Asteroids are the small rocky objects in the Solar System. The largest asteroid is Ceres, which is 933 kilometers (580 miles) across. Many asteroids, including all of the largest asteroids, orbit the sun between Mars and Jupiter in the Asteroid Belt. The asteroid belt is a doughnut-shaped concentration of asteroids. Most asteroids orbit from between 186 million to 370 million miles from the Sun. The asteroids in the asteroid belt have a slightly elliptical orbit. The time for one revolution around the Sun varies from about three to six Earth years.

In general, neither asteroids nor comets are a threat to Earth. This is because their orbits stay the same year in and year out, just as the Earth does. Once an asteroid is identified and its orbit determined, its future path can be predicted very accurately. Most asteroids don’t come anywhere near the Earth. But a few have been nudged from their originally circular orbits by a close encounter with Jupiter or a collision with other asteroids. Their new orbits which are also predictable bring them to the inner Solar System where they can threaten Earth. These are the so-called Earth-crossing asteroid families; Apollos, Amors, and Atens.

Asteroids are believed to be leftover fragments from the formation of the solar system about 4.6 billion years ago. Scientists suspect that these fragments were prevented from accreting into larger objects because of Jupiter’s massive gravitational pull. As Jupiter orbited outside the asteroid belt, its gravity would tug on the small bodies, disturbing their orbits and preventing them from coalescing into a single body. Between the main concentrations of asteroids are relatively empty regions known as the Kirkwood gaps. These are regions where an object’s orbital period would be a simple fraction of that of Jupiter. An object in such an orbit is very likely to be accelerated by Jupiter into a different orbit.

Interest in asteroids has increased significantly over the past several years for many reasons such as looking for clues to the origin and primordial composition of the solar system, the dynamics of small solar system bodies, and the potential of asteroid collisions with the Earth.

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