In addition to the eight known planets, countless smaller bodies orbit the Sun. These objects range from dust grains and small coherent rocks with insignificant gravity to dwarf planets that have sufficient gravity to make them quite spherical in shape. Most are very faint, but some, the comets, release gas and dust when they approach the Sun and can be quite spectacular in appearance (Fig. 10.1); comets are discussed in Chapter 10. In this chapter, we describe the orbital and physical properties of the great variety of non-cometary small bodies ranging in radius from a few meters to over 1000 km that orbit the Sun. We refer to these bodies collectively as minor planets.

Minor planets occupy a wide variety of orbital niches (see Fig. 1.2). Most travel in the relatively stable regions between the orbits of Mars and Jupiter (known as the asteroid belt), exterior to Neptune's orbit (the Kuiper belt), or near the triangular Lagrangian points of Jupiter (the Trojan asteroids). The Kuiper belt is by far the most massive of these reservoirs, and contains the largest objects. However, the largest members of the asteroid belt appear much brighter than any Kuiper belt objects (KBOs) by virtue of their proximity to both the Earth and the Sun (see eq. 10.3 with ζ = 2).

Smaller numbers of minor planets are found in unstable regions. Most of these cross or closely approach the orbits of one or more of the eight planets, which control their dynamics. Those that come near our home planet are known as near-Earth asteroids (NEAs); those orbiting among the giant planets are called centaurs.