Other than the moons of the various planets, the chief small bodies of the solar system are comets and asteroids.

In general, a comet is a kilometer-size chunk of ice and associated dust and debris. The *Oort cloud is an apparent spherical shell of comets 10,000 to 100,000 *astronomical units (AU) from the Sun and the proposed source of comets that orbit the Sun. The cloud is at the extreme edge of the Sun's influence, halfway to the nearest star, and it is believed that when the cloud is perturbed by passing stars, comets may be sent into a solar orbit. The size and structure of the Oort cloud have been deduced from statistical studies of the orbits of comets; there is no direct evidence for the cloud's existence. Approximately 900 comets are known.

Asteroids (also called "minor planets") are small rocky objects, most of which orbit the Sun in a belt between the orbits of Mars and Jupiter. A few asteroids follow orbits that bring them into the inner Solar System, and several asteroids occasionally pass within a few tens of millions of miles of Earth. Some asteroids are located in the orbit of Jupiter, and some asteroids have been detected as far away as the orbit of Saturn. There are approximately 7200 known asteroids, and a million asteroids are believed resident in the Solar System. The consensus view is that asteroids are composed of material that failed to build a planet at a distance of 2.8 astronomical units from the Sun, perhaps due to the influence of massive Jupiter just outside the asteroid belt. Until recently, the shapes and surface features of asteroids were a matter of conjecture; during the past decade, however, significant direct observations of asteroids have been relayed back to Earth from spacecraft.

Classical astronomers have categorized comets and asteroids as distinctly different entities with different histories and compositions, but recent evidence is blurring the conceptual boundary between these two groups of small Solar System bodies, and there are several newly discovered objects that are considered to be both comets and asteroids on the basis of their characteristics. Don Yeomans (California Institute of Technology, US) presents a review of recent research on comets and asteroids, the author making the following points:

1) Recent observations have revealed comets in asteroid-like orbits and asteroids in comet-like orbits. Both comets and asteroids can evolve from the Oort cloud into highly inclined, even *retrograde, orbits about the Sun, so orbital behavior is no better than physical behavior for distinguishing comets from asteroids. The author suggests that attempts to categorize comets and asteroids as distinctly separate entities have failed, and that astronomers should now consider these objects as members of highly diverse family: the small bodies of the Solar System.

2) If all comets were solid dirty balls of water ice, then their bulk densities would be approximately 1 gram per cubic centimeter. But some comets have apparent low-density structures that are made from several bits held together by little more than their own self-gravity. This conclusion arose after some comets were observed to break up as a result of tidal forces from either the Sun or Jupiter, and more than two dozen other comets have split apart for no obvious reason at all. In addition, comets that have apparently transformed from active to quiescent objects suggest that some cometary bodies do become defunct and join the ranks of the asteroids. Low-density extinct comets can probably explain a significant fraction of the near-Earth asteroid population, "so we cannot assume that all objects that threaten Earth will have the same composition or structure."

3) Asteroids have been classified according to the light reflected from their surfaces -- their optical spectra. Although no two spectra are exactly alike, most asteroids fall into one of two groups, the C-type (carbonaceous) and S-type (silicaceous). C-type asteroids have low reflectance (albedo) and may contain mixtures of hydrated silicates, carbon, and organic compounds. S-type asteroids have higher albedos and can contain pyroxene (silicates containing magnesium, iron, and calcium), olivine (magnesium and iron silicates), and nickel-iron metal. The C-type asteroids are most common in the outer part of the main asteroid belt, and the S-type asteroids are mostly found in the inner asteroid belt.

4) Meteorites are asteroid collision fragments that have fallen to Earth, and as such are thought to hold clues regarding the early history of asteroids. Because most asteroid fragments are rocky, they can survive the passage through the atmosphere of the Earth. In contrast, debris from comet streams nearly always burns up in the atmosphere, sometimes producing spectacular meteor showers in the sky, but leaving little evidence on the surface of the Earth. The most common meteorite is the ordinary chondrite, which is composed mostly of rocky silicates, and so has not experienced the chemical differentiation associated with melting. Such chondrites are thought to be some of the most primitive rocks in the Solar System, although their parent asteroid type is not clear. On 22 March 1998, and ordinary chondrite was observed to fall to Earth by 7 boys in Monahans, Texas (US), and within 48 hours the meteorite was under examination at the Johnson Space Center in Houston, Texas. Laboratory analysis of the Monahans meteorite detected salt crystals embedded with water in the form of brine, and the salt crystals were dated to the very beginning of the Solar System, approximately 4.6 billion years ago. This suggests the presence of liquid water on the parent asteroid of this meteorite, and unless this water derived from a collision with a salt-bearing icy comet, the parent asteroid itself must have had flowing water within its interior structure. Far from being the dry rocky bodies they were once believed to be, it would seem that some asteroids, along with comets, might be significant sources of water.