Asteroids and comets are small fragments of material that did not become major planets, left over from the formation of the solar system. When, rarely, comets or asteroids collide with the Earth, they can influence the course of evolution. Most of them, like the planets, revolve around the Sun.

Asteroids are bodies of metal and rock, without atmospheres. If all the asteroids in the solar system accreted into one planet, it would be less than half the size of the Moon. There was brief controversy over whether Pluto was a mere asteroid rather than a planet. Since Pluto is about as large as all the asteroids combined, most astronomers still consider it to be a planet. The largest known asteroid is Ceres, about 600 miles (1,000 km) in diameter, while the smallest asteroids are pebbles. Sixteen asteroids exceed 150 miles (240 km) in diameter.

Comets consist mainly of wet and dry ice (H2O and CO2) but also contain many organic chemicals that are similar to those involved in the origin of life. Many comets revolve around the Sun in extremely slow and very elliptical circuits, in one of two main groups: The Kuiper Belt and The Oort cloud.

Solar gravitation is weak, especially in the Oort cloud, but only the tiniest perturbation in gravity is enough to send the comets in a path toward the Sun. Comets can die (for example, when they collide with a planet), but there is a virtually unlimited supply of them in the Kuiper Belt and Oort cloud. The tail of the comet consists of water and CO2 molecules that vaporize as the comet approaches the Sun; therefore, the tail of the comet points away from the Sun rather than trailing behind the comet.

A large number of comets and asteroids roamed in the paths of planets and moons early in the history of the solar system. Many of these objects collided with planets and moons until about four billion years ago. The craters from these impacts can still be seen on the Moon. The influence of the tremendous gravitational field of Jupiter has profoundly affected the asteroids of the solar system: Jupiter cleared many of them out of the plane of the ecliptic and stabilized a band of asteroids between its orbit and that of Mars that might otherwise have formed into a planet. Humans have Jupiter to thank for the fact that asteroid collisions with Earth are so rare. . .

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