A dead NASA satellite will soon fall to Earth, but the space agency says there is very little chance that a piece of it will hit someone.
NASA says the 20-year-old satellite will probably fall sometime between late September and October. Pieces of it could land anywhere in the six inhabited continents in a worldwide swath from south of Juneau, Alaska, to just north of the tip of South America. NASA scientists estimate a 1-in-3,200 chance a satellite part could hit someone. Most of it will burn up after entering Earth's atmosphere.
The 6-ton Upper Atmosphere Research Satellite (UARS) ran out of fuel in 2005 and will fall uncontrolled out of orbit. Only about 1,200 pounds of metal should survive, NASA said.
This satellite is far smaller than the 135-ton Russian space station Mir, which fell to Earth in 2001 or the 100-ton Skylab that fell in 1979. Mir fell into the South Pacific, while Skylab hit the Indian Ocean and parts of sparsely populated western Australia. Because two-thirds of the Earth is ocean, space debris usually hits water
"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," said NASA orbital debris chief Gene Stansbery. "That doesn't mean we're not concerned."
NASA now has a rule that the chance of any of its satellites hitting someone has to be more than 1 in 10,000. But UARS, which measured chemicals in the air, was launched in 1991 before that rule was adopted. The agency usually tries to put dead satellites into "a graveyard orbit" or steer them down to the ocean, Stansbery said. But there was not enough fuel in this one to fire engines that would move it to a higher orbit or steer it down safely.
The 1-in-3,200 odds of being hit pertain to any of the nearly 7 billion people on Earth. But any one individual's odds of being struck are about 1 in 21 trillion.
Space debris bigger than 5 tons doesn't often fall to Earth. But this will be the third time this year for something that big to reach Earth, according to Jonathan McDowell, a Harvard University astrophysicist who tracks objects in orbit.
The UARS satellite travels over a large band of Earth, avoiding only areas close to the poles. NASA calculates that when the satellite does fall it will scatter pieces over a 500-mile-wide region.
Stansbery said the agency doesn't know exactly when and where those will fall because it depends on the orientation of the satellite in the atmosphere, solar storm activity and other variables.
There probably is no hazardous material left in the falling pieces, but people should not touch any fallen satellite parts just in case, he said.
NASA will be tracking the satellite on a weekly and later daily basis until it falls.
NASA's UARS satellite re-entry tracking site: http://www.nasa.gov/mission_pages/uars/index.html