Asteroids


 * //Asteroids are rocky fragments left over from the formation of the solar system about 4.6 billion years ago. Most of these fragments of ancient space rubble - sometimes referred to by scientists as minor planets - can be found orbiting the Sun in a belt between//** Mars **//and//** Jupiter**//. This region in our solar system, called the Asteroid Belt or Main Belt, probably contains millions of asteroids ranging widely in size from Ceres, which at 940 km in diameter is about one-quarter the diameter of our Moon, to bodies that are less than 1 km across. There are more than 90,000 numbered asteroids.

As asteroids revolve around the Sun in elliptical orbits, giant Jupiter's gravity and occasional close encounters with Mars or with another asteroid change the asteroids' orbits, knocking them out of the Main Belt and hurling them into space across the orbits of the planets. For example, Mars' moons//** Phobos **//and//** Deimos **//may be captured asteroids. Scientists believe that stray asteroids or fragments of asteroids have slammed into Earth in the past, playing a major role both in altering the geological history of our planet and in the evolution of life on it. The extinction of the dinosaurs 65 million years ago has been linked to a devastating impact near the Yucatan peninsula in Mexico.

Asteroids were first observed with telescopes in the early 1800s, and in 1802, the astronomer William Herschel first used the word "asteroid," which means "starlike" in Greek, to describe these celestial bodies. Most of what we have learned about asteroids in the past 200 years has been derived from telescopic observations. Ground-based telescopes are used to watch asteroids that orbit close to Earth, not only to detect new ones or keep track of them, but also to watch for any asteroids that might collide with Earth in the future. Scientists define near-Earth asteroids (NEAs) as those whose orbits never take them farther than about 195 million kilometers from the Sun.//**