September 24, 2011 / 1:15 AM / 8 years ago

UPDATE 2-Falling six-ton satellite headed toward Earth

* Spacecraft’s re-entry likely late Friday, early Saturday

* Canada, Africa, Australia may be in debris impact zone

* Impacts most likely will be in ocean (Updates with new time for re-entry, details, background)

By Irene Klotz

CAPE CANAVERAL, Fla., Sept 23 (Reuters) - Predictions for the timing and path of a defunct NASA satellite falling to Earth shifted on Friday and officials put North America back in a potential area where the debris could come crashing down.

The research satellite — about the size of a bus — is now likely to tumble to Earth between 11 p.m. EDT Friday and 3 a.m. EDT Saturday (0300 GMT to 0700 GMT Saturday), showering pieces over an as-yet unknown part of the planet, NASA said.

The satellite is expected to pass over Canada, Africa, Australia, and large stretches of the Pacific, Atlantic and Indian oceans during that time, the U.S. space agency said.

Scientists are unable to pinpoint the exact time and place where the Upper Atmosphere Research Satellite, or UARS, will return to Earth due to the satellite’s unpredictable tumbles.

“The satellite’s orientation apparently has changed, and that is now slowing its descent,” NASA said.

Stretching 35 feet (10.6 metres) long and 15 feet (4.5 metres) in diameter, UARS will be among the largest spacecraft to plummet uncontrollably through the atmosphere, although it is a slim cousin to NASA’s 75-ton Skylab station, which crashed to Earth in 1979.

Russia’s last space station, the 135-ton 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 13,000-pound (5,897 kg) 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, pulled by the planet’s gravity.

As of 7 p.m. EDT Friday (2300 GMT), NASA said UARS was about 90 miles (145 km) above the Earth.

Most of the spacecraft will burn up during the fiery plunge through the atmosphere, but about 26 individual pieces, weighing a total of about 1,100 pounds (500 kg) are expected to survive the incineration and land somewhere on Earth.

The debris field spans about 500 miles (805 km), but exactly where will depend on when UARS descends.

Initial predictions that took into account increased solar activity, which heats up and spreads the atmosphere, proved inaccurate, as the satellite shifted position and slowed its descent. North America, which was out of the probable impact zone, is now considered a possible target, although an extremely unlikely one.

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 will be hit by falling debris is 1-in-3,200, NASA said.

“The risk to public safety is very remote,” it said.

The satellite flies over most of the planet, traveling between 57 degrees north and 57 degrees south of the equator.

The satellite is one of about 20,000 pieces of space debris loitering in orbit around Earth. Something the size of UARS falls back into the atmosphere about once a year.

NASA held a news conference earlier this month about UARS’ re-entry and has been posting daily updates on its website. (Editing by Kevin Gray and Todd Eastham)

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