In the vast space among the nine known major planets move countless chunks of rock and ice, all orbiting the Sun, many on highly eccentric paths. This final component of the solar system is the collection of interplanetary matter—cosmic "debris" ranging in size from the relatively large asteroids, through the smaller comets and even smaller meteoroids, down to the smallest grains of interplanetary dust that litter our cosmic environment.
The dust arises when larger bodies collide and break apart into smaller pieces that, in turn, collide again and are slowly ground into microscopic fragments, which eventually settle into the Sun or are swept away by the solar wind, a stream of energetic charged particles that continually flows outward from the Sun and pervades the entire solar system. The dust is quite difficult to detect in visible light, but infrared studies reveal that interplanetary space contains surprisingly large amounts of it. Our solar system is an extremely good vacuum by terrestrial standards, but positively dirty by the standards of interstellar or intergalactic space.
Asteroids (Figure 6.8a) and meteoroids are generally rocky in composition, somewhat like the outer layers of the terrestrial planets. The distinction between the two is simply a matter of size—anything larger than 100 m in diameter (corresponding to a mass of about 10,000 tons) is conventionally termed an asteroid, anything smaller is a meteoroid. Their total mass is much less than that of Earth's Moon, so these objects play no important role in the present-day workings of the planets or their moons. Yet they are of crucial importance to our studies, for they are the keys to answering some very fundamental questions about our planetary environment and what the solar system was like soon after its birth. Many of these bodies are made of material that has evolved hardly at all since the early days of the solar system. (In addition, they often conveniently deliver themselves right to our doorstep, in the form of meteorites, allowing us to study them in detail without having to fetch them from space.)
Figure 6.8 (a) Asteroids, like meteoroids, are generally composed of rocky material. This asteroid, Gaspra, is about 20 km long; it was photographed by the Galileo spacecraft on its way to Jupiter. (b) Comet Hale-Bopp, seen as it approached the Sun in 1997. Most comets are composed largely of ice, and so tend to be relatively fragile. Its vaporized gas and dust form the tail, here extending away from the Sun for nearly a quarter of the way across the sky. (See also the chapter-opening photo for Chapter 14.)
Comets are quite distinct from the other small bodies in the solar system. They are generally icy rather than rocky in composition (although they do contain some rocky material) and typically have diameters in the 1–10 km range. They are quite similar in chemical makeup to some of the icy moons of the outer planets. Even more so than the asteroids and meteoroids, comets represent truly ancient material—the vast majority have probably not changed in any significant way since their formation long ago along with the rest of the solar system. Comets striking Earth's atmosphere do not reach the surface intact, so we do not have actual samples of cometary material. However, they do vaporize and emit radiation as their highly elongated orbits take them near the Sun (see Figure 6.8b). Astronomers can determine a comet's makeup by spectroscopic study of the radiation it gives off as it is destroyed. 
(Sec. 4.2)
