Scientists continuously monitor Earth-crossing asteroids, whose paths intersect Earth's orbit, and near-Earth asteroids that approach Earth's orbital distance to within about 28 million miles (45 million kilometers) and may pose an impact danger. Stray asteroids and asteroid fragments have slammed into Earth and the other planets in the past, playing a major role in altering the geological history of the planets and in the evolution of life on Earth. These encounters can knock asteroids out of the main belt, and hurl them into space in all directions across the orbits of the other planets. The orbits of asteroids can be changed by Jupiter's massive gravity – and by occasional close encounters with Mars or other objects. Some experienced high temperatures after they formed and partly melted, with iron sinking to the center and forcing basaltic (volcanic) lava to the surface. The asteroids' compositional differences are related to how far from the Sun they formed. The S-types ("stony") are made up of silicate materials and nickel-iron. They are among the most ancient objects in the solar system. They probably consist of clay and silicate rocks, and are dark in appearance. The C-type (chondrite) asteroids are most common. The three broad composition classes of asteroids are C-, S-, and M-types. There are also binary (double) asteroids, in which two rocky bodies of roughly equal size orbit each other, as well as triple asteroid systems.
More than 150 asteroids are known to have a small companion moon (some have two moons). As they revolve around the Sun in elliptical orbits, the asteroids also rotate, sometimes quite erratically, tumbling as they go. Most asteroids are irregularly shaped, though a few are nearly spherical, and they are often pitted or cratered. The total mass of all the asteroids combined is less than that of Earth's Moon. Asteroids range in size from Vesta – the largest at about 329 miles (530 kilometers) in diameter – to bodies that are less than 33 feet (10 meters) across. Most of this ancient space rubble can be found orbiting our Sun between Mars and Jupiter within the main asteroid belt. Asteroids, sometimes called minor planets, are rocky remnants left over from the early formation of our solar system about 4.6 billion years ago.
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