Unless otherwise noted, images were obtained from various NASA web sites.

Asteroids are variously-shaped masses of rock, metal, and ice. Approximately 10,000 of these objects have been discovered to date. Most orbit between Mars and Jupiter, forming the Asteroid Belt. This debris zone is thought to represent the remains of a proto-planet that failed to form as a result of constant gravitational disruption by Jupiter. The image above shows asteroid Ida, which is unusual in that it has a very small asteroid as a satellite (far left).

Other asteroids have highly elliptical orbits and somewhat over 150 of these have orbits that intersect the orbit of the Earth. These "near-Earth asteroids" pose a definite collision hazard with the planet and asteroids of this type are thought to be responsible for a number of mass extinctions in the history of life.

Jupiter is, by far, the largest planet in the solar system (diameter ~ 89,000 miles) and has an escape velocity of over 133,000 mph. Temperature at the cloud-tops in the upper atmosphere average -244^oF. Major atmospheric components include hydrogen (86%), helium (14%, and methane (0.2%).

While Jupiter is not a promising environment for life, it has a total of 17 known satellites, four of which are comparable in size to some of the inner planets of the solar system. One of these, Europa, (left) is completely covered by an ocean, frozen to a depth of approximately 60 miles. Below that level it is thought that liquid water exists, heated by internal tidal stresses within Europa, as a result of the gravitational attraction of Jupiter.

With a diameter of approximately 75,000 miles, Saturn is the second largest planet in the solar system. It has an escape velocity of slightly more than 79,000 mph and cloud-top temperatures average -300^oF. Its atmosphere is dominated by hydrogen (97%), followed in importance by helium (3%) and methane (0.2%).

Saturn has 18 known moons, with the smaller satellites serving to stabilize the magnificent ring system (diameter 170,000 miles but with a thickness of less than 500 feet!). One of the moons, Titan, is larger than Mercury and has a significant methane atmosphere.

Uranus is quite large, compared with the Earth, with a diameter of approximately 32,000 miles and an escape velocity of ~47,500 mph. The cold atmosphere (-350^oF) is dominated by hydrogen (79%), helium (18%), and methane (3%). Uranus is quite odd in that its axis of rotation (and the orbits of its satellites and faint ring system) is in the plane of its orbit, meaning that the planet rotates "on its side". There are also significant brightness variations in the two hemispheres that are not understood.

Neptune, orbiting at the outer fringes of the solar system is a cold world (-370^oF) with the "typical" hydrogen (79%), helium (18%), and methane (3%) atmosphere of the "gas giants". Neptune has a diameter of approximately 31,000 miles and an escape velocity of 52,600 mph.

Neptune has eight known moons, two of which are of substantial size. Triton, the larger of the two moons, may have been captured by Neptune early in its history. Triton is covered by frozen water, carbon dioxide, and carbon monoxide and has both methane and nitrogen in its atmosphere.

Pluto is the smallest planet in the solar system (diameter of ~1500 miles) with an escape velocity of just 2500 mph. It is the only solar planet that has not been imaged by spacecraft and the best views we have are provided by the Hubble Space Telescope. It is extremely cold (-390^oF). It may have a slight methane/nitrogen atmosphere with frozen methane on the surface. Some astronomers now argue that Pluto should be reclassified as an asteroid - a dispute that can be expected to go on for some time. Based on its size (smaller than seven of the larger satellites within the solar system) and its highly elliptical/out-of-plane orbit, this reclassification proposal is certainly rational.