A six-tonne NASA science satellite plunged through the atmosphere early this morning, breaking up and possibly scattering debris in Canada, NASA said.

There were reports on Twitter of debris falling over Okotoks, a town south of Calgary in western Canada, most likely the remains of the Upper Atmosphere Research Satellite, or UARS, which had been in orbit for 20 years.

Scientists were unable to pinpoint the exact time and place where UARS would return to Earth due to the satellite's unpredictable tumbles as it plowed through the upper atmosphere. Re-entry was believed to have occurred today at some time between 3.45am and 4.45am GMT).

At 35 feet long and 15 feet in diameter, UARS was among the largest spacecraft to plummet uncontrollably through the atmosphere, although it is a slim cousin to NASA's 75-tonnes skylab station, which crashed to Earth in 1979.

Russia's last space station, the 135-tonnes 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 had been slowly losing altitude ever since, pulled by the planet's gravity.

Most of the spacecraft burned up during the fiery plunge through the atmosphere, but about 26 individual pieces, weighing a total of about 500 kg could have survived the incineration and landed somewhere on Earth.

The debris field spans about 500 miles, but exactly where it is located depends on when UARS descended.

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 1-in-3,200, NASA said.

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

The satellite flew over most of the planet, traveling between 57 degrees north and 57 degrees 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