The word asteroid means "star-like", even if these minor bodies of the Solar System don't emit light on their own, but are visible only because they reflect sunlight. The size of asteroids range from dust particles to significant bodies hundreds of miles in diameter (Ceres, the largest observed is 913 km of diameter). Globally, the total mass of all the asteroids is less than that of the Moon.
Asteroids are found in different places in the solar system: most of them orbit around the Sun, grouped in the main belt, while others are farther objects, with highly unpredictable orbits, such as the Trojans, which lie on the orbit of Jupiter, or such as the Centaurs, in the very outer solar system. Asteroids that, for some dynamical mechanism, closely approach the Earth are named Near-Earth Asteroids (NEAs) and form a class of particular interest.


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