A nearly 6-ton satellite is gradually falling from Earth orbit, and parts of it could crash to the surface as early as Sept. 23, 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 it enters the Earth's atmosphere, moving at 5 miles per second.
What are your chances of being hit by debris from it? About one in 21 trillion, NASA said. When you add up the 7 billion people now alive, the chances that someone, somewhere on the planet could get hit are 1 in 3,200.
The "productive science life" of the satellite ended in 2005 when it ran out of fuel, according to NASA's website. Other satellites launched more recently have used their last fuel to ditch themselves in the Pacific. The vast majority burn up as they enter the atmosphere.
UARS is unusually large, though. NASA's orbital debris office said it will break into pieces as it crashes toward Earth but not all of it will burn up. NASA scientists have identified 26 separate components, with a total mass of 1,200 lbs., that will likely survive, spreading out over 400 to 500 miles.
"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 said last week.
The satellite has been in an orbit that takes it 57 degrees north and south of the equator. So for 20 years, UARS has been occasionally passing over 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. We think of ourselves as living on a crowded planet, they say, but 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," said Air Force Maj. Michael Duncan of the U.S. Strategic Command last week.