How did asteroids form and what is the difference between an asteroid and a comet? (Beginner)

Hi, I am a 2nd grader doing a science project on asteroids. Can you please tell me how asteroids are formed and what is the difference between an asteroid and a comet.

Asteroids formed from small pieces of rock and metal, just like the rest of the inner solar system. Small particles run into each other, and if they don't hit too hard, they'll stick together. The reason why the asteroids didn't continue to grow into a planet like all the rest of the planets is that they are too close to Jupiter. Jupiter's gravity "stirs them up" and makes them go so fast that when they run into each other, they usually bounce off or break apart instead of sticking together.

Comets formed farther away from the Sun. They're made of ice, which would melt if it was as close to the Sun as the asteroids. In fact, when comets leave the outer parts of the solar system to travel close to the Sun, they do melt, and the ice that was turned to water vapor forms the long tail that we see in the sky. After a few trips into the inner solar system, the comet will melt so much that it breaks into little pieces!

This page was last updated on July 18, 2015.

About the Author

Dave Kornreich

Dave was the founder of Ask an Astronomer. He got his PhD from Cornell in 2001 and is now an assistant professor in the Department of Physics and Physical Science at Humboldt State University in California. There he runs his own version of Ask the Astronomer. He also helps us out with the odd cosmology question.

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