While striking the early Earth billions of years ago, comets are thought to have created major changes to Earth's early oceans, atmosphere and climate, and may have delivered the first carbon-based molecules to our planet, triggering the process of the origins of life. Most asteroids are made of rock, but some are composed of metal, mostly nickel and iron. They range in size from small boulders to objects that are hundreds of miles in diameter. A small portion of the asteroid population may be burned-out comets whose ices have evaporated away and been blown off into space. Almost all asteroids are part of the Main Asteroid Belt, with orbits in the vast region of space between Mars and Jupiter.
Some asteroids pass very close to Earth's orbit around the Sun.
Scientists have found evidence that asteroids have hit our planet in the past. Usually, asteroids and smaller debris called meteoroids are too small to survive the passage through Earth's atmosphere. Larger asteroids occasionally crash into Earth, however, and create craters, such as Arizona's mile-wide Meteor Crater near Flagstaff. Another impact site off the coast of the Yucatan Peninsula in Mexico, which is buried by ocean sediments today, is believed to be a record of the event that led to the extinction of the dinosaurs 65 million years ago. Fortunately for us, these big asteroid impacts are rare.