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"My own feelings went from disbelief to excitement to downright fear," says Carl Hergenrother, 23, an Arizona undergraduate who verified a large asteroid barreling toward Earth with a 230em telescope atop nearby Kitt Peak. "It was scary, because there was the possibility that we were confirming the demise of some city somewhere, or some state or small country."
Well, not quite. Early last week, his celestial interloper whizzed by Earth, missing the planet by 450620 km--a hairbreadth in astronomical terms. Perhaps half a kilometer across, it was the largest object ever observed to pass that close to Earth.
Duncan Steel, an Australian astronomer, has calculated that if the asteroid had struck Earth, it would have hit at some 93450 km/h. The resulting explosion, scientists estimate, would have been in the 3000-to-12000-megaton range. That, says astronomer Eugene Shoemaker, a pioneer asteroid and comet hunter, "is like taking all of the
A. are different in nature.
B. serve similar functions.
C. are identical to each other.
D. pose real challenges.

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