Impact craters are evidence of the many cosmic collisions that have occurred in our solar system. Some of these collisions may have aided the development of Earth's physical characteristics. For example, some scientists believe that comet impacts may be responsible for the water in our oceans and for the original organic molecules from which life evolved. Other cosmic collisions have probably had a role in species extinctions on our planet. For example, the final dinosaur extinction may have resulted from environmental changes associated with a massive asteroid impact on the Yucatan Peninsula about 65 million years ago.

The asteroids and comets that cause these impacts are remnants from the birth of the solar system. According to current theory, rocky asteroids were formed in the inner solar system together with the terrestrial planets, while icy comets were formed in the outer solar system beyond Neptune. Today, millions of asteroids circle the Sun in relatively stable orbits between Mars and Jupiter. Comets are found in two regions: within the plane of the solar system just past the orbit of Neptune, in an area known as the Kuiper belt, and on the outskirts of the solar system as part of the Oort cloud.

Just as engineers use the gravity of planets to adjust the path of a spacecraft, the orbits of asteroids and comets can also be altered by a gravitational encounter. Jupiter, whose mass is more than twice the mass of all the other planets in the solar system combined, has a strong gravitational field. It is believed that during the development of the solar system, Jupiter's gravity perturbed the orbits of asteroids and comets, helping to clear away much of the debris in the solar system.

In the early solar system, these gravitational interactions probably increased the number of collisions. Countless asteroids and comets most likely vanished into the Sun or crashed into planets. But as the number of small bodies decreased in the asteroid belt, and as most comets were ejected into distant orbits to form the Oort cloud, the rate of impacts lessened. Eventually, Jupiter's immense gravity caused it to act as a shield for Earth, protecting Earth from being overly bombarded. Without the particular size and placement of Jupiter, Earth could still be subjected to intense pummeling by asteroids and comets, and life may not have been able to develop as it did.

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