Introduction
In this chapter, you will be taking a closer look at the minor bodies of the Solar System. Although most asteroids and comets may seem tiny compared to the planets, they have an important role to play in shaping the appearance of planetary surfaces. You considered this when looking at impact cratering processes in Chapter 4. Furthermore, the study of fragments of asteroids that land on the Earth as meteorites can give us crucial information on the elemental abundances of the material that formed the solar nebula from which the planets were made. This was discussed in Chapter 2, and will be considered again in Chapter 9. Similarly, the study of comets (remotely using telescopes, via the dust particles that they release, and by space probe encounters) gives us information about the processes involved in the formation of the Solar System. For these reasons, the minor bodies of the Solar System can be of major importance.
Before we look at the minor bodies themselves, we need to consider the orbits of bodies in the Solar System in more detail than we have so far. Understanding orbits is key to understanding the motion of the planets, their moons, tidal heating process, and even the structure of the ring systems around the giant planets. Subject to minor variations driven by mutual interactions, planetary orbits are stable over hundreds of millions of years. However the gravitational influence of the planets can cause the orbits of minor bodies to change significantly, enabling them to migrate from one region of the Solar System to another, or even to be put on a collision course with the Earth.
Orbits and Kepler's laws
The German astronomer Johannes Kepler (1571-1630) worked at a time when it was generally believed that the orbits of celestial bodies must be based on circles. Complex schemes using smaller circles superimposed on larger ones were devised to try to account for the apparent paths of the planets across the heavens. However, Kepler realized that the motion of Mars could be described simply by an ellipse. From this starting point, Kepler went on to formulate three laws of planetary motion, which remain fundamental to understanding the functioning of the Solar System. They apply not only to the movements of planets around the Sun, but to all bodies orbiting under the influence of gravity.