Meteoroids are small pieces of interplanetary debris and are commonly fragments of asteroids. Most meteoroids that become visible as meteors, or “shooting stars,” when they enter the Earth’s atmosphere are the size of pebbles or smaller. Remnants of meteoroids and asteroids that are slowed by the Earth’s atmosphere and reach the surface are all called meteorites. Larger bodies called bolides are not slowed as much and are
vaporized upon impact, but for the purpose of this article, the term asteroid will encompass bodies of all sizes that strike the Earth. The surface rock that sustains impact is called target rock. Craters formed by asteroid impact are the
most common landforms in the solar system.1 However, impact craters are comparatively uncommon on Earth, where the atmosphere causes most falling bodies to disintegrate. A few, known as fireballs, explode quite near the surface,
Winter 2009/2010
and others undergo glancing or full-on collisions. Frequency of impact is inversely proportional to asteroid size; while several pebble-sized meteor- ites may land per year, and asteroids that cause craters 20 to 50 km [12 to 31 mi] across occur every million years or so, asteroids large enough to cause widespread catastrophe intersect the Earth’s orbit only every 100 million years or more (previous page).2
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