Asteroids-Fact sheet.
Asteroids are one of the many small or minor rocky planetoids that are a member of the solar system and that move in elliptical orbits primarily between the orbits of Mars and Jupiter. The largest representatives of asteroids are 1 Ceres, with a diameter of about 1,003 km, 2 Pallas and 4 Vesta, with diameters of about 550 km. About 200 asteroids have diameters of more than 97 km, and thousands of smaller ones exist. The total mass of all asteroids in the solar system is much less than the mass of the Moon. The larger bodies are roughly spherical, but elongated and irregular shapes are common for those with diameters of less than 160 km. Most asteroids, regardless of size, rotate on their axes every 5 to 20 hours. Certain asteroids may be binary, or have satellites of their own.

Few scientists believe that asteroids may be remains or remnants of former planet. It is more likely that asteroids occupy in a place in the solar system where a sizable planet could have formed but was prevented from doing so by the disruptive gravitational influences of the nearby giant planet Jupiter. Some asteroid orbits intersect with earth’s orbit and they are known as Apollos. Astronomers have found more than 300 asteroids with orbits that approach Earth’s orbit. Some scientists think that several thousand of these near-Earth asteroids may exist and as many as 1,500 could be large enough to cause a global catastrophe if they collided with Earth. Still, the chances of such a collision average out to only one collision about every 300,000 years.

Many scientists believe that a collision with an asteroid or a comet may have been responsible for at least one mass extinction of life on Earth over the planet’s history. A giant crater on the Yucatan Peninsula in Mexico marks the spot where a comet or asteroid struck Earth at the end of the Cretaceous Period; about 65 million years ago, which scientists think, caused the death of the dinosaurs.