Solar System: Other Orbiting Bodies

The asteroids are small rocky bodies that move in orbits primarily between the orbits of Mars and Jupiter. Numbering in the thousands, asteroids range in size from Ceres, which has a diameter of 1,003 km (623 mi), to microscopic grains. Some asteroids are perturbed, or pulled by forces other than their attraction to the Sun, into eccentric orbits that can bring them closer to the Sun. If the orbits of such bodies intersect that of Earth, they are called meteoroids. When they appear in the night sky as streaks of light, they are known as meteors, and recovered fragments are termed meteorites. Laboratory studies of meteorites have revealed much information about primitive conditions in our solar system. The surfaces of Mercury, Mars, and several satellites of the planets (including Earth’s Moon) show the effects of an intense bombardment by asteroidal objects early in the history of the solar system. On Earth that record has eroded away, except for a few recently found impact craters.

Some meteors and interplanetary dust may also come from comets, which are basically aggregates of dust and frozen gases typically 5 to 10 km (about 3 to 6 mi) in diameter. Comets orbit the Sun at distances so great that they can be perturbed by stars into orbits that bring them into the inner solar system. As comets approach the Sun, they release their dust and gases to form a spectacular coma and tail. Under the influence of Jupiter’s strong gravitational field, comets can sometimes adopt much smaller orbits. The most famous of these is Halley’s Comet, which returns to the inner solar system at 75-year periods. Its most recent return was in 1986. In July 1994 fragments of Comet Shoemaker-Levy 9 bombarded Jupiter’s dense atmosphere at speeds of about 210,000 km/h (130,000 mph). Upon impact, the tremendous kinetic energy of the fragments was released through massive explosions, some resulting in fireballs larger than Earth.

Comets circle the Sun in two main groups, within the Kuiper Belt or within the Oort cloud. The Kuiper Belt is a ring of debris that orbits the Sun beyond the planet Neptune. Many of the comets with periods of less than 500 years come from the Kuiper Belt. In 2002 astronomers discovered a planetoid in the Kuiper Belt, and they named it Quaoar.

Many of the objects that do not fall into the asteroid belts, the Kuiper Belt, or the Oort cloud may be comets that will never make it back to the Sun. The surfaces of the icy satellites of the outer planets are scarred by impacts from such bodies. The asteroid-like object Chiron, with an orbit between Saturn and Uranus, may itself be an extremely large inactive comet. Similarly, some of the asteroids that cross the path of Earth’s orbit may be the rocky remains of burned-out comets. Chiron and similar objects called the Centaurs probably escaped from the Kuiper Belt and were drawn into their irregular orbits by the gravitational pull of the giant outer planets, Jupiter, Saturn, Neptune, and Uranus.

The Sun was also found to be encircled by rings of interplanetary dust. One of them, between Jupiter and Mars, has long been known as the cause of zodiacal light, a faint glow that appears in the east before dawn and in the west after dusk. Another ring, lying only two solar widths away from the Sun, was discovered in 1983.