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The solar system is crowded with small objects like
asteroids and comets. Most have stable orbits which keep them out of harm’s
way, but a small proportion of them are in orbits that risk them colliding with
planets.
The smaller the objects, the more numerous they are, and the
more frequent these collisions should occur. Collisions like the recent meteor
seen over Chelyabinsk, Russia, in February 2013, are rare because the object
was relatively large, around 17 meters across.