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25 Sep 2011 06:27
A six-tonne Nasa science satellite crashed to Earth on Saturday, leaving a mystery about where a tonne of space debris may have landed.
The U.S. space agency said it believes the debris ended up in the Pacific Ocean, but the precise time of the bus-sized satellite’s re-entry and the location of its debris field have not been determined.
The Upper Atmosphere Research Satellite, or UARS, ended 20 years in orbit with a suicidal plunge into the atmosphere sometime between 3.23am to 5.09am GMT on Saturday, Nasa said.
The satellite would have been torn apart during the fiery re-entry, but about 26 pieces, the largest of which was estimated to have weighed 150kg, probably survived the fall, officials said.
As it fell to Earth, UARS passed from the east coast of Africa over the Indian Ocean, then the Pacific Ocean, across northern Canada and the northern Atlantic Ocean to a point over West Africa.
Most of the transit was over water, with some flight over northern Canada and West Africa, Nasa said.
“Because we don’t know where the re-entry point actually was, we don’t know where the debris field might be,” said Nicholas Johnson, chief orbital debris scientist at Nasa’s Johnson Space Centre in Houston.
Stretching 10.7m long and 4.6m in diameter, UARS was among the largest spacecraft to plummet uncontrollably through the atmosphere, although it is a slim cousin to Nasa’s 68 000 Skylab station, which crashed to Earth in 1979.
Russia’s last space station, the 122 000km Mir, crashed into the Pacific Ocean in 2001, but it was a guided descent.
Nasa now plans for the controlled re-entry of large spacecraft, but it did not when UARS was designed.
The 5 897kg satellite was dispatched into orbit by a space shuttle crew in 1991 to study ozone and other chemicals in Earth’s atmosphere.
It completed its mission in 2005 and has been slowly losing altitude ever since.
With most of the planet covered in water and vast uninhabited deserts and other land directly beneath the satellite’s flight path, the chance that someone would be hit by falling debris was one-in-3 200, Nasa said.
“The risk to public safety is very remote,” it said.
The satellite flew over most of the planet, travelling between 57° north and 57° south of the equator.
UARS was one of about 20 000 pieces of space debris in orbit around Earth. Something the size of UARS falls back into the atmosphere about once a year. - Reuters
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