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The sky is falling. US officials report a nuclear-powered Soviet spy satellite may crash to Earth by the end of January, perhaps causing radiation problems if it lands in a densely populated area.

But space debris is not a new problem. Since the Soviet's Sputnik was launched in 1957, about 13,000 objects have decayed in orbit, according to the North American Air Defense Command. Most vaporized in the atmosphere before falling to Earth.

The most well-known US space object to crash was Skylab, which blazed through the atmosphere in 1979, scattering pieces off the coast of Western Australia. Nearly five years ago a nuclear-powered Soviet satellite came down in Canada's Northwest Territories, spreading radioactive debris over several hundred square miles.

About 4,800 objects now orbit Earth, says Col. Fred Watkins of NORAD. Several of those decay out of orbit every week. But ''very few things ever survive reentry into Earth's atmosphere.''m