Asteroids are debris left over from the formation of the inner planets in our solar system. Asteroids escaped the erosion, heat and chemicals that have altered ancient rocks on Earth. They represent our best chance for glimpsing the primordial solar system.

Most asteroids orbit between Mars and Jupiter, crashing into each other in a silent version of 3-D bumper cars that slowly grinds them to bits. Occasionally, an asteroid's orbit becomes more oval-shaped. As the orbit grows more elliptical, it may eventually cross Earth's orbit and collide with the planet.

During the first billion years of the solar system, the asteroids and comets that constantly bombarded the Earth hampered the "building blocks of life". This includes water and complex organic molecules.

Only after the bombardment ended about 3.8 billion years ago did the first fossil evidence of life -- in the form of bacteria -- appear. But scientists like Donald Yeomans, Jet Propulsion Laboratory expert on the sub-planetary orbiters, believe asteroids also supplied the water and carbon molecules that later permitted life to begin. There's also the possibility that they brought stray DNA to the planet -- thus seeding a barren planet with life.

:: Relics of the solar system
:: Celestia