Ranging from six to 60 miles across, the craters appear to have formed about 214 million years ago, perhaps within the span of a few hours, according to John Spray, a geologist at the University of New Brunswick, Canada. A train of eight craters follows a nearly straight line across the center of the United States from Illinois through Missouri to Kansas. NASA geologist Michael Rampino believes they might be the wounds left by a comet or asteroid 320 million years ago. Another possible three-crater chain is being studied by Ocampo and her colleagues in the African nation of Chad. Comets are balls of dust and ice that periodically wander toward the sun from their nesting ground beyond Pluto.
Asteroids are chunks of rock and metal left over from the formation of the solar system.
Baby asteroids, commonly known as meteors, constantly pepper the Earth. On June 13, for example, a meteor exploded over Portales, N.M., scattering 143 pounds of bits and pieces over four miles. And a 27-pound meteor smashed the fender of a Chevrolet in Peekskill, N.Y., on Oct. 9, 1992. But the big destroyers, half a mile or more in diameter, can wreak enormous damage. The best-known of these is the huge asteroid or comet, an estimated five to eight miles wide, that slammed into Mexico's Yucatan Peninsula 65 million years ago.