Key Ideas

Discovery of the Asteroids: Astronomers first discovered the asteroids while searching for a “missing planet.”

Origin of the Asteroids: The asteroids are the relics of planetesimals that failed to accrete into a full-sized planet, thanks to the effects of Jupiter and other Mars-sized objects.

Asteroid Collisions: Asteroids undergo collisions with one another, causing them to break up into smaller fragments.

Meteoroids, Meteors, and Meteorites: Small rocks in space less than 1 m in size are called meteoroids. If a meteoroid enters Earth’s atmosphere, it produces a bright trail called a meteor (or shooting star). A rock that does not burn up in the atmosphere and reaches Earth’s surface is called a meteorite; most of the meteoroids are too small to make it to Earth before being completely vaporized.

Comets: A comet is a chunk of ice with imbedded rock fragments that generally moves in a highly elliptical orbit about the Sun.

Origin and Fate of Comets: Comets are thought to originate from two regions, the Kuiper belt and the Oort cloud.