neptunian System Nomenclature Tables
more on the 2002 moons
data on the moons discovered in 2002
Data on the newly discovered moons can be found at JPL and Scott Sheppard's site.





Open Issues


Neptune's magnetic field is off center and at a large angle to its rotation axis. What processes in the interior generate this oddly shaped field?
What accounts for the relative lack of hydrogen and helium in Neptune (and Uranus)?
Why are Neptune's winds so strong in spite of the fact that it is so far from the Sun and has a relatively weak internal heat source?
What happened to the Great Dark Spot?
Can we design a useful Neptune orbiter mission cheap enough to be funded?

Uranus

2011-01-14T03:19:00.000-08:00

Uranus is the seventh planet from the Sun and the third largest (by diameter). Uranus is larger in diameter but smaller in mass than Neptune.

        orbit:    2,870,990,000 km (19.218 AU) from Sun
        diameter: 51,118 km (equatorial)
        mass:     8.683e25 kg




Careful pronunciation may be necessary to avoid embarrassment; say "YOOR a nus", not "your anus" or "urine us".

Uranus is the ancient Greek deity of the Heavens, the earliest supreme god. Uranus was the son and mate of Gaia the father of Cronus (Saturn) and of the Cyclopes and Titans (predecessors of the Olympian gods).

Uranus, the first planet discovered in modern times, was discovered by William Herschel while systematically searching the sky with his telescope on March 13, 1781. It had actually been seen many times before but ignored as simply another star (the earliest recorded sighting was in 1690 when John Flamsteed cataloged it as 34 Tauri). Herschel named it "the Georgium Sidus" (the Georgian Planet) in honor of his patron, the infamous (to Americans) King George III of England; others called it "Herschel". The name "Uranus" was first proposed by Bode in conformity with the other planetary names from classical mythology but didn't come into common use until 1850.

Uranus has been visited by only one spacecraft, Voyager 2 on Jan 24 1986.

Most of the planets spin on an axis nearly perpendicular to the plane of the ecliptic but Uranus' axis is almost parallel to the ecliptic. At the time of Voyager 2's passage, Uranus' south pole was pointed almost directly at the Sun. This results in the odd fact that Uranus' polar regions receive more energy input from the Sun than do its equatorial regions. Uranus is nevertheless hotter at its equator than at its poles. The mechanism underlying this is unknown.

Actually, there's an ongoing battle over which of Uranus' poles is its north pole! Either its axial inclination is a bit over 90 degrees and its rotation is direct, or it's a bit less than 90 degrees and the rotation is retrograde. The problem is that you need to draw a dividing line *somewhere*, because in a case like Venus there is little dispute that the rotation is indeed retrograde (not a direct rotation with an inclination of nearly 180).

Uranus is composed primarily of rock and various ices, with only about 15% hydrogen and a little helium (in contrast to Jupiter and Saturn which are mostly hydrogen). Uranus (and Neptune) are in many ways similar to the cores of Jupiter and Saturn minus the massive liquid metallic hydrogen envelope. It appears that Uranus does not have a rocky core like Jupiter and Saturn but rather that its material is more or less uniformly distributed.

Uranus' atmosphere is about 83% hydrogen, 15% helium and 2% methane.

Like the other gas planets, Uranus has bands of clouds that blow around rapidly. But they are extremely faint, visible only with radical image enhancement of the Voyager 2 pictures (right). Recent observations with HST (left) show larger and more pronounced streaks. Further HST observations show even more activity. Uranus is no longer the bland boring planet that Voyager saw! It now seems clear that the differences are due to seasonal effects since the Sun is now at a lower Uranian latitude which may cause more pronounced day/night weather effects. By 2007 the Sun will be directly over Uranus's equator.





Uranus' blue color is the result of absorption of red light by methane in the upper atmosphere. There may be colored bands like Jupiter's but they are hidden from view by the overlaying methane layer.

Like the other gas planets, Uranus has rings. Like Jupiter's, they are very dark but like Saturn's they are composed of fairly large particles ranging up to 10 meters in diameter in addition to fine dust. There are 13 known rings, all very faint; the brightest is known as the Epsilon ring. The Uranian rings were the first after Saturn's to be discovered. This was of considerable importance since we now know that rings are a common feature of planets, not a peculiarity of Saturn alone.

Voyager 2 discovered 10 small moons in addition to the 5 large ones already known. It is likely that there are several more tiny satellites within the rings.

Uranus' magnetic field is odd in that it is not centered on the center of the planet and is tilted almost 60 degrees with respect to the axis of rotation. It is probably generated by motion at relatively shallow depths within Uranus.

Uranus is sometimes just barely visible with the unaided eye on a very clear night; it is fairly easy to spot with binoculars (if you know exactly where to look). A small astronomical telescope will show a small disk. There are several Web sites that show the current position of Uranus (and the other planets) in the sky, but much more detailed charts will be required to actually find it. Such charts can be created with a planetarium program.


Uranus' Satellites
Uranus has 27 named moons:
Unlike the other bodies in the solar system which have names from classical mythology, Uranus' moons take their names from the writings of Shakespeare and Pope.
They form three distinct classes: the 11 small very dark inner ones discovered by Voyager 2, the 5 large ones (right), and the newly discovered much more distant ones.
Most have nearly circular orbits in the plane of Uranus' equator (and hence at a large angle to the plane of the ecliptic); the outer 4 are much more elliptical.

           Distance  Radius    Mass
Satellite  (000 km)   (km)     (kg)   Discoverer   Date
---------  --------  ------  -------  ----------  -----
Cordelia         50      13    ?      Voyager 2    1986
Ophelia          54      16    ?      Voyager 2    1986
Bianca           59      22    ?      Voyager 2    1986
Cressida         62      33    ?      Voyager 2    1986
Desdemona        63      29    ?      Voyager 2    1986
Juliet           64      42    ?      Voyager 2    1986
Portia           66      55    ?      Voyager 2    1986
Rosalind         70      27    ?      Voyager 2    1986
Cupid            75       6    ?      Showalter    2003
Belinda          75      34    ?      Voyager 2    1986
Perdita          76      40    ?      Voyager 2    1986
Puck             86      77    ?      Voyager 2    1985
Mab              98       8    ?      Showalter    2003
Miranda         130     236  6.30e19  Kuiper       1948
Ariel           191     579  1.27e21  Lassell      1851
Umbriel         266     585  1.27e21  Lassell      1851
Titania         436     789  3.49e21  Herschel     1787
Oberon          583     761  3.03e21  Herschel     1787
Francisco      4281       6    ?      Sheppard     2003
Caliban        7169      40    ?      Gladman      1997
Stephano       7948      15    ?      Gladman      1999
Trinculo       8578       5    ?      Holman       2001
Sycorax       12213      80    ?      Nicholson    1997
Margaret      14689       6    ?      Sheppard     2003
Prospero      16568      20    ?      Holman       1999
Setebos       17681      20    ?      Kavelaars    1999
Ferdinand     21000       6    ?      Sheppard     2003



Uranus' Rings
         Distance   Width
Ring       (km)      (km)
-------  --------   -----
1986U2R    38000    2,500
6          41840    1-3
5          42230    2-3
4          42580    2-3
Alpha      44720    7-12
Beta       45670    7-12
Eta        47190    0-2
Gamma      47630    1-4
Delta      48290    3-9
1986U1R    50020    1-2
Epsilon    51140    20-100

(distance is from Uranus' center to the ring's inner edge)

Saturn

2011-01-14T02:49:00.000-08:00

Saturn is the sixth planet from the Sun and the second largest:

orbit: 1,429,400,000 km (9.54 AU) from Sun
diameter: 120,536 km (equatorial)
mass: 5.68e26 kg

In Roman mythology, Saturn is the god of agriculture. The associated Greek god, Cronus, was the son of Uranus and Gaia and the father of Zeus (Jupiter). Saturn is the root of the English word "Saturday" (see Appendix 5).

Saturn has been known since prehistoric times. Galileo was the first to observe it with a telescope in 1610; he noted its odd appearance but was confused by it. Early observations of Saturn were complicated by the fact that the Earth passes through the plane of Saturn's rings every few years as Saturn moves in its orbit. A low resolution image of Saturn therefore changes drastically. It was not until 1659 that Christiaan Huygens correctly inferred the geometry of the rings. Saturn's rings remained unique in the known solar system until 1977 when very faint rings were discovered around Uranus (and shortly thereafter around Jupiter and Neptune).

Saturn was first visited by NASA's Pioneer 11 in 1979 and later by Voyager 1 and Voyager 2. Cassini (a joint NASA / ESA project) arrived on July 1, 2004 and will orbit Saturn for at least four years.

Saturn is visibly flattened (oblate) when viewed through a small telescope; its equatorial and polar diameters vary by almost 10% (120,536 km vs. 108,728 km). This is the result of its rapid rotation and fluid state. The other gas planets are also oblate, but not so much so.

Saturn is the least dense of the planets; its specific gravity (0.7) is less than that of water.

Like Jupiter, Saturn is about 75% hydrogen and 25% helium with traces of water, methane, ammonia and "rock", similar to the composition of the primordial Solar Nebula from which the solar system was formed.

Saturn's interior is similar to Jupiter's consisting of a rocky core, a liquid metallic hydrogen layer and a molecular hydrogen layer. Traces of various ices are also present.

Saturn's interior is hot (12000 K at the core) and Saturn radiates more energy into space than it receives from the Sun. Most of the extra energy is generated by the Kelvin-Helmholtz mechanism as in Jupiter. But this may not be sufficient to explain Saturn's luminosity; some additional mechanism may be at work, perhaps the "raining out" of helium deep in Saturn's interior.

The bands so prominent on Jupiter are much fainter on Saturn. They are also much wider near the equator. Details in the cloud tops are invisible from Earth so it was not until the Voyager encounters that any detail of Saturn's atmospheric circulation could be studied. Saturn also exhibits long-lived ovals (red spot at center of image at right) and other features common on Jupiter. In 1990, HST observed an enormous white cloud near Saturn's equator which was not present during the Voyager encounters; in 1994 another, smaller storm was observed (left).

Two prominent rings (A and B) and one faint ring (C) can be seen from the Earth. The gap between the A and B rings is known as the Cassini division. The much fainter gap in the outer part of the A ring is known as the Encke Division (but this is somewhat of a misnomer since it was very likely never seen by Encke). The Voyager pictures show four additional faint rings. Saturn's rings, unlike the rings of the other planets, are very bright (albedo 0.2 - 0.6).

Though they look continuous from the Earth, the rings are actually composed of innumerable small particles each in an independent orbit. They range in size from a centimeter or so to several meters. A few kilometer-sized objects are also likely.

Saturn's rings are extraordinarily thin: though they're 250,000 km or more in diameter they're less than one kilometer thick. Despite their impressive appearance, there's really very little material in the rings -- if the rings were compressed into a single body it would be no more than 100 km across.

The ring particles seem to be composed primarily of water ice, but they may also include rocky particles with icy coatings.

Voyager confirmed the existence of puzzling radial inhomogeneities in the rings called "spokes" which were first reported by amateur astronomers (left). Their nature remains a mystery, but may have something to do with Saturn's magnetic field.

Saturn's outermost ring, the F-ring, is a complex structure made up of several smaller rings along which "knots" are visible. Scientists speculate that the knots may be clumps of ring material, or mini moons. The strange braided appearance visible in the Voyager 1 images (right) is not seen in the Voyager 2 images perhaps because Voyager 2 imaged regions where the component rings are roughly parallel. They are prominent in the Cassini images which also show some as yet unexplained wispy spiral structures.

There are complex tidal resonances between some of Saturn's moons and the ring system: some of the moons, the so-called "shepherding satellites" (i.e. Atlas, Prometheus and Pandora) are clearly important in keeping the rings in place; Mimas seems to be responsible for the paucity of material in the Cassini division, which seems to be similar to the Kirkwood gaps in the asteroid belt; Pan is located inside the Encke Division and S/2005 S1 is in the center of the Keeler Gap. The whole system is very complex and as yet poorly understood.

The origin of the rings of Saturn (and the other jovian planets) is unknown. Though they may have had rings since their formation, the ring systems are not stable and must be regenerated by ongoing processes, perhaps the breakup of larger satellites. The current set of rings may be only a few hundred million years old.

Like the other jovian planets, Saturn has a significant magnetic field.

When it is in the nighttime sky, Saturn is easily visible to the unaided eye. Though it is not nearly as bright as Jupiter, it is easy to identify as a planet because it doesn't "twinkle" like the stars do. The rings and the larger satellites are visible with a small astronomical telescope. There are several Web sites that show the current position of Saturn (and the other planets) in the sky. More detailed and customized charts can be created with a planetarium program.

Saturn's Satellites

Saturn has 53 named satellites (as of spring 2010):
The three pairs Mimas-Tethys, Enceladus-Dione and Titan-Hyperion interact gravitationally in such a way as to maintain stable relationships between their orbits: the period of Mimas' orbit is exactly half that of Tethys, they are thus said to be in a 1:2 resonance; Enceladus-Dione are also 1:2; Titan-Hyperion are in a 3:4 resonance.
See Scott Sheppard's site for the latest about recently discovered moons (there are lots).
There are 9 more that have been discovered but as yet not named.

Major moons: 
           Distance  Radius    Mass
Satellite  (000 km)   (km)     (kg)   Discoverer   Date
---------  --------  ------  -------  ----------  -----
Pan             134      10     ?     Showalter    1990
Atlas           138      14     ?     Terrile      1980
Prometheus      139      46  2.70e17  Collins      1980
Pandora         142      46  2.20e17  Collins      1980
Epimetheus      151      57  5.60e17  Walker       1980
Janus           151      89  2.01e18  Dollfus      1966
Mimas           186     196  3.80e19  Herschel     1789
Enceladus       238     260  8.40e19  Herschel     1789
Tethys          295     530  7.55e20  Cassini      1684
Telesto         295      15     ?     Reitsema     1980
Calypso         295      13     ?     Pascu        1980
Dione           377     560  1.05e21  Cassini      1684
Helene          377      16     ?     Laques       1980
Rhea            527     765  2.49e21  Cassini      1672
Titan          1222    2575  1.35e23  Huygens      1655
Hyperion       1481     143  1.77e19  Bond         1848
Iapetus        3561     730  1.88e21  Cassini      1671
Phoebe        12952     110  4.00e18  Pickering    1898






Saturn's Rings
                  Radius   Radius             approx.   approx.
Name               inner    outer     width  position  mass (kg)
----              ------   ------     -----  --------  --------
D-Ring            67,000   74,500     7,500    (ring)
Guerin Division   
C-Ring            74,500   92,000    17,500    (ring)  1.1e18
Maxwell Division  87,500   88,000       500  (divide)
B-Ring            92,000  117,500    25,500    (ring)  2.8e19
Cassini Division 115,800  120,600     4,800  (divide)
Huygens Gap      117,680    (n/a)   285-440  (subdiv)
A-Ring           122,200  136,800    14,600    (ring)  6.2e18
Encke Minima     126,430  129,940     3,500   29%-53%
Encke Division   133,410  133,740
Keeler Gap       136,510  136,550
F-Ring           140,210             30-500   (ring)
G-Ring           165,800  173,800     8,000    (ring)  1e7?
E-Ring           180,000  480,000   300,000    (ring)

Notes:
  * distance is kilometers from Saturn's center
  * the "Encke Minima" is a slang term used by amateur astronomers, not an official IAU designation

Jupiter

2011-01-13T16:13:00.000-08:00

Jupiter is the fifth planet from the Sun and by far the largest. Jupiter is more than twice as massive as all the other planets combined (the mass of Jupiter is 318 times that of Earth).
   orbit: 778,330,000 km (5.20 AU) from Sun
   diameter: 142,984 km (equatorial)
   mass: 1.900e27 kg

Jupiter (a.k.a. Jove; Greek Zeus) was the King of the Gods, the ruler of Olympus and the patron of the Roman state. Zeus was the son of Cronus (Saturn).

Jupiter is the fourth brightest object in the sky (after the Sun, the Moon and Venus). It has been known since prehistoric times as a bright "wandering star". But in 1610 when Galileo first pointed a telescope at the sky he discovered Jupiter's four large moons Io, Europa, Ganymede and Callisto (now known as the Galilean moons) and recorded their motions back and forth around Jupiter. This was the first discovery of a center of motion not apparently centered on the Earth. It was a major point in favor of Copernicus's heliocentric theory of the motions of the planets (along with other new evidence from his telescope: the phases of Venus and the mountains on the Moon). Galileo's outspoken support of the Copernican theory got him in trouble with the Inquisition. Today anyone can repeat Galileo's observations (without fear of retribution :-) using binoculars or an inexpensive telescope.

Jupiter was first visited by Pioneer 10 in 1973 and later by Pioneer 11, Voyager 1, Voyager 2 and Ulysses. The spacecraft Galileo orbited Jupiter for eight years. It is still regularly observed by the Hubble Space Telescope.

The gas planets do not have solid surfaces, their gaseous material simply gets denser with depth (the radii and diameters quoted for the planets are for levels corresponding to a pressure of 1 atmosphere). What we see when looking at these planets is the tops of clouds high in their atmospheres (slightly above the 1 atmosphere level).

Jupiter is about 90% hydrogen and 10% helium (by numbers of atoms, 75/25% by mass) with traces of methane, water, ammonia and "rock". This is very close to the composition of the primordial Solar Nebula from which the entire solar system was formed. Saturn has a similar composition, but Uranus and Neptune have much less hydrogen and helium.

Our knowledge of the interior of Jupiter (and the other gas planets) is highly indirect and likely to remain so for some time. (The data from Galileo's atmospheric probe goes down only about 150 km below the cloud tops.)

Jupiter probably has a core of rocky material amounting to something like 10 to 15 Earth-masses.

Above the core lies the main bulk of the planet in the form of liquid metallic hydrogen. This exotic form of the most common of elements is possible only at pressures exceeding 4 million bars, as is the case in the interior of Jupiter (and Saturn). Liquid metallic hydrogen consists of ionized protons and electrons (like the interior of the Sun but at a far lower temperature). At the temperature and pressure of Jupiter's interior hydrogen is a liquid, not a gas. It is an electrical conductor and the source of Jupiter's magnetic field. This layer probably also contains some helium and traces of various "ices".

The outermost layer is composed primarily of ordinary molecular hydrogen and helium which is liquid in the interior and gaseous further out. The atmosphere we see is just the very top of this deep layer. Water, carbon dioxide, methane and other simple molecules are also present in tiny amounts.

Recent experiments have shown that hydrogen does not change phase suddenly. Therefore the interiors of the jovian planets probably have indistinct boundaries between their various interior layers.

Three distinct layers of clouds are believed to exist consisting of ammonia ice, ammonium hydrosulfide and a mixture of ice and water. However, the preliminary results from the Galileo probe show only faint indications of clouds (one instrument seems to have detected the topmost layer while another may have seen the second). But the probe's entry point (left) was unusual -- Earth-based telescopic observations and more recent observations by the Galileo orbiter suggest that the probe entry site may well have been one of the warmest and least cloudy areas on Jupiter at that time.

Data from the Galileo atmospheric probe also indicate that there is much less water than expected. The expectation was that Jupiter's atmosphere would contain about twice the amount of oxygen (combined with the abundant hydrogen to make water) as the Sun. But it now appears that the actual concentration much less than the Sun's. Also surprising was the high temperature and density of the uppermost parts of the atmosphere.

Jupiter and the other gas planets have high velocity winds which are confined in wide bands of latitude. The winds blow in opposite directions in adjacent bands. Slight chemical and temperature differences between these bands are responsible for the colored bands that dominate the planet's appearance. The light colored bands are called zones; the dark ones belts. The bands have been known for some time on Jupiter, but the complex vortices in the boundary regions between the bands were first seen by Voyager. The data from the Galileo probe indicate that the winds are even faster than expected (more than 400 mph) and extend down into as far as the probe was able to observe; they may extend down thousands of kilometers into the interior. Jupiter's atmosphere was also found to be quite turbulent. This indicates that Jupiter's winds are driven in large part by its internal heat rather than from solar input as on Earth.

The vivid colors seen in Jupiter's clouds are probably the result of subtle chemical reactions of the trace elements in Jupiter's atmosphere, perhaps involving sulfur whose compounds take on a wide variety of colors, but the details are unknown.

The colors correlate with the cloud's altitude: blue lowest, followed by browns and whites, with reds highest. Sometimes we see the lower layers through holes in the upper ones.

The Great Red Spot (GRS) has been seen by Earthly observers for more than 300 years (its discovery is usually attributed to Cassini, or Robert Hooke in the 17th century). The GRS is an oval about 12,000 by 25,000 km, big enough to hold two Earths. Other smaller but similar spots have been known for decades. Infrared observations and the direction of its rotation indicate that the GRS is a high-pressure region whose cloud tops are significantly higher and colder than the surrounding regions. Similar structures have been seen on Saturn and Neptune. It is not known how such structures can persist for so long.

Jupiter radiates more energy into space than it receives from the Sun. The interior of Jupiter is hot: the core is probably about 20,000 K. The heat is generated by the Kelvin-Helmholtz mechanism, the slow gravitational compression of the planet. (Jupiter does NOT produce energy by nuclear fusion as in the Sun; it is much too small and hence its interior is too cool to ignite nuclear reactions.) This interior heat probably causes convection deep within Jupiter's liquid layers and is probably responsible for the complex motions we see in the cloud tops. Saturn and Neptune are similar to Jupiter in this respect, but oddly, Uranus is not.

Jupiter is just about as large in diameter as a gas planet can be. If more material were to be added, it would be compressed by gravity such that the overall radius would increase only slightly. A star can be larger only because of its internal (nuclear) heat source. (But Jupiter would have to be at least 80 times more massive to become a star.)

Jupiter has a huge magnetic field, much stronger than Earth's. Its magnetosphere extends more than 650 million km (past the orbit of Saturn!). (Note that Jupiter's magnetosphere is far from spherical -- it extends "only" a few million kilometers in the direction toward the Sun.) Jupiter's moons therefore lie within its magnetosphere, a fact which may partially explain some of the activity on Io. Unfortunately for future space travelers and of real concern to the designers of the Voyager and Galileo spacecraft, the environment near Jupiter contains high levels of energetic particles trapped by Jupiter's magnetic field. This "radiation" is similar to, but much more intense than, that found within Earth's Van Allen belts. It would be immediately fatal to an unprotected human being. The Galileo atmospheric probe discovered a new intense radiation belt between Jupiter's ring and the uppermost atmospheric layers. This new belt is approximately 10 times as strong as Earth's Van Allen radiation belts. Surprisingly, this new belt was also found to contain high energy helium ions of unknown origin.

Jupiter has rings like Saturn's, but much fainter and smaller (right). They were totally unexpected and were only discovered when two of the Voyager 1 scientists insisted that after traveling 1 billion km it was at least worth a quick look to see if any rings might be present. Everyone else thought that the chance of finding anything was nil, but there they were. It was a major coup. They have since been imaged in the infra-red from ground-based observatories and by Galileo.

Unlike Saturn's, Jupiter's rings are dark (albedo about .05). They're probably composed of very small grains of rocky material. Unlike Saturn's rings, they seem to contain no ice.

Particles in Jupiter's rings probably don't stay there for long (due to atmospheric and magnetic drag). The Galileo spacecraft found clear evidence that the rings are continuously resupplied by dust formed by micrometeor impacts on the four inner moons, which are very energetic because of Jupiter's large gravitational field. The inner halo ring is broadened by interactions with Jupiter's magnetic field.

In July 1994, Comet Shoemaker-Levy 9 collided with Jupiter with spectacular results (left). The effects were clearly visible even with amateur telescopes. The debris from the collision was visible for nearly a year afterward with HST.

When it is in the nighttime sky, Jupiter is often the brightest "star" in the sky (it is second only to Venus, which is seldom visible in a dark sky). The four Galilean moons are easily visible with binoculars; a few bands and the Great Red Spot can be seen with a small astronomical telescope. There are several Web sites that show the current position of Jupiter (and the other planets) in the sky. More detailed and customized charts can be created with a planetarium program.

Jupiter's Satellites

Jupiter has 63 known satellites (as of Feb 2004): the four large Galilean moons plus many more small ones some of which have not yet been named:
Jupiter is very gradually slowing down due to the tidal drag produced by the Galilean satellites. Also, the same tidal forces are changing the orbits of the moons, very slowly forcing them farther from Jupiter.
Io, Europa and Ganymede are locked together in a 1:2:4 orbital resonance and their orbits evolve together. Callisto is almost part of this as well. In a few hundred million years, Callisto will be locked in too, orbiting at exactly twice the period of Ganymede (eight times the period of Io).
Jupiter's satellites are named for other figures in the life of Zeus (mostly his numerous lovers).
Many more small moons have been discovered recently but have not as yet been officially confirmed or named. The most up to date info on them can be found at Scott Sheppard's site.

 Distance  Radius    Mass
Satellite  (000 km)   (km)     (kg)   Discoverer   Date
---------  --------  ------  -------  ----------  -----
Metis           128      20  9.56e16  Synnott      1979
Adrastea        129      10  1.91e16  Jewitt       1979
Amalthea        181      98  7.17e18  Barnard      1892
Thebe           222      50  7.77e17  Synnott      1979
Io              422    1815  8.94e22  Galileo      1610
Europa          671    1569  4.80e22  Galileo      1610
Ganymede       1070    2631  1.48e23  Galileo      1610
Callisto       1883    2400  1.08e23  Galileo      1610
Leda          11094       8  5.68e15  Kowal        1974
Himalia       11480      93  9.56e18  Perrine      1904
Lysithea      11720      18  7.77e16  Nicholson    1938
Elara         11737      38  7.77e17  Perrine      1905
Ananke        21200      15  3.82e16  Nicholson    1951
Carme         22600      20  9.56e16  Nicholson    1938
Pasiphae      23500      25  1.91e17  Melotte      1908
Sinope        23700      18  7.77e16  Nicholson    1914

Values for the smaller moons are approximate. Many more small moons are not listed here.

Jupiter's Rings

          Distance   Width   Mass
Ring      (km)        (km)   (kg)
----      --------   -----  ------
Halo      100000     22800   ?
Main      122800      6400  1e13
Gossamer  129200    214200   ?

(distance is from Jupiter's center to the ring's inner edge)

The day the moon is gone

2011-01-13T13:04:00.001-08:00

When the Moon first formed, it was very close to the Earth. It was possibly only 20 to 30 thousands of kilometers away, and it would have looked extremely large in the sky, at least 20 to 10 times bigger. But there were no living creatures on the Earth at that time to witness this beautiful scene.

The tidal effect of a body increases as a cube of the distance, so the effect of the Moon’s tidal forcing on the Earth was extremely high at this time, to the point that the early magma ocean was affected. This provided some additional energy to the heating from radioactive elements present, but after the radiogenic heating decayed, the Moon still was a source of heating that may have had some geological effect, keeping the Earth’s magma hot and perhaps forcing additional convection in the Earth’s mantle.

After the Earth started to cool, the first crust started to float on top of the magma. During this period the Earth was subjected to increased meteor bombardment. The bombardment had been very intense at the beginning of the solar system and then had started to decline, but about 500 million years after the birth of the Earth, or about 2 hours and 40 minutes into our clock of 24 hours, there was a burst of impactors. This lasted for about hundred million years, and we call this “the late heavy bombardment.” Many of the large basins on the Moon are evidence of this late heavy bombardment period. In this way, the Moon is a history book for the inner solar system and the Earth. We have studied these basins with the SMART-1 mission.

The Earth was hit more often than the Moon, however, because Earth is larger and has more gravity. This increased gravity also caused the impactors to be accelerated to higher velocities towards the Earth. That must have been a catastrophic time to be here. So many bombardments would have sterilized the planet. If life had appeared before this period, it would have been extinguished unless it found a way to retreat into niches where it could be protected from these global catastrophes.

When some of these impactors hit the Earth, the explosion caused rocks and dirt from Earth to shoot up and away from our planet. Some of that projected material flew all over the solar system, and some of it landed on the Moon. There could be a few hundred kilograms of Earth material per square kilometer of the Moon’s surface, buried under a few meters of lunar soil. It would be interesting to retrieve those rocks and bring back samples of the early Earth. Almost nothing from this time period has survived on the Earth because of tectonic recycling of the crust plates or because of atmospheric weathering. We would try to detect some organics within those rocks, and that could tell us about the history of organic chemistry on Earth. Some of these rocks could even have preserved fossils of life. Such rocks could help us look further back into the fossil record, which now stops at 3.5 billion years ago. This way, we could possibly learn about the emergence of life on Earth.

By exploring the Moon, we also can get clues on how the Earth has evolved. We can study processes on the Moon that have also shaped the Earth, like volcanism and tectonics. Because the Moon is smaller than the Earth, the Moon’s radiogenic heating dissipated much faster. After about one billion years, the interior of the Moon didn’t evolve much, and surface changes mostly were due to impacts. There was a brief period of magmatic activity from the subsurface -- a few plumes of magma made their way up to the surface and filled newly formed impact basins with basalt, creating what we call the Maria. This happened up to about 2 billion years ago. Because the Moon offers different conditions than the Earth, we can better understand how physical processes work generally by studying a larger range of parameters than just the Earth’s.

The Moon affects the liquid envelope of the Earth, and the oceanic tides in particular. The Moon affects the ocean tides more in some areas than others. For instance, in the channel between the British Isles and the European continent, the tidal range can be 10 meters, compared to what you see in the Pacific, where it is below a meter.

The crust of the Earth is also affected. The Moon’s tidal forcing causes significant heating and dissipation of energy to take place. Part of this energy is heating the Earth, and part of it is dissipated by forcing the Moon to recede from the Earth over time. There are people who propose that the tidal effect of the Moon may have helped trigger the convection on the Earth that led to the multi-plate tectonics. The other planets don’t have the same tectonic cycle. For most of them, the crust is like a lid that doesn’t move much horizontally, and the magma and heat are blocked by this lid on the surface. The Earth instead has rolling convective motion that drags the crust, and then the crust plunges back down into the mantle and gets recycled.

There are some very subtle effects of the Moon in the climate and the oceans. One pattern that has been found recently is related to the Pacific Ocean’s El Niño phenomenon. You have a cold undersea current coming from the Antarctic sea, and that creates the Humboldt stream which keeps the sea around the South American coast near Peru and Chile quite cold. Because of this, there are fewer clouds and less precipitation there. Sometimes this current drifts away from the coast, and then you have much more cloud formation and a period of very bad weather over South America. Satellites have monitored this stream over the Pacific Ocean and they have found some streams which were not known before. They can connect some of these streams with how the Moon’s tidal effect influences the mixing of the deep ocean. There was a French-American mission called TOPEX/Poseidon that accurately measured the altitude of the sea and detected a little stream a few centimeters high. That doesn’t seem like a lot, but over the whole area of the Pacific Ocean it represents a huge amount of water transferred from one place to another.

If you would take away the Moon suddenly, it would change the global altitude of the ocean. Right now there is a distortion which is elongated around the equator, so if we didn’t have this effect, suddenly a lot of water would be redistributed toward the polar regions.

The Moon has been a stabilizing factor for the axis of rotation of the Earth. If you look at Mars, for instance, that planet has wobbled quite dramatically on its axis over time due to the gravitational influence of all the other planets in the solar system. Because of this obliquity change, the ice that is now at the poles on Mars would sometimes drift to the equator. But the Earth’s moon has helped stabilize our planet so that its axis of rotation stays in the same direction. For this reason, we had much less climatic change than if the Earth had been alone. And this has changed the way life evolved on Earth, allowing for the emergence of more complex multi-cellular organisms compared to a planet where drastic climatic change would allow only small, robust organisms to survive.

The Moon has influenced biology in other ways as well. For species living near the coast, the tide is an important factor. When you look at the shorelines, you can recognize different layers of organisms that have adapted to the salt water conditions based on the ebb and flow of the tide.

The eyesight of many mammals is sensitive to moonlight. The level of adaptation of night vision would be very different without the Moon. Many of these species have evolved in such a way that their night vision could work in even partial lunar illumination, because that’s when they are most active. But they can be more subjected to predators, too, so there is a balance between your ability to see and your ability not to be seen. The Moon has completely changed evolution in that aspect.

Human vision is so sensitive that we are almost able to work by the light of the Milky Way. The full Moon has more light than we need to see at night. For most of our history, we were hunting and fishing or doing agriculture, and we organized our lives by using the Moon. It determined the time for hunting, or the time where we could harvest. That’s why most of our calendars are based on the Moon.

In a recent workshop called “Earth-Moon Relationships,” psychologists discussed the relation between the lunar phases and several aspects of life. There was a very interesting correlation, not with the birth of children, but with the time of conception. Perhaps that is due to some social or sentimental value of the Moon. We tend to forget the impact the Moon has on our lives because we use electric lights, but for most of our history we had to adapt our behavior to the lunar phases.

Finally, the Moon had a key role in the emergence of science, and in our understanding of our place in the universe. We saw the repetition of the phenomena of lunar phases, and we observed solar and lunar eclipses. These were big challenges to our understanding of nature, and a few astronomers were put to death because they weren’t able to predict the eclipses. This challenged us to develop accurate predictions for the motion of the sun and the motion of the Moon.

Studying the Moon helped us determine distances in the solar system and the size of celestial objects. By studying lunar phases, for example, people were able to determine how far the Moon is from the Earth, the size of the Earth, and our distance from the sun. More recently, the Moon was the terrain where the space race took place between two political systems, allowing for great technical and scientific achievements. The Moon has inspired humankind to learn how to travel to space, and to bring life beyond Earth’s cradle.

NASA Aids in Characterizing Super-Earth Atmosphere

2011-01-04T19:15:00.001-08:00

PASADENA, Calif. -- A team of astronomers, including two NASA Sagan Fellows, has made the first characterizations of a super-Earth's atmosphere, by using a ground-based telescope. A super-Earth is a planet up to three times the size of Earth and weighing up to 10 times as much. The findings, reported in the Dec. 2 issue of the journal Nature, are a significant milestone toward eventually being able to probe the atmospheres of Earth-like planets for signs of life.

The team determined the planet, GJ 1214b, is either blanketed with a thin layer of water steam or surrounded by a thick layer of high clouds. If the former, the planet itself would have an icy composition. If the latter, the planet would be rocky or similar to the composition of Neptune, though much smaller.

"This is the first super-Earth known to have an atmosphere," said Jacob Bean, a NASA Sagan Fellow and astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. "But even with these new measurements, we can't say yet what that atmosphere is made of. This world is being very shy and veiling its true nature from us."

GJ 1214b, first discovered in December 2009, is 2.7 times the size of Earth and 6.5 times as massive. Previous observations of the planet's size and mass demonstrated it has a low density for its size, leading astronomers to conclude the planet is some kind of solid body with an atmosphere.

The planet orbits close to its dim star, at a distance of 0.014 astronomical units. An astronomical unit is the distance between Earth and the sun, approximately 93 million miles. GJ 1214b circles too close to its star to be habitable by any life forms.

Bean and his team observed infrared light as the planet crossed in front of its star. During such transits, the star's light filters through the atmosphere. Gases absorb the starlight at particular wavelengths, leaving behind chemical fingerprints detectable from Earth. This same type of technique has been used to study the atmospheres of distant "hot Jupiters," or Jupiter-like planets orbiting close to their stars, and found gases like hydrogen, methane and sodium vapor.

In the case of the super-Earth, no chemical fingerprints were detected; however, this doesn't mean there are no chemicals present. Instead, this information ruled out some possibilities for GJ 1214b's atmosphere, and narrowed the scope to either an atmosphere of water steam or high clouds. Astronomers believe it's more likely the atmosphere is too thin around the planet to let enough light filter through and reveal chemical fingerprints.

"A steamy atmosphere would have to be very dense – about one-fifth water vapor by volume -- compared to our Earth, with an atmosphere that's four-fifths nitrogen and one-fifth oxygen with only a touch of water vapor," Bean said. "During the next year, we should have some solid answers about what this planet is truly like."

The team, which included Bean's co-authors -- Eliza Miller-Ricci Kempton, a NASA Sagan Fellow at the University of California in Santa Cruz, and Derek Homeier of the Institute for Astrophysics in Gottingen, Germany -- examined GJ 1214b using the ground-based Very Large Telescope at Paranal Observatory in Chile.

"This is an important step forward, narrowing our understanding of the atmosphere of this planet," said NASA Exoplanet Exploration Program Scientist Douglas Hudgins at NASA Headquarters in Washington. "Bizarre worlds like this make exoplanet science one of the most compelling areas in astrophysics today."

The Sagan Fellowship Program is administered by the NASA Exoplanet Science Institute at the California Institute of Technology in Pasadena. Its purpose is to advance the scientific and technical goals of NASA's Exoplanet Exploration Program. The program is managed for NASA by the Jet Propulsion Laboratory in Pasadena, Calif. Caltech manages JPL for NASA.

Double Vision: New Instrument Casts Its Eyes to the Sky

2011-01-04T19:12:00.001-08:00

The Large Binocular Telescope Interferometer has taken its first images of the star Beta Peg in the constellation Pictor -- an encouraging start for an instrument designed to probe the cosmic neighborhoods where Earth-like planets could exist.

Eight years in development, the NASA-funded instrument combines beams of light from twin 8.4-meter (28-foot) mirrors mounted atop the Large Binocular Telescope on Mount Graham, Ariz. "By combining the light of the telescopes, we're able to realize its full potential," said Project Manager Tom McMahon of the University of Arizona, Tucson. "Together, the two mirrors form the largest single-mount telescope in the world."

"The quality of the first-light images is wonderful," said the principal investigator for the project, Phil Hinz of the University of Arizona. "The telescope was stable and the instrument was working properly."

With this high-resolution imaging capability, astronomers hope to probe nearby solar systems -- specifically, the areas in these systems where Earth-like planets with liquid water could exist. Though the Large Binocular Telescope Interferometer won't be able to detect Earth-size planets, it will be able to see dust disks that are indicative of planet formation, in addition to detecting large, Jupiter-size planets farther out from the star. These findings will help future, space-based exoplanet missions know where to search for Earth-like planets in our own galactic neighborhood.

With its ability to probe this "habitable zone" of other solar systems, the Large Binocular Telescope Interferometer will also complement the capabilities of other NASA missions -- the Keck Interferometer, which can find dust very close to stars; and the Spitzer Space Telescope, which is adept at observing planet-forming dust that is much more distant.

"This instrument will help complete our picture of what planetary systems look like and be a pathfinder for finding Earth-like planets that are close by," Hinz said.

With a major upgrade of the Large Binocular Telescope's adaptive optics system scheduled for next year, the interferometer will undergo testing and commissioning for the majority of 2011, and during that time, scientific observations will begin.

"This is the highest-resolution instrument of its kind in the world," McMahon said. "We won't just be able to image exoplanets, but extragalactic objects, nebulae and galaxies. It's taken time to make sure it works as envisioned, but now it's time to do science."

The Large Binocular Telescope Interferometer is funded by NASA and managed by Ben Parvin at NASA's Jet Propulsion Laboratory, Pasadena, Calif., as part of NASA's Exoplanet Exploration Program. The instrument and product development are provided by the University of Arizona, Tucson.

Small Asteroid to Pass Within Earth-Moon System

2011-01-04T19:05:00.001-08:00

PASADENA, Calif. – A small asteroid will fly past Earth early Tuesday within the Earth-moon system. The asteroid, 2010 TD54, will have its closest approach to Earth’s surface at an altitude of about 45,000 kilometers (27,960 miles) at 6:50 EDT a.m. (3:50 a.m. PDT). At that time, the asteroid will be over southeastern Asia in the vicinity of Singapore. During its flyby, Asteroid 2010 TD54 has zero probability of impacting Earth. A telescope of the NASA-sponsored Catalina Sky Survey north of Tucson, Arizona discovered 2010 TD54 on Oct. 9 at (12:55 a.m. PDT) during routine monitoring of the skies.

2010 TD54 is estimated to be about 5 to 10 meters (16 to 33 feet) wide. Due to its small size, the asteroid would require a telescope of moderate size to be viewed. A five-meter-sized near-Earth asteroid from the undiscovered population of about 30 million would be expected to pass daily within a lunar distance, and one might strike Earth's atmosphere about every 2 years on average. If an asteroid of the size of 2010 TD54 were to enter Earth’s atmosphere, it would be expected to burn up high in the atmosphere and cause no damage to Earth’s surface.

The distance used on the Near Earth Object page is always the calculated distance from the center of Earth. The distance stated for 2010 TD54 is 52,000 kilometers (32,000 miles). To get the distance it will pass from Earth’s surface you need to subtract the distance from the center to the surface (which varies over the planet), or about one Earth radii. That puts the pass distance at about 45,500 kilometers (28,000 miles) above the planet.

NASA detects, tracks and characterizes asteroids and comets passing close to Earth using both ground- and space-based telescopes. The Near-Earth Object Observations Program, commonly called "Spaceguard," discovers these objects, characterizes a subset of them, and plots their orbits to determine if any could be potentially hazardous to our planet.

JPL manages the Near-Earth Object Program Office for NASA's Science Mission Directorate in Washington. JPL is a division of the California Institute of Technology in Pasadena.

NASA's Space Shuttle Discovery

2011-01-04T19:00:00.001-08:00

CAPE CANAVERAL, Fla. -- Space shuttle Discovery is scheduled to roll back from Launch Pad 39A to the Vehicle Assembly Building (VAB) at NASA's Kennedy Space Center in Florida for additional work related to its final scheduled mission. The first motion of the shuttle is targeted for 12:30 a.m. EST on Tuesday, Dec. 21.

In the VAB, technicians will scan below the foam insulation surrounding the intertank section of Discovery's external tank to look for any issues, such as cracks. They also will reapply foam after removing 89 sensors from the tank's aluminum skin following an instrumented tanking test on Dec. 17.

The sensors were used to measure changes in the tank as super-cold propellants were pumped in and drained out during the test. Data and analysis from the test will be used to determine what caused the tops of two, 21-foot-long support beams, called stringers, on the outside of the intertank to crack during fueling on Nov. 5.

Discovery's next launch attempt is no earlier than 1:37 a.m. on Feb. 3.

The fully assembled space shuttle, consisting of orbiter Discovery, the external fuel tank and twin solid rocket boosters, is mounted on a mobile launcher platform and will be returned to the VAB atop a crawler transporter. The crawler will travel slower than 1 mph during the 3.4-mile journey, which is expected to take about six hours.

Video highlights of the rollback will air on the NASA Television Video File segment Tuesday.

Media are invited to a photo opportunity of the shuttle's move back into the VAB. Reporters must arrive at Kennedy's News Center by 6 a.m. for transportation to the viewing area.

As dates and times of this event are subject to change, updates are available at 321-867-2525.

For NASA TV downlink information, schedules and links to streaming video, visit:

NASA Seeks Proposals For Technology Flight

2011-01-04T18:58:00.001-08:00

NASA Seeks Proposals For Technology Flight Demonstrations And Information About Suborbital Flight Services

WASHINGTON -- NASA is seeking proposals from researchers interested in testing new technologies during suborbital flights. The agency also is requesting information from commercial suborbital reusable launch vehicle providers and commercial payload integrators about carrying the technology payloads.

The selected payloads will fly on aircraft that provide parabolic flight trajectories and on suborbital reusable launch vehicles capable of flying to altitudes above 62 miles. The flights will expose the payloads to reduced gravity and near-zero gravity environments.

"During these flights, researchers will be able to test their technologies in a range of microgravity environments and share their data with NASA -- data NASA can use when planning future missions," said Bobby Braun, NASA chief technologist at the agency's headquarters in Washington.

The solicitation is being made by NASA's Flight Opportunities Program, which is designed to foster development of a commercial reusable suborbital transportation industry while developing new technologies and improving microgravity research. When available, such reusable vehicles will provide lower-cost, more frequent, and more reliable access to space.

Technology flights are expected to reduce risks associated with emerging technologies and procedures, and overall space operations in future missions, by demonstrating applications in a relevant environment.

The solicitation for suborbital flights is open until Dec. 31, 2014. Applications for the first round of flights are due by Jan. 31, 2011. Responses to the request for information about services of commercial suborbital reusable launch vehicle providers are due Jan. 20, 2011. Participation in both activities is open to all categories of organizations.

The Flight Opportunities Program, part of NASA's Office of the Chief Technologist, integrates the existing Commercial Reusable Suborbital Research and Facilitated Access to the Space Environment for Technology programs. The Flight Opportunities Program is managed at NASA's Dryden Flight Research Center in Edwards, Calif.

Universe's biggest known star discovered by British astronomers

2011-01-04T18:52:00.000-08:00

Scientists at the University of Sheffield found the stellar giant – named R136a1 – using the European southern Observatory's Very Large Telescope in Chile and data from the Hubble Space Telescope.

The star is located in the Tarantula Nebula in the Large Magellanic Cloud, a small "satellite" galaxy which orbits the Milky Way.

Previously, the heaviest known stars were around 150 times the mass of the Sun, and this was believed to be close to the cosmic size limit.

As stars get more massive the amount of energy created in their cores grows at a faster rate than the force of gravity which holds them together. The torrents of energy produced eventually become so powerful that the stars are torn apart.

This is known as the "Eddington Limit", after the British physicist Arthur Eddington who, in 1919, proved Einstein's theory of relativity by showing that light is bent by gravity.

It was believed that the Eddington Limit was reached at around 150 solar masses.

However, R136a1 has been measured at 265 solar masses. Since heavy stars rapidly lose mass as they grow older by converting it into energy, R136a1 has already lost 20 per cent of its mass in its short million-year life. It is believed originally to have been a colossal 320 solar masses.

The Sun, by comparison, has been burning for 4.57 billion years, and has converted only 0.03 per cent of its mass into energy.

The chief researcher in the Sheffield team, Professor Paul Crowther,told Astronomy Now: "Because of their proximity to the Eddington Limit they lose mass at a pretty high rate." This means that they are incredibly bright and hot – R136a1 is believed to have a surface temperature of more than 40,000 degrees Celsius, and is 10 million times brighter than the Sun.

Among the largest known stars previously known were the Pistol Star, between 80 and 150 solar masses, and Eta Carinae, around 100 solar masses. The Pistol Star radiates as much energy in 20 seconds as our Sun does in a year. However, both are utterly dwarfed by the vast new discovery.

According to Astronomy Now, R136a1 gives off more energy than all the stars in the Orion Nebula, and if it were in our solar system would be as much brighter than the Sun as the Sun is than the Moon.

There are four stars in the cluster RCM 136a, where R136a1 sits, with a mass over 150 that of the Sun. Those four stars alone give off half the energy of the entire cluster, which contains 100,000 stars in total.

Hyper-stars like R136a1 are believed to be formed from several young stars merging together, and are only found in the very heart of stellar clusters.

Prof Crowther suspects that this is about as big as stars can get. He says: "Owing to the rarity of these monsters, I think it is unlikely that this new record will be broken any time soon."

The team’s work can be found in the latest issue of Monthly Notices of the Royal Astronomical Society.

Safety Warning

2011-01-04T18:48:00.000-08:00

Eye safety warning for January 4 partial eclipse

It can lead to permanent damage to eyesight or even blindness, said Dame Sally Davies.

Observing the eclipse through a telescope, binoculars, sunglasses, photographic film or camera is not safe, said The Department of Health.

The safest way is via the television or live webcasts, it added.

Dame Sally Davies said: "Under no circumstances should people look directly at the sun during a partial eclipse.

"The risks of doing so are very real and could lead to irreversible damage to eyesight and even blindness.

"Children are particularly vulnerable as they may be tempted to take a peek.

"We would urge parents to explain the danger to their children.

The partial eclipse of 4 January will be visible in much of the UK between 0800 and 0930 GMT.

The moon will pass across the front of the sun but not cover it completely, creating a partial eclipse.

The amount of sun obscured in the middle of the eclipse varies from around 75% in London to barely 40% in Glasgow.

Eclipse safety code

It is dangerous to look at the sun at any time

Viewing the sun during a partial eclipse requires special protection or indirect viewing methods

Only properly designed and certified solar filters should be used for direct viewing of the sun's disk

These must be identified as suitable for direct viewing of the sun, bear the CE mark, and a statement that it conforms to EC Directive 89/686/EEC

Demios

2011-01-04T16:41:00.000-08:00

Deimos ("DEE mos") is the smaller and outermost of Mars' two moons. It is one of the smallest known moons in the solar system.

         orbit:    23,459 km from Mars
         diameter: 12.6 km (15 x 12.2 x 11)
         mass:     1.8e15 kg

In Greek mythology, Deimos is one of the sons of Ares (Mars) and Aphrodite (Venus); "deimos" is Greek for "panic".

Discovered 1877 August 12 by Hall, photographed by Viking 1 in 1977.

Deimos and Phobos are composed of carbon-rich rock like C-type asteroids and ice. Both are heavily cratered.

Deimos and Phobos are probably asteroids perturbed by Jupiter into orbits that allowed them to be captured by Mars.

Phobos

2011-01-04T16:37:00.000-08:00

Phobos ("FOH bus") is the larger and innermost of Mars' two moons. Phobos is closer to its primary than any other moon in the solar system, less than 6000 km above the surface of Mars. It is also one of the smallest moons in the solar system.

        orbit:    9378 km from the center of Mars
        diameter: 22.2 km (27 x 21.6 x 18.8)
        mass:     1.08e16 kg

In Greek mythology, Phobos is one of the sons of Ares (Mars) and Aphrodite (Venus). "phobos" is Greek for "fear" (the root of "phobia").

Discovered 1877 August 18 by Hall; photographed by Mariner 9 in 1971, Viking 1 in 1977, and Phobos in 1988.

Phobos orbits Mars below the synchronous orbit radius. Thus it rises in the west, moves very rapidly across the sky and sets in the east, usually twice a day. It is so close to the surface that it cannot be seen above the horizon from all points on the surface of Mars.

And Phobos is doomed: because its orbit is below synchronous altitude tidal forces are lowering its orbit (current rate: about 1.8 meters per century). In about 50 million years it will either crash onto the surface of Mars or (more likely) break up into a ring. (This is the opposite effect to that operating to raise the orbit of the Moon.)

Phobos and Deimos may be composed of carbon-rich rock like C-type asteroids. But their densities are so low that they cannot be pure rock. They are more likely composed of a mixture of rock and ice. Both are heavily cratered. New images from Mars Global Surveyor indicate that Phobos is covered with a layer of fine dust about a meter thick, similar to theregolith on the Earth's Moon.

The Soviet spacecraft Phobos 2 detected a faint but steady outgassing from Phobos. Unfortunately, Phobos 2 died before it could determine the nature of the material; water is the best bet. Phobos 2 also returned a few images (right).

The most prominent feature on Phobos is the large crater named Stickney, the maiden name of Hall's wife (above). Like Mimas' crater Herschel (on a smaller scale) the impact that created Stickney must have almost shattered Phobos. The grooves and streaks on the surface were probably also caused by the Stickney impact.

Phobos and Deimos are widely believed to be captured asteroids. There is some speculation that they originated in the outer solar system rather than in the main asteroid belt.

Phobos and Deimos may someday be useful as "space stations" from which to study Mars or as intermediate stops to and from the Martian surface; especially if the presence of ice is confirmed.

Mars

2011-01-04T16:33:00.000-08:00

Mars is the fourth planet from the Sun and the seventh largest:

      orbit: 227,940,000 km (1.52 AU) from Sun
      diameter: 6,794 km
      mass: 6.4219e23 kg

Mars (Greek: Ares) is the god of War. The planet probably got this name due to its red color; Mars is sometimes referred to as the Red Planet. (An interesting side note: the Roman god Mars was a god of agriculture before becoming associated with the Greek Ares; those in favor of colonizing and terraforming Mars may prefer this symbolism.) The name of the month March derives from Mars.

Mars has been known since prehistoric times. Of course, it has been extensively studied with ground-based observatories. But even very large telescopes find Mars a difficult target, it's just too small. It is still a favorite of science fiction writers as the most favorable place in the Solar System (other than Earth!) for human habitation. But the famous "canals" "seen" by Lowell and others were, unfortunately, just as imaginary as Barsoomian princesses.

Viking 2 Landing Site

Pathfinder Landing Site

The first spacecraft to visit Mars was Mariner 4 in 1965. Several others followed including Mars 2, the first spacecraft to land on Mars and the two Viking landers in 1976. Ending a long 20 year hiatus, Mars Pathfinder landed successfully on Mars on 1997 July 4. In 2004 the Mars Expedition Rovers "Spirit" and "Opportunity" landed on Mars sending back geologic data and many pictures; they are still operating after more than three years on Mars. In 2008, Phoenix landed in the northern plains to search for water. Three Mars orbiters (Mars Reconnaissance Orbiter, Mars Odyssey, and Mars Express) are also currently in operation.

Mars' orbit is significantly elliptical. One result of this is a temperature variation of about 30 C at the subsolar point between aphelion and perihelion. This has a major influence on Mars' climate. While the average temperature on Mars is about 218 K (-55 C, -67 F), Martian surface temperatures range widely from as little as 140 K (-133 C, -207 F) at the winter pole to almost 300 K (27 C, 80 F) on the day side during summer.

Though Mars is much smaller than Earth, its surface area is about the same as the land surface area of Earth.

Olympus Mons

Mars has some of the most highly varied and interesting terrain of any of the terrestrial planets, some of it quite spectacular:

Olympus Mons: the largest mountain in the Solar System rising 24 km (78,000 ft.) above the surrounding plain. Its base is more than 500 km in diameter and is rimmed by a cliff 6 km (20,000 ft) high.
Tharsis: a huge bulge on the Martian surface that is about 4000 km across and 10 km high.
Valles Marineris: a system of canyons 4000 km long and from 2 to 7 km deep (top of page);
Hellas Planitia: an impact crater in the southern hemisphere over 6 km deep and 2000 km in diameter.

Much of the Martian surface is very old and cratered, but there are also much younger rift valleys, ridges, hills and plains. (None of this is visible in any detail with a telescope, even the Hubble Space Telescope; all this information comes from the spacecraft that we've sent to Mars.)

Southern Highlands

The southern hemisphere of Mars is predominantly ancient cratered highlands somewhat similar to the Moon. In contrast, most of the northern hemisphere consists of plains which are much younger, lower in elevation and have a much more complex history. An abrupt elevation change of several kilometers seems to occur at the boundary. The reasons for this global dichotomy and abrupt boundary are unknown (some speculate that they are due to a very large impact shortly after Mars' accretion). Mars Global Surveyor has produced a nice 3D map of Mars that clearly shows these features.

The interior of Mars is known only by inference from data about the surface and the bulk statistics of the planet. The most likely scenario is a dense core about 1700 km in radius, a molten rocky mantle somewhat denser than the Earth's and a thin crust. Data from Mars Global Surveyor indicates that Mars' crust is about 80 km thick in the southern hemisphere but only about 35 km thick in the north. Mars' relatively low density compared to the other terrestrial planets indicates that its core probably contains a relatively large fraction of sulfur in addition to iron (iron and iron sulfide).

Like Mercury and the Moon, Mars appears to lack active plate tectonics at present; there is no evidence of recent horizontal motion of the surface such as the folded mountains so common on Earth. With no lateral plate motion, hot-spots under the crust stay in a fixed position relative to the surface. This, along with the lower surface gravity, may account for the Tharis bulge and its enormous volcanoes. There is no evidence of current volcanic activity. However, data from Mars Global Surveyor indicates that Mars very likely did have tectonic activity sometime in the past.

Valley Network

There is very clear evidence of erosion in many places on Mars including large floods and small river systems. At some time in the past there was clearly some sort of fluid on the surface. Liquid water is the obvious fluid but other possibilities exist. There may have been large lakes or even oceans; the evidence for which was strenghtened by some very nice images of layered terrain taken by Mars Global Surveyor and the mineralology results from MER Opportunity. Most of these point to wet episodes that occurred only briefly and very long ago; the age of the erosion channels is estimated at about nearly 4 billion years. However, images from Mars Express released in early 2005 show what appears to be a frozen sea that was liquid very recently (maybe 5 million years ago). Confirmation of this interpretation would be a very big deal indeed! (Valles Marineris was NOT created by running water. It was formed by the stretching and cracking of the crust associated with the creation of the Tharsis bulge.)

Early in its history, Mars was much more like Earth. As with Earth almost all of its carbon dioxide was used up to form carbonate rocks. But lacking the Earth's plate tectonics, Mars is unable to recycle any of this carbon dioxide back into its atmosphere and so cannot sustain a significant greenhouse effect. The surface of Mars is therefore much colder than the Earth would be at that distance from the Sun.

Mars has a very thin atmosphere composed mostly of the tiny amount of remaining carbon dioxide (95.3%) plus nitrogen (2.7%), argon (1.6%) and traces of oxygen (0.15%) and water (0.03%). The average pressure on the surface of Mars is only about 7 millibars (less than 1% of Earth's), but it varies greatly with altitude from almost 9 millibars in the deepest basins to about 1 millibar at the top of Olympus Mons. But it is thick enough to support very strong winds and vast dust storms that on occasion engulf the entire planet for months. Mars' thin atmosphere produces a greenhouse effect but it is only enough to raise the surface temperature by 5 degrees (K); much less than what we see on Venus and Earth.

South Polar Cap

Early telescopic observations revealed that Mars has permanent ice caps at both poles; they're visible even with a small telescope. We now know that they're composed of water ice and solid carbon dioxide ("dry ice"). The ice caps exhibit a layered structure with alternating layers of ice with varying concentrations of dark dust. In the northern summer the carbon dioxide completely sublimes, leaving a residual layer of water ice. ESA's Mars Express has shown that a similar layer of water ice exists below the southern cap as well. The mechanism responsible for the layering is unknown but may be due to climatic changes related to long-term changes in the inclination of Mars' equator to the plane of its orbit. There may also be water ice hidden below the surface at lower latitudes. The seasonal changes in the extent of the polar caps changes the global atmospheric pressure by about 25% (as measured at the Viking lander sites).

Mars by HST

Recent observations with the Hubble Space Telescope have revealed that the conditions during the Viking missions may not have been typical. Mars' atmosphere now seems to be both colder and dryer than measured by the Viking landers (more details from STScI).

The Viking landers performed experiments to determine the existence of life on Mars. The results were somewhat ambiguous but most scientists now believe that they show no evidence for life on Mars (there is still some controversy, however). Optimists point out that only two tiny samples were measured and not from the most favorable locations. More experiments will be done by future missions to Mars.

A small number of meteorites (the SNC meteorites) are believed to have originated on Mars.

On 1996 Aug 6, David McKay et al announced what they thought might be evidence of ancient Martian microorganisms in the meteorite ALH84001. Though there is still some controversy, the majority of the scientific community has not accepted this conclusion. If there is or was life on Mars, we still haven't found it.

Large, but not global, weak magnetic fields exist in various regions of Mars. This unexpected finding was made by Mars Global Surveyor just days after it entered Mars orbit. They are probably remnants of an earlier global field that has since disappeared. This may have important implications for the structure of Mars' interior and for the past history of its atmosphere and hence for the possibility of ancient life.

When it is in the nighttime sky, Mars is easily visible with the unaided eye. Mars is a difficult but rewarding target for an amateur telescope though only for the three or four months each martian year when it is closest to Earth. Its apparent size and brightness varies greatly according to its relative position to the Earth. There are several Web sites that show the current position of Mars (and the other planets) in the sky. More detailed and customized charts can be created with a planetarium program.

Moon

2011-01-04T15:54:00.000-08:00

The Moon is the only natural satellite of Earth:

        orbit:    384,400 km from Earth
        diameter: 3476 km
        mass:     7.35e22 kg

Called Luna by the Romans, Selene and Artemis by the Greeks, and many other names in other mythologies.

The Moon, of course, has been known since prehistoric times. It is the second brightest object in the sky after the Sun. As the Moon orbits around the Earth once per month, the angle between the Earth, the Moon and the Sun changes; we see this as the cycle of the Moon's phases. The time between successive new moons is 29.5 days (709 hours), slightly different from the Moon's orbital period (measured against the stars) since the Earth moves a significant distance in its orbit around the Sun in that time.

Due to its size and composition, the Moon is sometimes classified as a terrestrial "planet" along with Mercury, Venus, Earth and Mars.

The Moon was first visited by the Soviet spacecraft Luna 2 in 1959. It is the only extraterrestrial body to have been visited by humans. The first landing was on July 20, 1969 (do you remember where you were?); the last was in December 1972. The Moon is also the only body from which samples have been returned to Earth. In the summer of 1994, the Moon was very extensively mapped by the little spacecraft Clementine and again in 1999 by Lunar Prospector.

The gravitational forces between the Earth and the Moon cause some interesting effects. The most obvious is the tides. The Moon's gravitational attraction is stronger on the side of the Earth nearest to the Moon and weaker on the opposite side. Since the Earth, and particularly the oceans, is not perfectly rigid it is stretched out along the line toward the Moon. From our perspective on the Earth's surface we see two small bulges, one in the direction of the Moon and one directly opposite. The effect is much stronger in the ocean water than in the solid crust so the water bulges are higher. And because the Earth rotates much faster than the Moon moves in its orbit, the bulges move around the Earth about once a day giving two high tides per day. (This is a greatly simplified model; actual tides, especially near the coasts, are much more complicated.)

But the Earth is not completely fluid, either. The Earth's rotation carries the Earth's bulges slightly ahead of the point directly beneath the Moon. This means that the force between the Earth and the Moon is not exactly along the line between their centers producing a torque on the Earth and an accelerating force on the Moon. This causes a net transfer of rotational energy from the Earth to the Moon, slowing down the Earth's rotation by about 1.5 milliseconds/century and raising the Moon into a higher orbit by about 3.8 centimeters per year. (The opposite effect happens to satellites with unusual orbits such as Phobos and Triton).

The asymmetric nature of this gravitational interaction is also responsible for the fact that the Moon rotates synchronously, i.e. it is locked in phase with its orbit so that the same side is always facing toward the Earth. Just as the Earth's rotation is now being slowed by the Moon's influence so in the distant past the Moon's rotation was slowed by the action of the Earth, but in that case the effect was much stronger. When the Moon's rotation rate was slowed to match its orbital period (such that the bulge always faced toward the Earth) there was no longer an off-center torque on the Moon and a stable situation was achieved. The same thing has happened to most of the other satellites in the solar system. Eventually, the Earth's rotation will be slowed to match the Moon's period, too, as is the case with Pluto and Charon.

Actually, the Moon appears to wobble a bit (due to its slightly non-circular orbit) so that a few degrees of the far side can be seen from time to time, but the majority of the far side (left) was completely unknown until the Soviet spacecraft Luna 3 photographed it in 1959. (Note: there is no "dark side" of the Moon; all parts of the Moon get sunlight half the time (except for a few deep craters near the poles). Some uses of the term "dark side" in the past may have referred to the far side as "dark" in the sense of "unknown" (eg "darkest Africa") but even that meaning is no longer valid today!)

The Moon has no atmosphere. But evidence from Clementine suggested that there may be water ice in some deep craters near the Moon's south pole which are permanently shaded. This has now been reinforced by data from Lunar Prospector. There is apparently ice at the north pole as well.

The Moon's crust averages 68 km thick and varies from essentially 0 under Mare Crisium to 107 km north of the crater Korolev on the lunar far side. Below the crust is a mantle and probably a small core (roughly 340 km radius and 2% of the Moon's mass). Unlike the Earth, however, the Moon's interior is no longer active. Curiously, the Moon's center of mass is offset from its geometric center by about 2 km in the direction toward the Earth. Also, the crust is thinner on the near side.

There are two primary types of terrain on the Moon: the heavily cratered and very old highlands and the relatively smooth and younger maria. The maria (which comprise about 16% of the Moon's surface) are huge impact craters that were later flooded by molten lava. Most of the surface is covered with regolith, a mixture of fine dust and rocky debris produced by meteor impacts. For some unknown reason, the maria are concentrated on the near side.

Most of the craters on the near side are named for famous figures in the history of science such as Tycho, Copernicus, and Ptolemaeus. Features on the far side have more modern references such as Apollo, Gagarin and Korolev (with a distinctly Russian bias since the first images were obtained by Luna 3). In addition to the familiar features on the near side, the Moon also has the huge craters South Pole-Aitken on the far side which is 2250 km in diameter and 12 km deep making it the the largest impact basin in the solar system and Orientale on the western limb (as seen from Earth; in the center of the image at left) which is a splendid example of a multi-ring crater.

A total of 382 kg of rock samples were returned to the Earth by the Apollo and Luna programs. These provide most of our detailed knowledge of the Moon. They are particularly valuable in that they can be dated. Even today, more than 30 years after the last Moon landing, scientists still study these precious samples.

Most rocks on the surface of the Moon seem to be between 4.6 and 3 billion years old. This is a fortuitous match with the oldest terrestrial rocks which are rarely more than 3 billion years old. Thus the Moon provides evidence about the early history of the Solar System not available on the Earth.

Prior to the study of the Apollo samples, there was no consensus about the origin of the Moon. There were three principal theories: co-accretion which asserted that the Moon and the Earth formed at the same time from the Solar Nebula; fission which asserted that the Moon split off of the Earth; and capture which held that the Moon formed elsewhere and was subsequently captured by the Earth. None of these work very well. But the new and detailed information from the Moon rocks led to the impact theory: that the Earth collided with a very large object (as big as Mars or more) and that the Moon formed from the ejected material. There are still details to be worked out, but the impact theory is now widely accepted.

The Moon has no global magnetic field. But some of its surface rocks exhibit remanent magnetism indicating that there may have been a global magnetic field early in the Moon's history.

With no atmosphere and no magnetic field, the Moon's surface is exposed directly to the solar wind. Over its 4 billion year lifetime many ions from the solar wind have become embedded in the Moon's regolith. Thus samples of regolith returned by the Apollo missions proved valuable in studies of the solar wind.

Planet Earth

2011-01-04T15:44:00.000-08:00

Earth is the third planet from the Sun and the fifth largest:

        orbit:    149,600,000 km (1.00 AU) from Sun
        diameter: 12,756.3 km
        mass:     5.972e24 kg

Earth is the only planet whose English name does not derive from Greek/Roman mythology. The name derives from Old English and Germanic. There are, of course, hundreds of other names for the planet in other languages. In Roman Mythology, the goddess of the Earth was Tellus - the fertile soil (Greek: Gaia, terra mater - Mother Earth).

It was not until the time of Copernicus (the sixteenth century) that it was understood that the Earth is just another planet.

Earth, of course, can be studied without the aid of spacecraft. Nevertheless it was not until the twentieth century that we had maps of the entire planet. Pictures of the planet taken from space are of considerable importance; for example, they are an enormous help in weather prediction and especially in tracking and predicting hurricanes. And they are extraordinarily beautiful.

The Earth is divided into several layers which have distinct chemical and seismic properties (depths in km):

      0-  40  Crust
     40- 400  Upper mantle
    400- 650  Transition region
    650-2700  Lower mantle
   2700-2890  D'' layer
   2890-5150  Outer core
   5150-6378  Inner core

The crust varies considerably in thickness, it is thinner under the oceans, thicker under the continents. The inner core and crust are solid; the outer core and mantle layers are plastic or semi-fluid. The various layers are separated by discontinuities which are evident in seismic data; the best known of these is the Mohorovicic discontinuity between the crust and upper mantle.

Most of the mass of the Earth is in the mantle, most of the rest in the core; the part we inhabit is a tiny fraction of the whole (values below x10^24 kilograms):

      atmosphere     = 0.0000051
      oceans         = 0.0014
      crust          = 0.026
      mantle         = 4.043
      outer core     = 1.835
      inner core     = 0.09675

The core is probably composed mostly of iron (or nickel/iron) though it is possible that some lighter elements may be present, too. Temperatures at the center of the core may be as high as 7500 K, hotter than the surface of the Sun. The lower mantle is probably mostly silicon, magnesium and oxygen with some iron, calcium and aluminum. The upper mantle is mostly olivene and pyroxene (iron/magnesium silicates), calcium and aluminum. We know most of this only from seismic techniques; samples from the upper mantle arrive at the surface as lava from volcanoes but the majority of the Earth is inaccessible. The crust is primarily quartz (silicon dioxide) and other silicates like feldspar. Taken as a whole, the Earth's chemical composition (by mass) is:

South America by Galileo

    34.6%  Iron
    29.5%  Oxygen
    15.2%  Silicon
    12.7%  Magnesium
     2.4%  Nickel
     1.9%  Sulfur
     0.05% Titanium

The Earth is the densest major body in the solar system.

The other terrestrial planets probably have similar structures and compositions with some differences: the Moon has at most a small core; Mercury has an extra large core (relative to its diameter); the mantles of Mars and the Moon are much thicker; the Moon and Mercury may not have chemically distinct crusts; Earth may be the only one with distinct inner and outer cores. Note, however, that our knowledge of planetary interiors is mostly theoretical even for the Earth.

Unlike the other terrestrial planets, Earth's crust is divided into several separate solid plates which float around independently on top of the hot mantle below. The theory that describes this is known as plate tectonics. It is characterized by two major processes: spreading and subduction. Spreading occurs when two plates move away from each other and new crust is created by upwelling magma from below. Subduction occurs when two plates collide and the edge of one dives beneath the other and ends up being destroyed in the mantle. There is also transverse motion at some plate boundaries (i.e. the San Andreas Fault in California) and collisions between continental plates (i.e. India/Eurasia). There are (at present) eight major plates:

North American Plate - North America, western North Atlantic and Greenland
Earth's Plate Boundaries delineated by earthquake epicenters
South American Plate - South America and western South Atlantic
Antarctic Plate - Antarctica and the "Southern Ocean"
Eurasian Plate - eastern North Atlantic, Europe and Asia except for India
African Plate - Africa, eastern South Atlantic and western Indian Ocean
Indian-Australian Plate - India, Australia, New Zealand and most of Indian Ocean
Nazca Plate - eastern Pacific Ocean adjacent to South America
Pacific Plate - most of the Pacific Ocean (and the southern coast of California!)

There are also twenty or more small plates such as the Arabian, Cocos, and Philippine Plates. Earthquakes are much more common at the plate boundaries. Plotting their locations makes it easy to see the plate boundaries.

The Earth's surface is very young. In the relatively short (by astronomical standards) period of 500,000,000 years or so erosion and tectonic processes destroy and recreate most of the Earth's surface and thereby eliminate almost all traces of earlier geologic surface history (such as impact craters). Thus the very early history of the Earth has mostly been erased. The Earth is 4.5 to 4.6 billion years old, but the oldest known rocks are about 4 billion years old and rocks older than 3 billion years are rare. The oldest fossils of living organisms are less than 3.9 billion years old. There is no record of the critical period when life was first getting started.

Space Shuttle view of the Strait of Gibraltar

71 Percent of the Earth's surface is covered with water. Earth is the only planet on which water can exist in liquid form on the surface (though there may be liquid ethane or methane on Titan's surface and liquid water beneath the surface of Europa). Liquid water is, of course, essential for life as we know it. The heat capacity of the oceans is also very important in keeping the Earth's temperature relatively stable. Liquid water is also responsible for most of the erosion and weathering of the Earth's continents, a process unique in the solar system today (though it may have occurred on Mars in the past).

Earth's atmosphere seen at the limb

The Earth's atmosphere is 77% nitrogen, 21% oxygen, with traces of argon, carbon dioxide and water. There was probably a very much larger amount of carbon dioxide in the Earth's atmosphere when the Earth was first formed, but it has since been almost all incorporated into carbonate rocks and to a lesser extent dissolved into the oceans and consumed by living plants. Plate tectonics and biological processes now maintain a continual flow of carbon dioxide from the atmosphere to these various "sinks" and back again. The tiny amount of carbon dioxide resident in the atmosphere at any time is extremely important to the maintenance of the Earth's surface temperature via the greenhouse effect. The greenhouse effect raises the average surface temperature about 35 degrees C above what it would otherwise be (from a frigid -21 C to a comfortable +14 C); without it the oceans would freeze and life as we know it would be impossible. (Water vapor is also an important greenhouse gas.)

View from Apollo 11

The presence of free oxygen is quite remarkable from a chemical point of view. Oxygen is a very reactive gas and under "normal" circumstances would quickly combine with other elements. The oxygen in Earth's atmosphere is produced and maintained by biological processes. Without life there would be no free oxygen.

The interaction of the Earth and the Moon slows the Earth's rotation by about 2 milliseconds per century. Current research indicates that about 900 million years ago there were 481 18-hour days in a year.

Earth has a modest magnetic field produced by electric currents in the outer core. The interaction of the solar wind, the Earth's magnetic field and the Earth's upper atmosphere causes the auroras (see the Interplanetary Medium). Irregularities in these factors cause the magnetic poles to move and even reverse relative to the surface; the geomagnetic north pole is currently located in northern Canada. (The "geomagnetic north pole" is the position on the Earth's surface directly above the south pole of the Earth's field.)

The Earth's magnetic field and its interaction with the solar wind also produce the Van Allen radiation belts, a pair of doughnut shaped rings of ionized gas (or plasma) trapped in orbit around the Earth. The outer belt stretches from 19,000 km in altitude to 41,000 km; the inner belt lies between 13,000 km and 7,600 km in altitude.

Earth has only one natural satellite, the Moon. But

thousands of small artificial satellites have also been placed in orbit around the Earth.
Asteroids 3753 Cruithne and 2002 AA29 have complicated orbital relationships with the Earth; they're not really moons, the term "companion" is being used. It is somewhat similar to the situation with Saturn's moons Janus and Epimetheus.
Lilith doesn't exist but it's an interesting story.

Venus

2011-01-04T15:17:00.000-08:00

Venus is the second planet from the Sun and the sixth largest. Venus' orbit is the most nearly circular of that of any planet, with an eccentricity of less than 1%.

        orbit:    108,200,000 km (0.72 AU) from Sun
        diameter: 12,103.6 km
        mass:     4.869e24 kg

Venus (Greek: Aphrodite; Babylonian: Ishtar) is the goddess of love and beauty. The planet is so named probably because it is the brightest of the planets known to the ancients. (With a few exceptions, the surface features on Venus are named for female figures.)

Venus has been known since prehistoric times. It is the brightest object in the sky except for the Sun and the Moon. Like Mercury, it was popularly thought to be two separate bodies: Eosphorus as the morning star and Hesperus as the evening star, but the Greek astronomers knew better. (Venus's apparition as the morning star is also sometimes called Lucifer.)

Since Venus is an inferior planet, it shows phases when viewed with a telescope from the perspective of Earth. Galileo's observation of this phenomenon was important evidence in favor of Copernicus's heliocentric theory of the solar system.

Venera 9 surface photo

The first spacecraft to visit Venus was Mariner 2 in 1962. It was subsequently visited by many others (more than 20 in all so far), including Pioneer Venus and the Soviet Venera 7 the first spacecraft to land on another planet, and Venera 9 which returned the first photographs of the surface. The first orbiter, the US spacecraft Magellan

Magellan radar map (false color)

produced detailed maps of Venus' surface using radar. ESA's Venus Express is now in orbit with a large variety of instruments.

Venus' rotation is somewhat unusual in that it is both very slow (243 Earth days per Venus day, slightly longer than Venus' year) and retrograde. In addition, the periods of Venus' rotation and of its orbit are synchronized such that it always presents the same face toward Earth when the two planets are at their closest approach. Whether this is a resonance effect or merely a coincidence is not known.

Venus is sometimes regarded as Earth's sister planet. In some ways they are very similar:

Venus is only slightly smaller than Earth (95% of Earth's diameter, 80% of Earth's mass).
Both have few craters indicating relatively young surfaces.
Their densities and chemical compositions are similar.

Because of these similarities, it was thought that below its dense clouds Venus might be very Earthlike and might even have life. But, unfortunately, more detailed study of Venus reveals that in many important ways it is radically different from Earth. It may be the least hospitable place for life in the solar system.

Venus in visible light from Galileo

The pressure of Venus' atmosphere at the surface is 90 atmospheres (about the same as the pressure at a depth of 1 km in Earth's oceans). It is composed mostly of carbon dioxide. There are several layers of clouds many kilometers thick composed of sulfuric acid. These clouds completely obscure our view of the surface. This dense atmosphere produces a run-away greenhouse effect that raises Venus' surface temperature by about 400 degrees to over 740 K (hot enough to melt lead). Venus' surface is actually hotter than Mercury's despite being nearly twice as far from the Sun.

Venus in ultra-violet light

There are strong (350 kph) winds at the cloud tops but winds at the surface are very slow, no more than a few kilometers per hour.

Venus probably once had large amounts of water like Earth but it all boiled away. Venus is now quite dry. Earth would have suffered the same fate had it been just a little closer to the Sun. We may learn a lot about Earth by learning why the basically similar Venus turned out so differently.

Most of Venus' surface consists of gently rolling plains with little relief. There are also several broad depressions: Atalanta Planitia, Guinevere Planitia, Lavinia Planitia. There two large highland areas: Ishtar Terra in the northern hemisphere (about the size of Australia) and Aphrodite Terra along the equator (about the size of South America). The interior of Ishtar consists mainly of a high plateau, Lakshmi Planum, which is surrounded by the highest mountains on Venus including the enormous Maxwell Montes.

Sif Mons (Magellan radar)

Data from Magellan's imaging radar shows that much of the surface of Venus is covered by lava flows. There are several large shield volcanoes (similar to Hawaii or Olympus Mons) such as Sif Mons. Recently announced findings indicate that Venus is still volcanically active, but only in a few hot spots; for the most part it has been geologically rather quiet for the past few hundred million years.

There are no small craters on Venus. It seems that small meteoroids burn up in Venus' dense atmosphere before reaching the surface. Craters on Venus seem to come in bunches indicating that large meteoroids that do reach the surface usually break up in the atmosphere.

The oldest terrains on Venus seem to be about 800 million years old. Extensive volcanism at that time wiped out the earlier surface including any large craters from early in Venus' history.

Coronae

Pancake volcanoes

Magellan's images show a wide variety of interesting and unique features including pancake volcanoes (left) which seem to be eruptions of very thick lava and coronae (right) which seem to be collapsed domes over large magma chambers.

The interior of Venus is probably very similar to that of Earth: an iron core about 3000 km in radius, a molten rocky mantle comprising the majority of the planet. Recent results from the Magellan gravity data indicate that Venus' crust is stronger and thicker than had previously been assumed. Like Earth, convection in the mantle produces stress on the surface. However on Venus the stress is relieved in many relatively small regions instead of being concentrated at the boundaries of large plates as is the case on Earth.

Venus has no magnetic field, perhaps because of its slow rotation.

Venus has no satellites, and thereby hangs a tale.

Venus is usually visible with the unaided eye. Sometimes (inaccurately) referred to as the "morning star" or the "evening star", it is by far the brightest "star" in the sky. There are several Web sites that show the current position of Venus (and the other planets) in the sky. More detailed and customized charts can be created with a planetarium program.

On June 8 2004, Venus passed directly between the Earth and the Sun, appearing as a large black dot travelling across the Sun's disk. This event is known as a "transit of Venus" and is very rare: the last one was in 1882, the next one is in 2012 but after than you'll have to wait until 2117. While no longer of great scientific importance as it was in the past, this event was the impetus for a major journey for many amateur astronomers.

Mercury

2011-01-04T14:55:00.000-08:00

Mercury is the closest planet to the Sun and the eighth largest. Mercury is slightly smaller in diameter than the moons Ganymede and Titan but more than twice as massive.

        orbit:    57,910,000 km (0.38 AU) from Sun
        diameter: 4,880 km
        mass:     3.30e23 kg

In Roman mythology Mercury is the god of commerce, travel and thievery, the Roman counterpart of the Greek god Hermes, the messenger of the Gods. The planet probably received this name because it moves so quickly across the sky.

Mercury has been known since at least the time of the Sumerians (3rd millennium BC). It was sometimes given separate names for its apparitions as a morning star and as an evening star. Greek astronomers knew, however, that the two names referred to the same body. Heraclitus even believed that Mercury and Venus orbit the Sun, not the Earth.

Since it is closer to the Sun than the Earth, the illumination of Mercury's disk varies when viewed with a telescope from our perspective. Galileo's telescope was too small to see Mercury's phases but he did see the phases of Venus.

Mercury has been now been visited by two spacecraft, Mariner 10 and MESSENGER. Marriner 10 flew by three times in 1974 and 1975. Only 45% of the surface was mapped (and, unfortunately, it is too close to the Sun to be safely imaged by HST). MESSENGER was launched by NASA in 2004 and will orbit Mercury starting in 2011 after several flybys. Its first flyby in Jan 2008 provided new high quality images of some of the terrain not seen by Marriner 10.

Mercury's orbit is highly eccentric; at perihelion it is only 46 million km from the Sun but at aphelion it is 70 million. The position of the perihelion precesses around the Sun at a very slow rate. 19th century astronomers made very careful observations of Mercury's orbital parameters but could not adequately explain them using Newtonian mechanics. The tiny differences between the observed and predicted values were a minor but nagging problem for many decades. It was thought that another planet (sometimes called Vulcan) slightly closer to the Sun than Mercury might account for the discrepancy. But despite much effort, no such planet was found. The real answer turned out to be much more dramatic: Einstein's General Theory of Relativity! Its correct prediction of the motions of Mercury was an important factor in the early acceptance of the theory.

Until 1962 it was thought that Mercury's "day" was the same length as its "year" so as to keep that same face to the Sun much as the Moon does to the Earth. But this was shown to be false in 1965 by doppler radar observations. It is now known that Mercury rotates three times in two of its years. Mercury is the only body in the solar system known to have an orbital/rotational resonance with a ratio other than 1:1 (though many have no resonances at all).

This fact and the high eccentricity of Mercury's orbit would produce very strange effects for an observer on Mercury's surface. At some longitudes the observer would see the Sun rise and then gradually increase in apparent size as it slowly moved toward the zenith. At that point the Sun would stop, briefly reverse course, and stop again before resuming its path toward the horizon and decreasing in apparent size. All the while the stars would be moving three times faster across the sky. Observers at other points on Mercury's surface would see different but equally bizarre motions.

Temperature variations on Mercury are the most extreme in the solar system ranging from 90 K to 700 K. The temperature on Venus is slightly hotter but very stable.

Mercury craters

Mercury is in many ways similar to the Moon: its surface is heavily cratered and very old; it has no plate tectonics. On the other hand, Mercury is much denser than the Moon (5.43 gm/cm3 vs 3.34). Mercury is the second densest major body in the solar system, after Earth. Actually Earth's density is due in part to gravitational compression; if not for this, Mercury would be denser than Earth. This indicates that Mercury's dense iron core is relatively larger than Earth's, probably comprising the majority of the planet. Mercury therefore has only a relatively thin silicate mantle and crust.

Mercury's interior is dominated by a large iron core whose radius is 1800 to 1900 km. The silicate outer shell (analogous to Earth's mantle and crust) is only 500 to 600 km thick. At least some of the core is probably molten.

Mercury actually has a very thin atmosphere consisting of atoms blasted off its surface by the solar wind. Because Mercury is so hot, these atoms quickly escape into space. Thus in contrast to the Earth and Venus whose atmospheres are stable, Mercury's atmosphere is constantly being replenished.

Southwest Mercury

The surface of Mercury exhibits enormous escarpments, some up to hundreds of kilometers in length and as much as three kilometers high. Some cut thru the rings of craters and other features in such a way as to indicate that they were formed by compression. It is estimated that the surface area of Mercury shrank by about 0.1% (or a decrease of about 1 km in the planet's radius).

Caloris Basin

One of the largest features on Mercury's surface is the Caloris Basin (right); it is about 1300 km in diameter. It is thought to be similar to the large basins (maria) on the Moon. Like the lunar basins, it was probably caused by a very large impact early in the history of the solar system.

Weird terrain opposite Caloris Basin

That impact was probably also responsible for the odd terrain on the exact opposite side of the planet (left).

In addition to the heavily cratered terrain, Mercury also has regions of relatively smooth plains. Some may be the result of ancient volcanic activity but some may be the result of the deposition of ejecta from cratering impacts.

A reanalysis of the Mariner data provides some preliminary evidence of recent volcanism on Mercury. But more data will be needed for confirmation.

Amazingly, radar observations of Mercury's north pole (a region not mapped by Mariner 10) show evidence of water ice in the protected shadows of some craters.

Mercury has a small magnetic field whose strength is about 1% of Earth's.

Mercury has no known satellites.

Mercury is often visible with binoculars or even the unaided eye, but it is always very near the Sun and difficult to see in the twilight sky. There are several Web sites that show the current position of Mercury (and the other planets) in the sky. More detailed and customized charts can be created with a planetarium program.

Our Sun

2011-01-04T14:49:00.000-08:00

Our Sun is a normal main-sequence G2 star, one of more than 100 billion stars in our galaxy.

        diameter:    1,390,000 km.
        mass:        1.989e30 kg
        temperature: 5800 K (surface)
                     15,600,000 K (core)

The Sun is by far the largest object in the solar system. It contains more than 99.8% of the total mass of the Solar System (Jupiter contains most of the rest).

It is often said that the Sun is an "ordinary" star. That's true in the sense that there are many others similar to it. But there are many more smaller stars than larger ones; the Sun is in the top 10% by mass. The median size of stars in our galaxy is probably less than half the mass of the Sun.

The Sun is personified in many mythologies: the Greeks called it Helios and the Romans called it Sol.

The Sun is, at present, about 70% hydrogen and 28% helium by mass everything else ("metals") amounts to less than 2%. This changes slowly over time as the Sun converts hydrogen to helium in its core.

The outer layers of the Sun exhibit differential rotation: at the equator the surface rotates once every 25.4 days; near the poles it's as much as 36 days. This odd behavior is due to the fact that the Sun is not a solid body like the Earth. Similar effects are seen in the gas planets. The differential rotation extends considerably down into the interior of the Sun but the core of the Sun rotates as a solid body.

Conditions at the Sun's core (approximately the inner 25% of its radius) are extreme. The temperature is 15.6 million Kelvin and the pressure is 250 billion atmospheres. At the center of the core the Sun's density is more than 150 times that of water.

The Sun's power (about 386 billion billion megaWatts) is produced by nuclear fusion reactions. Each second about 700,000,000 tons of hydrogen are converted to about 695,000,000 tons of helium and 5,000,000 tons (=3.86e33 ergs) of energy in the form of gamma rays. As it travels out toward the surface, the energy is continuously absorbed and re-emitted at lower and lower temperatures so that by the time it reaches the surface, it is primarily visible light. For the last 20% of the way to the surface the energy is carried more by convection than by radiation.

The surface of the Sun, called the photosphere, is at a temperature of about 5800 K. Sunspots are "cool" regions, only 3800 K (they look dark only by comparison with the surrounding regions). Sunspots can be very large, as much as 50,000 km in diameter. Sunspots are caused by complicated and not very well understood interactions with the Sun's magnetic field.

A small region known as the chromosphere lies above the photosphere.

The highly rarefied region above the chromosphere, called the corona, extends millions of kilometers into space but is visible only during a total solar eclipse (left). Temperatures in the corona are over 1,000,000 K.

It just happens that the Moon and the Sun appear the same size in the sky as viewed from the Earth. And since the Moon orbits the Earth in approximately the same plane as the Earth's orbit around the Sun sometimes the Moon comes directly between the Earth and the Sun. This is called a solar eclipse; if the alignment is slighly imperfect then the Moon covers only part of the Sun's disk and the event is called a partial eclipse. When it lines up perfectly the entire solar disk is blocked and it is called a total eclipse of the Sun. Partial eclipses are visible over a wide area of the Earth but the region from which a total eclipse is visible, called the path of totality, is very narrow, just a few kilometers (though it is usually thousands of kilometers long). Eclipses of the Sun happen once or twice a year. If you stay home, you're likely to see a partial eclipse several times per decade. But since the path of totality is so small it is very unlikely that it will cross you home. So people often travel half way around the world just to see a total solar eclipse. To stand in the shadow of the Moon is an awesome experience. For a few precious minutes it gets dark in the middle of the day. The stars come out. The animals and birds think it's time to sleep. And you can see the solar corona. It is well worth a major journey.

The Sun's magnetic field is very strong (by terrestrial standards) and very complicated. Its magnetosphere (also known as the heliosphere) extends well beyond Pluto.

In addition to heat and light, the Sun also emits a low density stream of charged particles (mostly electrons and protons) known as the solar wind which propagates throughout the solar system at about 450 km/sec. The solar wind and the much higher energy particles ejected by solar flares can have dramatic effects on the Earth ranging from power line surges to radio interference to the beautiful aurora borealis.

Recent data from the spacecraft Ulysses show that during the minimum of the solar cycle the solar wind emanating from the polar regions flows at nearly double the rate, 750 kilometers per second, than it does at lower latitudes. The composition of the solar wind also appears to differ in the polar regions. During the solar maximum, however, the solar wind moves at an intermediate speed.

Further study of the solar wind will be done by the recently launched Wind, ACE and SOHO spacecraft from the dynamically stable vantage point directly between the Earth and the Sun about 1.6 million km from Earth.

The solar wind has large effects on the tails of comets and even has measurable effects on the trajectories of spacecraft.

Spectacular loops and prominences are often visible on the Sun's limb (left).

The Sun's output is not entirely constant. Nor is the amount of sunspot activity. There was a period of very low sunspot activity in the latter half of the 17th century called the Maunder Minimum. It coincides with an abnormally cold period in northern Europe sometimes known as the Little Ice Age. Since the formation of the solar system the Sun's output has increased by about 40%.

The Sun is about 4.5 billion years old. Since its birth it has used up about half of the hydrogen in its core. It will continue to radiate "peacefully" for another 5 billion years or so (although its luminosity will approximately double in that time). But eventually it will run out of hydrogen fuel. It will then be forced into radical changes which, though commonplace by stellar standards, will result in the total destruction of the Earth (and probably the creation of a planetary nebula).