Millions of asteroids tumble in a belt around the Sun. They live between the orbits of Mars and Jupiter. They are mostly odd-shaped rocks way too small to be planets. Asteroids are the dinosaur bones of the solar system – the fossils left after all the planets and moons were formed. And they haven't changed much since this beginning.

The largest asteroid is Ceres, 592 miles (952 kilometres) across. The smallest are less than about a half mile across. All of them put together wouldn't make a rock pile big enough to build our Moon.

There are good reasons to care about asteroids. Just as fossils help us figure out the history of Earth, asteroids give us clues about how the solar system formed 4.6 billion years ago. Scientists want to study them up close whenever possible. So even a spacecraft headed to another destination may take a little side trip to check out a nearby asteroid.

Rosetta is a good example. This spacecraft's main mission is to meet up with Comet Churyumov-Gerasimenko in 2014. But on July 10, 2010, it flew close to an asteroid named Lutetia. Rosetta took detailed pictures from about 1,965 miles away. The pictures show that Lutetia is more or less potato-shaped, about 80 miles long. It has many craters and a big bowl-shaped dent on one side. It looks as if it's been beaten and battered by smaller objects for billions of years. Scientists are studying the images and other data to see what else they can learn.

Now Rosetta will continue to its final destination, the comet. Rosetta will orbit the comet nucleus for two years as it travels toward the Sun. Rosetta will also drop a small lander on the nucleus. Rosetta is an international mission lead by the European Space Agency (ESA). NASA provides support and several important science instruments.

Find out more about asteroids and the solar system. Play the Solar System Game, and help Rosetta on its mission. Visit The Space Place, http://spaceplace.nasa.gov/en/kids/solar-system.

This article was written by Diane K. Fisher and provided by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration