Comets

The paths of comets and asteroids cross the orbital path of the planets and are believed to be the cause of some impact craters on Earth and the Moon. In 1979, United States Air Force satellite P78–1 took the first photograph of a comet colliding with the Sun. Late in 1994, comet Shoemaker-Levy collided with Jupiter. Asteroidal impacts on Earth may have caused the extinction of many species, including the dinosaurs, while making the development of new species—including ourselves—possible.

For millennia, humans have predicted the "end of the world" from the impact of a giant comet. Now, however, some scientists argue that molecules released by comets' vaporized gases may have supplied important molecules in Earth's early atmosphere. When exposed to the Sun's radiation, these molecules undergo the formation of organic compounds. During the recent passage of Hale-Bopp, for example, scientists discovered a variety of complex organic chemicals in the comet.

The theory gained evidence from data gathered by the Polar spacecraft, launched by NASA in 1996. According to observations by the probe, cometlike objects 30–40 ft (9.1–12.1 m) in diameter are hitting the atmosphere at the rate of 43,000 per day. These cosmic slushballs are too small to vaporize and provide the standard show we associate with comets; most disintegrate in the upper atmosphere, entering the weather cycle and eventually reaching the terrestrial surface as precipitation. According to estimates by scientists associated with the study, this cosmic rain has added one inch of water to the earth's surface each 10,000–20,000 years, supplying a large quantity of water over geologic time.