Both asteroids and comets are rocky or icy solar system objects ranging in size from less than a half a mile to many hundreds of miles wide. Traditional descriptions of asteroids and comets make them sound very different. Asteroids are pieces of rock that orbit the sun, mostly located in the Asteroid belt between the orbits of Mars and Jupiter in the inner solar system. Comets are dirty snowballs orbiting the solar system in an outer region called the Oort cloud. Closer studies indicate that comets and asteroids aren’t very different from each other, having more in common than originally believed.

When a comet passes close to the sun, surface ice warms and sublimates (turns from a solid into a gas), forming a cloud and a tail of gas and dust. A tail was an indication of a comet. Asteroids weren’t supposed to be able to form tails, being rock and not ice. But now a few asteroids have been discovered that have sporadic tails, making them look like comets. Comets get a lot of dark gunk on their surface when they shed material after passing near the sun, which makes them look like dark, rocky asteroids.