A nearly 6-ton satellite is heading toward Earth and could crash into the planet as early as Sept. 24, NASA officials said.
The UARS -- short for Upper Atmosphere Research Satellite -- has been in orbit since the space shuttle Discovery launched it in 1991, but it's gradually coming closer and closer to the ground as it encounters friction from the upper reaches of the atmosphere.
NASA officials told ABC News overnight that they won't know where the satellite will hit until two hours before the satellite enters the earth, moving at 5 mph.
The chances of anyone getting hit by the UARS satellite are 1 in 3,200, NASA said.
The "productive science life" of the satellite ended in 2005 when it ran out of fuel, according to NASA's website. That fuel could have been used by the satellite to ditch itself in the Pacific.
The satellite will break into pieces as it crashes toward Earth but not all of it will burn up. Scientists have identified 26 separate components that will likely survive with the debris, spreading out over 400 to 500 miles. Engineers say 1,200 pounds of metal chunks could make it down to the surface.
ASA Marshall Space Center/Rex Features via AP Images
"Things have been re-entering ever since the dawn of the Space Age; to date nobody has been injured by anything that's re-entered," Gene Stansbery of NASA's orbital debris office told ABCNews.com last week.
The target range for where the satellite might fall is broad: 57 degrees north of the equator to 57 degrees south of the equator.
For 20 years, UARS has been occasionally passing over you anyone living south of, say, Juneau, Alaska, or Inverness, Scotland, or anywhere north of Punta Arenas, Chile or the southern tip of New Zealand.
Engineers have quietly said in the past that falling space junk is more of a public relations problem than an actual threat. Even though the planet's population is passing 7 billion, it's perhaps surprising how much elbow room there is.
Seventy percent of the surface is water; most of the rest is mountain, desert, tundra or open farmland. By some estimates, humans really use only about 5 percent of the land on the planet.
Robert Kunzig of National Geographic pointed out this year that the world's 7 billion people, standing shoulder-to shoulder, would fit in an area the size of the city of Los Angeles.
Places such as Los Angeles -- to say nothing of Chicago, New Delhi, Shanghai, Lagos and countless other cities -- are indeed crowded; hence the worry. But there's no saying it will come at all close to one of them, or end up leaving pieces in eastern Siberia or the southern Pacific.
"We simply will not know where it's going to come down until it comes down," Air Force Maj. Michael Duncan of the U.S. Strategic Command told ABCNews.com last week.
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