Planetary Science Spacecraft

Past Missions
Luna 2
impact on the surface of the Moon 1959 (USSR)
Luna 3
first photos of the far side of the Moon 1959 (USSR)
Mariner 2
the first successful probe to flyby Venus in December of 1962, and it returned information which confirmed that Venus is a very hot (800 degrees Fahrenheit, now revised to 900 degrees F.) world with a cloud-covered atmosphere composed primarily of carbon dioxide.
(more info from NASA Spacelink)
Mariner 3
launched on November 5, 1964, was lost when its protective shroud failed to eject as the craft was placed into interplanetary space. Unable to collect the Sun's energy for power from its solar panels, the probe soon died when its batteries ran out and is now in solar orbit. It was intended for a Mars flyby with Mariner 4.
Mariner 4
the sister probe to Mariner 3, did reach Mars in 1965 and took the first close-up images of the Martian surface (22 in all) as it flew by the planet. The probe found a cratered world with an atmosphere much thinner than previously thought. Many scientists concluded from this preliminary scan that Mars was a "dead" world in both the geological and biological sense.
Mariner 9
Mariner 9, the sister probe to Mariner 8 which failed on launch, became the first craft to orbit Mars in 1971. It returned information on the Red Planet that no other probe had done before, revealing huge volcanoes on the Martian surface, as well as giant canyon systems, and evidence that water once flowed across the planet. The probe also took the first detailed closeup images of Mars' two small moons, Phobos and Deimos.
Apollo
6 manned landings on the Moon and sample returns 1969-72. (The seventh landing, Apollo 18, was canceled for political reasons)
(Apollo "home page"; Apollo Missions)
Luna 16
automated sample return from the Moon 1970 (USSR)
Pioneer 10 and Pioneer 11
Pioneer 10 was the first spacecraft to flyby Jupiter in 1973. Pioneer 11 followed it in 1974, and then went on to become the first probe to study Saturn in 1979. The Pioneers were designed to test the ability of spacecraft to survive passage thru the asteroid belt and Jupiter's magnetosphere. The asteroid belt was easy, but they were nearly fried by ions trapped in Jupiter's magnetic field. This information was crucial to the success of the Voyager missions.
Pioneer 11's RTG power supply is dead. Its last communication with Earth was in November 1995. Pioneer 10 is still functioning (barely) but is no longer being tracked regularly due to budget cutbacks. The last data was received from it on 1997 March 31. They are heading off into interstellar space, the first craft ever to do so.
As the first two spacecraft to leave our solar system, Pioneer 10 & 11 carry a graphic message in the form of a 6- by 9-inch gold anodized plaque bolted to the spacecraft's main frame.
(Pioneer Project Home Page and more about Pioneer 10 and Pioneer 11 from NASA Spacelink )
Mariner 10
used Venus as a gravity assist to Mercury in 1974. The probe did return the first close-up images of the Venusian atmosphere in ultraviolet, revealing previously unseen details in the cloud cover, plus the fact that the entire cloud system circles the planet in four Earth days. Mariner 10 eventually made three flybys of Mercury from 1974 to 1975 before running out of attitude control gas. The probe revealed Mercury as a heavily cratered world with a mass much greater than thought. This would seem to indicate that Mercury has an iron core which makes up 75 percent of the entire planet.
(more from JPL and JPL)
Venera 7
First probe to return data from the surface of another planet (Venus) in 1970.
Venera 9
soft landing on Venus, pictures of the surface 1975. (USSR) This was the first spacecraft to land on the surface of another planet.
Pioneer Venus
1978; orbiter and four atmospheric probes; made the first high-quality map of the surface of Venus.
(more info from NASA Spacelink; and NSSDC a tutorial from UCLA)
Viking 1
Viking 1 was launched from Cape Canaveral, Florida on August 20, 1975 on a TITAN 3E-CENTAUR D1 rocket. The probe went into Martian orbit on June 19, 1976, and the lander set down on the western slopes of Chryse Planitia on July 20, 1976. It soon began its programmed search for Martian micro-organisms (there is still debate as to whether the probes found life there or not), and sent back incredible color panoramas of its surroundings. One thing scientists learned was that Mars' sky was pinkish in color, not dark blue as they originally thought (the sky is pink due to sunlight reflecting off the reddish dust particles in the thin atmosphere). The lander set down among a field of red sand and boulders stretching out as far as its cameras could image.
Viking 2
Viking 2 was launched on September 9, 1975, and arrived in Martian orbit on August 7, 1976. The lander touched down on September 3, 1976 in Utopia Planitia. It accomplished essentially the same tasks as its sister lander, with the exception that its seismometer worked, recording one marsquake.
The last data from Viking (Lander 1) made its final transmission to Earth Nov. 11, 1982. Controllers at JPL tried unsuccessfully for another six and one-half months to regain contact with Viking Lander 1. The overall mission came to an end May 21, 1983.
An interesting side note: Viking 1's lander has been designated the Thomas A. Mutch Memorial Station in honor of the late leader of the lander imaging team. The National Air and Space Museum in Washington, DC is entrusted with the safekeeping of the Mutch Station Plaque until it can be attached to the lander by a manned expedition.
(more info (pdf) and an web page from JPL)
Voyager 1
Voyager 1 (image at top) was launched September 5, 1977, and flew past Jupiter on March 5, 1979 and by Saturn on November 13, 1980. Voyager 2 was launched August 20, 1977 (before Voyager 1), and flew by Jupiter on August 7, 1979, by Saturn on August 26, 1981, by Uranus on January 24, 1986, and by Neptune on August 8, 1989. Voyager 2 took advantage of a rare once-every-189-years alignment to slingshot its way from outer planet to outer planet. Voyager 1 could, in principle, have headed towards Pluto, but JPL opted for the sure thing of a Titan close up.
Between the two probes, our knowledge of the 4 giant planets, their satellites, and their rings has become immense. Voyager 1 & 2 discovered that Jupiter has complicated atmospheric dynamics, lightning and aurorae. Three new satellites were discovered. Two of the major surprises were that Jupiter has rings and that Io has active sulfurous volcanoes, with major effects on the Jovian magnetosphere.
When the two probes reached Saturn, they discovered over 1000 ringlets and 7 satellites, including the predicted shepherd satellites that keep the rings stable. The weather was tame compared with Jupiter: massive jet streams with minimal variance (a 33-year great white spot/band cycle is known). Titan's atmosphere was smoggy. Mimas's appearance was startling: one massive impact crater gave it the Death Star appearance. The big surprise here was the stranger aspects of the rings. Braids, kinks, and spokes were both unexpected and difficult to explain.
Voyager 2
Voyager 2, thanks to heroic engineering and programming efforts, continued the mission to Uranus and Neptune. Uranus itself was highly monochromatic in appearance. One oddity was that its magnetic axis was found to be highly skewed from the already completely skewed rotational axis, giving Uranus a peculiar magnetosphere. Icy channels were found on Ariel, and Miranda was a bizarre patchwork of different terrains. 10 satellites and one more ring were discovered.
In contrast to Uranus, Neptune was found to have rather active weather, including numerous cloud features. The ring arcs turned out to be bright patches on one ring. Two other rings, and 6 other satellites, were discovered. Neptune's magnetic axis was also skewed. Triton had a canteloupe appearance and geysers. (What's liquid at 38K?)
If no unforeseen failures occur, we will be able to maintain communications with both spacecraft until at least the year 2030. Both Voyagers have plenty of hydrazine fuel -- Voyager 1 is expected to have enough propellant until 2040 and Voyager 2 until 2034. The limiting factor is the RTGs (radio-isotope thermal generators). The power output from the RTGs is slowly dropping each year. By 2000, there won't be enough power for the UVS (ultraviolet spectrometer) instrument. By 2010, the power will have dropped low enough such that not all of the fields and particles instruments can be powered on at the same time. A power sharing plan will go into effect then, where some of the F & P instruments are powered on, and others off. The spacecraft can last in this mode for about another 10 years, and after that the power will probably be too low to maintain the spacecraft.
(the Voyager Project Home Page from JPL; another nice "home page" at NSSDC; fact sheets and a web page from JPL )
Vega
International project VENUS-HALLEY, launched in 1984, carried a Venus orbiter and lander and did a fly-by of Comet Halley.
(Vega Mission Home page
Phobos
Two spacecraft were launched by the USSR in 1988. One failed without a trace. A few images were returned before the second one failed, too.
(Phobos Mission Home page
Giotto
Giotto was launched by an Ariane-1 by ESA on July 2 1985, and approached within 540 km +/- 40 km of the nucleus of Comet Halley on March 13, 1986. The spacecraft carried 10 instruments including a multicolor camera, and returned data until shortly before closest approach, when the downlink was temporarily lost. Giotto was severely damaged by high-speed dust encounters during the flyby and was placed into hibernation shortly afterwards.
In April, 1990, Giotto was reactivated. 3 of the instruments proved fully operational, 4 partially damaged but usable, and the remainder, including the camera, were unusable. On July 2, 1990, Giotto made a close encounter with Earth and was retargeted to a successful flyby of comet Grigg-Skjellerup on July 10, 1992.
(more info from NSSDC)
Clementine
a joint mission of the Ballistic Missile Defense Organization (formerly SDIO) and NASA to flight test sensors developed by Lawrence Livermore for BMDO. The spacecraft, built by the Naval Research Lab, was launched on January 25 1994 to a 425 km by 2950 km orbit of the Moon for a 2 month mapping mission. Instruments onboard include UV to mid-IR imagers, including an imaging lidar that may be able to also obtain altimetric data for the middle latitudes of the Moon. In early May the spacecraft was to have been sent out of lunar orbit toward a flyby of the asteroid 1620 Geographos but a failure prevented the attempt.
Ground controllers have regained control of the spacecraft, however. Its potential future mission is being considered.
(for more information see the Clementine Mission Home page from USGS and the Clementine page from NASA PDS or The Clementine Mission from LPI.)
Mars Observer
Mars orbiter including 1.5 m/pixel resolution camera. Launched 9/25/92 on a Titan III/TOS booster. Contact was lost with MO on 8/21/93 while it was preparing for entry into Mars orbit. The spacecraft has been written off (postmortem analysis). Mars Global Surveyor, a replacement mission to achieve most of MO's science goals, has been very successful.
Magellan
Launched in May 1989, Magellan has mapped 98% of the surface of Venus at better than 300 meter resolution and obtained a comprehensive gravity field map for 95 percent of the planet. Magellan recently executed an 80-day aerobraking program to lower and circularize its orbit. Magellan has completed its radar mapping and gravity data collection. In the fall of 1994, just before it would have failed due to deterioration in its solar panels, Magellan was deliberately sent into Venus' atmosphere to further study aerobraking techniques which can make major savings in fuel for future missions.
(more info (pdf), a web page and another web page from JPL; fact sheet from NSSDC)
Galileo
Jupiter orbiter and atmosphere probe. It made extensive surveys of the Jovian moons and the probe descended into Jupiter's atmosphere to provide our first direct evidence of the interior of a gas giant.
In addition, Galileo has returned the first resolved images of two asteroids, 951 Gaspra and 243 Ida, while in transit to Jupiter. It also returned pictures of the impact of Comet SL9 onto Jupiter from its unique vantage point.
Galileo was deliberately crashed in to Jupiter in 2003 to prevent any possibility that it might crash into Europa and contaminate any life that might be there.
(Education and Public Outreach (images!); Galileo Home Page; Galileo Probe Home Page from ARC; web page; NSSDC page; preliminary Galileo Probe Results from JPL and LANL)
Mars 96
a large orbiter with several landers originally known as Mars 94. Launch failed 1996 November 17. (The original Mars 96 was known for a while as Mars 98 and then cancelled.) (more info from MSSS and from IKI (Russia))
Pathfinder
The Mars Pathfinder (formerly known as the Mars Environmental Survey, or MESUR, Pathfinder) is the second of NASA's low-cost planetary Discovery missions. The mission consists of a stationary lander and a surface rover known as Sojourner. The mission has the primary objective of demonstrating the feasibility of low-cost landings on and exploration of the Martian surface. This objective will be met by tests of communications between the rover and lander, and the lander and Earth, and tests of the imaging devices and sensors.
The scientific objectives include atmospheric entry science, long-range and close-up surface imaging, with the general objective being to characterize the Martian environment for further exploration. The spacecraft will enter the Martian atmosphere without going into orbit around the planet and land on Mars with the aid of parachutes, rockets and airbags, taking atmospheric measurements on the way down. Prior to landing, the spacecraft will be enclosed by three triangular solar panels (petals), which will unfold onto the ground after touchdown.
Mars Pathfinder was launched 1996 December 4 and landed successfully on Mars on 1997 July 4.
( MPF Home Page from JPL; more info from NSSDC; images and press releases from MSFC; Mars Watch, Linking Amateur and Professional Mars Observing Communities for Observational Support of the Mars Pathfinder Mission)
NEAR
The Near Earth Asteroid Rendezvous (NEAR) mission promises to answer fundamental questions about the nature of near-Earth objects such as asteroids and comets.
Launched on 1996 February 17 aboard a Delta 2 rocket, the NEAR spacecraft should arrive in orbit around asteroid 433 Eros in early January 1999. It will then survey the rocky body for a minimum of one year, at altitudes as close as 15 miles (24 kilometers). Eros is one of the largest and best-observed asteroids whose orbits cross Earth's path. These asteroids are closely related to the more numerous "Main Belt" asteroids that orbit the Sun in a vast doughnut-shaped ring between Mars and Jupiter.
(NEAR Home Page; more info from NSSDC; more from John Hopkins Univ.; Curriculum materials)
Lunar Prospector
Lunar Prospector, the first NASA mission to the Moon in almost 30 years, was launched Jan 6th, 1998. Within a month it will begin returning answers to long-standing questions about the Moon, its resources, its structure and its origins. (Welcome to the Moon, Lunar Prospector home page); more from NSSDC
Ongoing Missions
Voyager 1 and 2
still operational after more than 15 years in space and are traveling out of the Solar System. The two Voyagers are expected to last until at least the year 2015 when their radioisotope thermoelectric generators (RTG) power supplies are expected for fail. Their trajectories give negative evidence about possible planets beyond Pluto. Their next major scientific discovery should be the location of the heliopause. Low-frequency radio emissions believed to originate at the heliopause have been detected by both Voyagers.
Both Voyagers are using their ultraviolet spectrometers to map the heliosphere and study the incoming interstellar wind. The cosmic ray detectors are seeing the energy spectra of interstellar cosmic rays in the outer heliosphere
Voyager 1 has passed the Pioneer 10 spacecraft and is now the most distant human-made object in space.
(more info from JPL)
Hubble Space Telescope
launched April 1990; fixed December 1993. HST can provide pictures and spectra over a long period of time. This provides an important extra dimension to the higher resolution data from the planetary probes. For example, recent HST data show that Mars is colder and drier than during the Viking missions; and HST images of Neptune indicate that its atmospheric features change rapidly.
Named for the American astronomer Edwin Hubble.
Much, much more information about HST and HST pictures are available at the Space Telescope Science Institute. HST's latest images are posted regularly. (Here is a brief history of the HST project. There's also some more HST info at JPL.)
Ulysses
now investigating the Sun's polar regions (European Space Agency/NASA). Ulysses was launched by the Space Shuttle Discovery in October 1990. In February 1992, it got a gravity boost from Jupiter to take it out of the plane of the ecliptic. It has now completed its main mission of surveying both of the Sun's poles. Its mission has been extended for another orbit so that it can survey the Sun's poles near the maximum of the sunspot cycle, too. Its aphelion is 5.2 AU, and, surprisingly, its perihelion is about 1.5 AU-- that's right, a solar-studies spacecraft that's always further from the Sun than the Earth is! It is expected to provide a much better understanding of the Sun's magnetic field and the solar wind.
(Ulysses Home Pages from JPL and ESA)
Wind
After its November 1, 1994, launch, NASA's Wind satellite will take up a vantage point between the Sun and the Earth, giving scientists a unique opportunity to study the enormous flow of energy and momentum known as the solar wind.
The main scientific goal of the mission is to measure the mass, momentum and energy of the solar wind that somehow is transferred into the space environment around the Earth. Although much has been learned from previous space missions about the general nature of this huge transfer, it is necessary to gather a great deal of detailed information from several strategic regions of space around the Earth before scientists understand the ways in which the planet's atmosphere responds to changes in the solar wind.
The launch also marks the first time a Russian instrument will fly on an American spacecraft. The Konus Gamma-Ray Spectrometer instrument, provided by the Ioffe Institute, Russia, is one of two instruments on Wind which will study cosmic gamma-ray bursts, rather than the solar wind. A French instruments is also aboard.
At first, the satellite will have a figure-eight orbit around the Earth with the assistance of the Moon's gravitational field. Its furthest point from the Earth will be up to 990,000 miles (1,600,000 kilometers), and its closest point will be at least 18,000 miles (29,000 kilometers).
Later in the mission, the Wind spacecraft will be inserted into a special halo orbit in the solar wind upstream from the Earth, at the unique distance which allows Wind to always remain between the Earth and the Sun (about 930,000 to 1,050,000 miles, or 1,500,000 to 1,690,000 kilometers, from the Earth).
Mars Surveyor Program
Launched with a Delta II expendable vehicle from Cape Canaveral, Fla., on November 7 1996, the spacecraft is now in orbit around Mars. The spacecraft circles Mars once every two hours, maintaining a "sun synchronous" orbit that will put the sun at a standard angle above the horizon in each image and allow the mid-afternoon lighting to cast shadows in such a way that surface features will stand out. The spacecraft will carry a portion of the Mars Observer instrument payload and will use these instruments to acquire data of Mars for a full Martian year, the equivalent of about two Earth years. The spacecraft will then be used as a data relay station for signals from U.S. and international landers and low-altitude probes for an additional three years.
(MGS Home Page from JPL; Planned Missions from 1996 to 2003)
Cassini
Saturn orbiter and Titan atmosphere probe. Cassini is a joint NASA/ESA project designed to accomplish an exploration of the Saturnian system with its Cassini Saturn Orbiter and Huygens Titan Probe. Cassini was launched aboard a Titan IV/Centaur 1997 Oct 15.
(Cassini Home Page from JPL; Huygens Home Page; another Cassini page from JPL)
Stardust
Scheduled for launch in February 1999, Stardust will fly close to a comet and, for the first time ever, bring material from the comets coma back to Earth for analysis by scientists worldwide. Scheduled to fly-by Comet Wild-2 in 2004, return to Earth in 2006.
(home page) (All missions not otherwise labeled are NASA)

Seeing the Solar System

You don't need your own Voyager to see the solar system. You can see much of it from your own back yard. Of course, you don't see the fantastic closeup views that NASA gets, but you can see it first-hand with your own eyes. If you enjoyed The Nine Planets, go outside and take a look at what you just read about. You'll be amazed how rewarding such a simple thing it can be.
Hardcopy
Touring the Universe through Binoculars A personal tour of the universe using nothing more than a pair of binoculars.
Turn Left at Orion A guide to the night sky perfect for those with no previous knowledge of astronomy and in any age group. Shows how to explore the sky with a small telescope.
Nightwatch: A Practical Guide to Viewing the Universe A classic handbook combines a text both meaty and hard to put down with graphics and dazzling full-color photos.
To find the planets, you'll need to know where to look. Refer to Sky & Telescope or a similar magazine for up to date positions or check one of the several Web sites that show planetary positions. A planetarium program can also be useful, especially for fast moving objects like moons and comets. A simple chart or planisphere is a nice way to find the bright stars and constellations but isn't much help for planets.
The tables below are ordered by visual magnitude ("Vo"; bigger numbers are dimmer); this is the maximum brightness that the object attains (approximately when it is closest to Earth). "Date" is the date of discovery.
Unaided Eye
You can see 99.99% of the mass of the solar system with no instruments whatsoever:
Name
Vo
Sun
-27
Earth

Moon
-13
Venus
-4.4
Jupiter
-2.7
Mars
-2.0
Mercury
-1.9
Saturn
+0.7
Never look directly at the Sun! Always use a special solar filter designed specifically for solar observing.
Solar Observing FAQ by Jeff Medkeff
Does the Earth really count? Only the Apollo astronauts have ever seen the Earth from far enough away to perceive it as a globe.
Those with good eyes (especially children) and dark skies may be able to see a few of the binocular objects below, too.
Binoculars
A simple pair of binoculars is by far the most cost-effective optical aid available. For $200 you can get a far better optical instrument than Galileo or Newton had. You will find it much easier if you arrange a stable support for your binoculars (such as a tripod):
Name
Date
Vo
Discoverer
Ganymede
1610
4.6
Galileo Galilei
Io
1610
5.0
Galileo Galilei
Europa
1610
5.3
Galileo Galilei
Uranus
1781
5.5
William Herschel
Callisto
1610
5.6
Galileo Galilei
Neptune
1846
7.8
Johann Gottfried Galle
Titan
1655
8.3
Christiaan Huygens
Looking at the Sun with binoculars even for a fraction of a second can burn a hole in your retina. Be very careful, especially when looking for Mercury.
Amateur Telescopes
If you're more serious a modest telescope will reveal many more moons. The first few below are pretty easy, the last few are considerably more difficult. Good dark skies are essential:
Name
Date
Vo
Discoverer
Rhea
1672
9.7
Giovanni Domenico Cassini
Tethys
1684
10.2
Giovanni Domenico Cassini
Iapetus
1671
10.2
Giovanni Domenico Cassini
Dione
1684
10.4
Giovanni Domenico Cassini
Phobos
1877
11.3
Asaph Hall
Enceladus
1789
11.7
William Herschel
Deimos
1877
12.4
Asaph Hall
Mimas
1789
12.9
William Herschel
Triton
1846
13.5
William Lassell
Pluto
1930
13.6
Clyde W. Tombaugh
Titania
1787
13.7
William Herschel
Oberon
1787
13.9
William Herschel
Amalthea
1892
14.1
Edward Emerson Barnard
Ariel
1851
14.2
William Lassell
Hyperion
1848
14.2
William Cranch Bond
Janus
1966
14.5
Audouin Dollfus
Umbriel
1851
14.8
William Lassell
Himalia
1904
14.8
C. Perrine
Phobos and Deimos are harder to see than it might appear since they are so close to Mars (and the above magnitudes are for a favorable opposition)
The same holds for Amalthea and Janus.
Iapetus' brightness varies greatly as it rotates, from 10.2 to 11.9 or less.
The order of discovery may be a better guide to what is easy to see than magnitude.
Mars FAQ for amateur astronomers
With a small telescope you can easily see the phases of Venus and even the phases of Mercury when conditions are right.
Don't buy your first telescope without first reading Information for Beginning Astronomers
Other objects
Of course, the solar system has more than just planets and moons. Every year there are comets that can be seen with small telescopes and usually one or two that can be seen with binoculars. Occasionally there are comets visible to the unaided eye such as Hale-Bopp which was so spectacular in 1997.
It's easy to see a few of the brighter asteroids with binoculars. Several hundred can be seen with small telescopes. And even today, many asteroids and comets are still discovered by amateur astronomers.
If you're out at night under a clear sky, you are likely to see a meteor. You may see dozens of meteors if you catch one of the regular meteor showers.
You can even see the interplanetary medium if you're close enough to the poles to see an aurora or if you see the zodiacal light or the gegenschein.
You can also see the stars 51 Pegasi, 70 Virginis and 47 Ursae Majoris which probably have their own planets, though of course, you can't see the planets themselves.
Photography

Other Planetary Systems?

Are there planets orbiting other stars beyond our solar system? We do not know for sure, but with the recent discoveries about 51 Pegasi, 70 Virginis and 47 Ursae Majoris the weight of evidence is now so strong that only a "devil's advocate" denies the conclusions. Here is some of what we do know (this is somewhat incomplete; please see the references below for more info):
Facts
Three small bodies have been found in orbit around the pulsar PSR 1257+12. They have been designated "PSR1257+12 A, ..B, and ..C". One is about the size of the Moon, the other two are about 2 to 3 times the mass of Earth.
They were discovered by measuring variations in the pulsation speed of the pulsar which can be interpreted as gravitational effects of three small planets. The original observation has been confirmed but, of course, no direct images have been made -- that is way beyond the capabilities of our best telescopes.
These planets are believed to have formed after the supernova that produced the pulsar. The present planets would have originally been within the envelope of the progenitor star and therefore wouldn't have stood much chance of surviving the supernova explosion, and wouldn't have remained in circular orbits after the explosion.
Several decades of timing data on the pulsar PSR 0329+54 (PKS B0329+54) by Tatiana Shabanova (Lebedev Physics Institute) shows evidence of a planet with a 16.9 year period and mass greater than 2 Earth masses.
But, while the evidence for these is pretty good, they aren't really what we're looking for when we talk about 'solar systems'.
It has been known since 1983 that the star Beta Pictoris is surrounded by a disk of gas and dust. Spectra of Beta Pictoris show absorption features which are currently believed to be due to cometary like clouds of gas occultating the star from the debris left over from planetary formation. Though it's far from certain it is believed by some that planets may already have formed around Beta Pictoris.
HST has observed Beta Pictoris (right) and found the disk to be significantly thinner than previously thought. Estimates based on the Hubble image place the disk's thickness as no more than one billion miles (1600 million kilometers), or about 1/4 previous estimates from ground-based observations. The disk is tilted nearly edge-on to Earth. Because the dust has had enough time to settle into a flat plane, the disk may be older than some previous estimates. A thin disk also increases the probability that comet-sized or larger bodies have formed through accretion in the disk. Both conditions are believed to be characteristic of a hypothesized circumstellar disk around our own Sun, which was a necessary precursor to the planet-building phase of our Solar Systems, according to current theory.
More recent HST observations have shown the disk to be slightly warped as might be expected from the gravitational influence of a planet. This has been confirmed by observations at ESO.
Recent observations at radio wavelengths of a gas cloud known as Bok Globule B335 have produced images of material collapsing onto a newly born star (only about 150,000 years old). These observations are helping to understand how stars and planets form. The phenomena observed matches the theory of the formation of the solar system -- that is, a large gas cloud collapsed to form a star with an attendant circumstellar disk in which, over time, planets accreted from the matter in the disk and orbited the Sun.
The IRAS satellite found that Vega had too much infrared emission, and that has been attributed to a dust shell (with a mass of maybe Earth's moon).
Observations of the very nearby Barnard's Star were once thought to be evidence of gravitational effects of planets but they now seem to have been in error.
The star Gl229 seems to contain a 20 Jupiter mass object orbiting at a distance of 44 AU. An object this large is probably a brown-dwarf rather than an ordinary planet.
What may be the first discovery of a planet orbiting a normal, Sun-like star other than our own has been announced by astronomers studying 51 Pegasi, a spectral type G2-3 V main-sequence star 42 light-years from Earth. At a recent conference in Florence, Italy, Michel Mayor and Didier Queloz of Geneva Observatory explained that they observed 51 Pegasi with a high-resolution spectrograph and found that the star's line-of-sight velocity changes by some 70 meters per second every 4.2 days. If this is due to orbital motion, these numbers suggest that a planet lies only 7 million kilometers from 51 Pegasi -- much closer than Mercury is to the Sun -- and that the planet has a mass at least half that of Jupiter. These physical characteristics hinge on the assumption that our line of sight is near the planet's orbital plane. However, other evidence suggests that this is a good bet. A world merely 7 million km from a star like 51 Pegasi should have a temperature of about 1,000 degrees Celsius, just short of red hot. It was initially thought that it might be a solid body like a very big Mercury but the concensus now seems to be that it is a "hot Jupiter", a gas planet formed much farther from its star that migrated inward.
These observations have now been confirmed by several independent observers. And there is some evidence for a second planet much farther out that is not yet confirmed.
[ The 5.5-magnitude 51 Pegasi is easily visible in binoculars between Alpha and Beta Pegasi, the western pair of stars in the Great Square of Pegasus. The star's equinox-2000 coordinates are R.A. 22 hours 57 minutes, Dec. +20 degrees 46 minutes. ]
On 1/17/96 Geoffrey Marcy andPaul Butler announced the discovery of planets orbiting the stars 70 Virginis and 47 Ursae Majoris. 70 Vir is a G5V (main sequence) star about 78 light-years from Earth; 47 UMa is a G0V star about 44 light-years away. These were discovered using the same doppler shift technique that found the planet orbiting 51 Pegasi.
The planet around 70 Vir orbits the star in an eccentric, elongated orbit every 116 days and has a mass about nine times that of Jupiter. Using standard formulas that balance the sunlight absorbed and the heat radiated, Marcy and Butler calculated the temperature of the planet at about 85 degrees Celsius (185 degrees Fahrenheit), cool enough to permit water and complex organic molecules to exist. The star 70 Vir is nearly identical to the Sun, though several hundred degrees cooler and perhaps three billion years older.
The planet around 47 UMa was discovered after analysis of eight years of observations at Lick Observatory. Its period is a little over three years (1100 days), its mass about three times that of Jupiter, and its orbital radius about twice the Earth's distance from the Sun. This planet too probably has a region in its atmosphere where the temperature would allow liquid water.
As of April 1996, Drs. Marcy and Butler have discovered yet another planet this time around the star HR3522 (aka Rho 1 Cancri, 55 Cancri) about 45 light years from the Earth. The planet is estimated to be about 0.8 Jupiter masses. It is likely that several more planets will show up in the initial set of 120 stars that they have monitored.
Several more extra-solar planets have now been discovered by the Butler/Marcy method. It seems likely that there are a very large number of such planets out there.
Another extra-solar planet has been discovered orbiting 16 Cygni B. But unlike all other previously known planets this one has a very large orbital eccentricity (0.6); its orbit carries it from a closest distance of 0.6 AU from its star to 2.7 AU. This calls into question many theories of planetary formation.
Detecting extra-solar planets directly is very difficult. Even the Hubble Space Telescope wouldn't be able to image planets at the expected sizes and distances from their suns.
What HST did find were disks of matter around stars seen in silhouette against the Orion Nebula (called 'proplyds', for 'proto-planetary disks' (right). This is great evidence for how common these objects are, but the scale is way too small to say anything directly about planets there. More detailed HST images are now available, too.
Nevertheless, it might be possible to detect the infra-red radiation of very large planets (Jupiter-sized or more) in some circumstances.
By a stroke of good luck, HST has taken an image of what appears to be a planet escaping from a double star system. See the 1998 May 28 announcement. If this is confirmed, the existence of extrasolar planets will be undeniable.

Small Solar-System Bodies

The title The Nine Planets is somewhat misleading. In addition to the (eight) planets and their satellites the solar system contains a large number of smaller but interesting objects.
There are thousands of known asteroids and comets and undoubtedly many more unknown ones. Most asteroids orbit between Mars and Jupiter. A few (e.g. 2060 Chiron) are farther out. There are also some asteroids whose orbits carry them closer to the Sun than the Earth (Aten, Icarus, Hephaistos). Most comets have highly elliptical orbits which spend most of their time in the outer reaches of the solar system with only brief passages close to the Sun. And there is a large and important class of Trans-Neptunian Objects or Kuiper Belt Objects (including Pluto) that orbit (mostly) beyond Neptune.
The distinction between comets and asteroids is somewhat controversial. The main distinction seems to be that comets have more volatiles and more elliptical orbits. But there are interesting ambiguous cases such as 2060 Chiron (aka 95 P/Chiron) and 3200 Phaethon which seem to share some aspects of both categories.
Asteroids are sometimes also referred to as minor planets or planetoids (not to be confused with "lesser planets" which refers to Mercury and Pluto). Some of the largest asteroids and Kuiper Belt objects may be classified as dwarf planets. Very small rocks orbiting the Sun are sometimes called meteoroids to distinguish them from the larger asteroids. When such a body enters the Earth's atmosphere it is heated to incandescence and the visible streak in the sky is known as a meteor. If a piece of it survives to reach the Earth's surface it is known as a meteorite.
Millions of meteors bright enough to see strike the Earth every day (amounting to hundreds of tons of material). All but a tiny fraction burn up in the atmosphere before reaching the ground. The few that don't are our major source of physical information about the rest of the solar system.
Finally, the space between the planets is not empty at all. It contains a great deal of microscopic dust and gas as well as radiation and magnetic fields.

Charon

Charon ( "KAIR en" ) is Pluto's largest satellite: orbit: 19,640 km from Pluto
diameter: 1212 km
mass: 1.90e21 kg
Charon is named for the mythological figure who ferried the dead across the River Acheron into Hades (the underworld).
(Though officially named for the mythological figure, Charon's discoverer was also naming it in honor of his wife, Charlene. Thus, those in the know pronounce it with the first syllable sounding like 'shard' ("SHAHR en").
Charon was discovered in 1978 by Jim Christy. Prior to that it was thought that Pluto was much larger since the images of Charon and Pluto were blurred together.
Charon is unusual in that it is the largest moon with respect to its primary planet in the Solar System (a distinction once held by Earth's Moon). Some prefer to think of Pluto/Charon as a double planet rather than a planet and a moon.
Charon's radius is not well known. JPL's value of 586 has an error margin of +/-13, more than two percent. Its mass and density are also poorly known.
Pluto and Charon are also unique in that not only does Charon rotate synchronously but Pluto does, too: they both keep the same face toward one another. (This makes the phases of Charon as seen from Pluto very interesting.)
Charon's composition is unknown, but its low density (about 2 gm/cm3) indicates that it may be similar to Saturn's icy moons (i.e. Rhea). Its surface seems to be covered with water ice. Interestingly, this is quite different from Pluto.
Unlike Pluto, Charon does not have large albedo features, though it may have smaller ones that have not been resolved.
It has been proposed that Charon was formed by a giant impact similar to the one that formed Earth's Moon.
It is doubtful that Charon has a significant atmosphere.

Pluto

Pluto orbits beyond the orbit of Neptune (usually). It is much smaller than any of the official planets and now classified as a "dwarf planet". Pluto is smaller than seven of the solar system's moons (the Moon, Io, Europa, Ganymede, Callisto, Titan and Triton). orbit: 5,913,520,000 km (39.5 AU) from the Sun (average)
diameter: 2274 km
mass: 1.27e22 kg
In Roman mythology, Pluto (Greek: Hades) is the god of the underworld. The planet received this name (after many other suggestions) perhaps because it's so far from the Sun that it is in perpetual darkness and perhaps because "PL" are the initials of Percival Lowell.
Pluto was discovered in 1930 by a fortunate accident. Calculations which later turned out to be in error had predicted a planet beyond Neptune, based on the motions of Uranus and Neptune. Not knowing of the error, Clyde W. Tombaugh at Lowell Observatory in Arizona did a very careful sky survey which turned up Pluto anyway.
After the discovery of Pluto, it was quickly determined that Pluto was too small to account for the discrepancies in the orbits of the other planets. The search for Planet X continued but nothing was found. Nor is it likely that it ever will be: the discrepancies vanish if the mass of Neptune determined from the Voyager 2 encounter with Neptune is used. There is no Planet X. But that doesn't mean there aren't other objects out there, only that there isn't a relatively large and close one like Planet X was assumed to be. In fact, we now know that there are a very large number of small objects in the Kuiper Belt beyond the orbit of Neptune, some roughly the same size as Pluto.
Pluto has not yet been visited by a spacecraft. Even the Hubble Space Telescope can resolve only the largest features on its surface (left and above). A spacecraft called New Horizons was launched in January 2006. If all goes well it should reach Pluto in 2015.
Fortunately, Pluto has a satellite, Charon. By good fortune, Charon was discovered (in 1978) just before its orbital plane moved edge-on toward the inner solar system. It was therefore possible to observe many transits of Pluto over Charon and vice versa. By carefully calculating which portions of which body would be covered at what times, and watching brightness curves, astronomers were able to construct a rough map of light and dark areas on both bodies.
In late 2005, a team using the Hubble Space Telescope discovered two additional tiny moons orbiting Pluto. Provisionally designated S/2005 P1 and S/2005 P2, they are now known as Nix and Hydra. They are estimated to be between 60 and 200 kilometers in diameter.
Pluto's radius is not well known. JPL's value of 1137 is given with an error of +/-8, almost one percent.
Though the sum of the masses of Pluto and Charon is known pretty well (it can be determined from careful measurements of the period and radius of Charon's orbit and basic physics) the individual masses of Pluto and Charon are difficult to determine because that requires determining their mutual motions around the center of mass of the system which requires much finer measurements -- they're so small and far away that even HST has difficulty. The ratio of their masses is probably somewhere between 0.084 and 0.157; more observations are underway but we won't get really accurate data until a spacecraft is sent.
Pluto is the second most contrasty body in the Solar System (after Iapetus).
There has recently been considerable controversy about the classification of Pluto. It was classified as the ninth planet shortly after its discovery and remained so for 75 years. But on 2006 Aug 24 the IAU decided on a new definition of "planet" which does not include Pluto. Pluto is now classified as a "dwarf planet", a class distict from "planet". While this may be controversial at first (and certainly causes confusion for the name of this website) it is my hope that this ends the essentially empty debate about Pluto's status so that we can get on with the real science of figuring out its physical nature and history.
Pluto has been assigned number 134340 in the minor planet catalog.
Pluto's orbit is highly eccentric. At times it is closer to the Sun than Neptune (as it was from January 1979 thru February 11 1999). Pluto rotates in the opposite direction from most of the other planets.
Pluto is locked in a 3:2 resonance with Neptune; i.e. Pluto's orbital period is exactly 1.5 times longer than Neptune's. Its orbital inclination is also much higher than the other planets'. Thus though it appears that Pluto's orbit crosses Neptune's, it really doesn't and they will never collide. (Here is a more detailed explanation.)
Like Uranus, the plane of Pluto's equator is at almost right angles to the plane of its orbit.
The surface temperature on Pluto varies between about -235 and -210 C (38 to 63 K). The "warmer" regions roughly correspond to the regions that appear darker in optical wavelengths.
Pluto's composition is unknown, but its density (about 2 gm/cm3) indicates that it is probably a mixture of 70% rock and 30% water ice much like Triton. The bright areas of the surface seem to be covered with ices of nitrogen with smaller amounts of (solid) methane, ethane and carbon monoxide. The composition of the darker areas of Pluto's surface is unknown but may be due to primordial organic material or photochemical reactions driven by cosmic rays.
Little is known about Pluto's atmosphere, but it probably consists primarily of nitrogen with some carbon monoxide and methane. It is extremely tenuous, the surface pressure being only a few microbars. Pluto's atmosphere may exist as a gas only when Pluto is near its perihelion; for the majority of Pluto's long year, the atmospheric gases are frozen into ice. Near perihelion, it is likely that some of the atmosphere escapes to space perhaps even interacting with Charon. NASA mission planners want to arrive at Pluto while the atmosphere is still unfrozen.
The unusual nature of the orbits of Pluto and of Triton and the similarity of bulk properties between Pluto and Triton suggest some historical connection between them. It was once thought that Pluto may have once been a satellite of Neptune's, but this now seems unlikely. A more popular idea is that Triton, like Pluto, once moved in an independent orbit around the Sun and was later captured by Neptune. Perhaps Triton, Pluto and Charon are the only remaining members of a large class of similar objects the rest of which were ejected into the Oort cloud. Like the Earth's Moon, Charon may be the result of a collision between Pluto and another body.
Pluto can be seen with an amateur telescope but it is not easy. There are several Web sites that show the current position of Pluto (and the other planets) in the sky, but much more detailed charts and careful observations over several days will be required to reliably find it. Suitable charts can be created with many planetarium programs.

Neptune

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
Hardcopy
The New Solar SystemSummarizes what we have learned from interplanetary explorations in the last 25 years. My primary reference for The Nine Planets.
Encyclopedia of the Solar SystemA more scholarly introduction the planetary science for those who want to dig a little deeper.
The Compact NASA Atlas of the Solar SystemThis road map of the solar system contains lots of maps and data as well as photos.
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 which have yet to be named. 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
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)

Uranus

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
Hardcopy
The New Solar System Summarizes what we've learned from interplanetary explorations in the last 25 years. My primary reference for The Nine Planets.
The Compact NASA Atlas of the Solar System This 'road map' of the solar system is the definitive guide for planetary science.
The Tempest by William Shakespeare
The Rape of the Lock by Alexander Pope
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 11 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' SatellitesUranus has 21 named moons and six unnamed ones:
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
2003U2 75 6 ? Showalter 2003
Belinda 75 34 ? Voyager 2 1986
1986U10 76 40 ? Voyager 2 1986
Puck 86 77 ? Voyager 2 1985
2003U1 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
2001U3 4281 6 ? Sheppard 2003
Caliban 7169 40 ? Gladman 1997
Stephano 7948 15 ? Gladman 1999
Trinculo 8578 5
Sycorax 12213 80 ? Nicholson 1997
2003U3 14689 6 ? Sheppard 2003
Prospero 16568 20 ? Holman 1999
Setebos 17681 20 ? Kavelaars 1999
2002U2 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)

Jupiter

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
Hardcopy
The New Solar System Summarizes what we've learned from interplanetary explorations in the last 25 years. My primary reference for The Nine Planets.
The Moons of Jupiter 106 images of the Galilean moons from Galileo with unusual descriptive text.
Symphony No. 41 in C Major K. 551 ("Jupiter") by W. A. Mozart; I am not sure what it has to do with the planet but it is one of Mozart's best. And that is saying a lot!
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 1914Values 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)

Mars

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
Hardcopy
The Case for Marsby Robert Zubrin. A realistic proposal for sending men to Mars cheaply. The ideas in this book are being taken seriously by NASA. There is hope!
Encyclopedia of the Solar SystemA more scholarly introduction the planetary science for those who want to dig a little deeper.
A Travelers Guide to MarsAccessible, heavily illustrated, presenting much of the astonishing information recently learned about Mars, written in a engaging, lively style.
Mars UndergroundFiction by a scientist who knows both how to tell a good story and the territory it is set in.
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 a year on Mars. Three Mars orbiters (Mars Global Surveyor, 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.
Mars' SatellitesMars has two tiny satellites which orbit very close to the martian surface: Distance Radius Mass
Satellite (000 km) (km) (kg) Discoverer Date
--------- -------- ------ ------- ---------- ----
Phobos 9 11 1.08e16 Hall 1877
Deimos 23 6 1.80e15 Hall 1877
("Distance" is measured from the center of Mars).

The Moon

The Moon is the only natural satellite of Earth: orbit: 384,400 km from Earth
diameter: 3476 km
mass: 7.35e22 kg
Hardcopy
The New Solar System Summarizes what we've learned from interplanetary explorations in the last 25 years. My primary reference for The Nine Planets.
Full Moon Very high quality reproductions of Apollo images of the Moon. If you think the Moon is boring ("been there, done that") then you haven't seen this book!
The Once and Future Moon An overview of what we know about our Moon, how we came to know it and how we might go back to learn more.
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. A final determination will probably come from NASA's Lunar Reconnaissance Orbiter, scheduled for 2008.
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.