The American space agency said decommissioned Upper Atmosphere Research Satellite fell back to Earth between 11.23pm and 1.09am on Saturday morning (3.23am GMT to 5.09am GMT).
Tracking of the satellite, which broke up during its re-entry through the atmosphere, showed it was passing eastwards over Canada and areas of open ocean, according to a NASA spokesman.
The Joint Space Operations Center at Vandenberg Air Force Base in California said it was still trying to determine the precise re-entry time and location. Unconfirmed reports on Twitter suggested some of the debris may have fallen near a town south of Calgary in western Canada.
A Nasa statement by on the UARS website said: “The satellite was passing eastward over Canada and Africa as well as vast portions of the Pacific, Atlantic and Indian oceans during that period. The precise re-entry time and location are not yet known with certainty.”
The UARS satellite was launched in 1991 from space shuttle Discovery to study the atmosphere and the ozone layer. At the time, the rules weren’t as firm for safe satellite disposal; now a spacecraft must be built to burn up upon re-entry or have a motor to propel it into a much higher, long-term orbit.
It was the biggest piece of US space junk to fall to Earth in 30 years. Most of the satellite was expected to have burned up during re-entry but 26 fragments weighing up to half a tonne in total are expected to hit the Earth’s surface.
The tumbling motion of the satellite has made it hard to accurately predict where the satellite would hit the Earth. Scientists were kept guessing as the object flipped position in its ever-lower orbit, temporarily stalling its death plunge.
Jonathan McDowell, of the Harvard-Smithsonian Centre for Astrophysics, said: “It just doesn’t want to come down.” He said the satellite’s delayed demise demonstrated how unreliable predictions could be. He added: “The best guess is that it will still splash in the ocean, just because there’s more ocean out there.”
The two dozen parts that survive the re-entry may weigh as little as two pounds or as much as 350 pounds, NASA said, and the debris field is expected to span 500 miles.
Mark Matney, an orbital debris scientist at Nasa, said: “In the entire 50 plus year history of the space program, no person has ever been injured by a piece of re-entering space debris.
“Keep in mind we have bits of debris re-entering the atmosphere every single day.” The US Department of Defence and Nasa were tracking the debris. The US Federal Aviation Administration issued a notice to pilots and flight crews of the potential hazard, and urged them to report any falling space debris and take note of its position and time.
Orbital debris experts say space junk of this size from broken-down satellites and spent rockets tends to fall back to Earth about once a year, though this is the biggest Nasa satellite to fall in three decades. Nasa’s 85-ton Skylab crashed into Western Australia in 1979.
The surviving chunks of the UARS satellite are likely to include titanium fuel tanks, beryllium housing and stainless steel batteries and wheel rims.
Nasa added: “Pieces of UARS landing on Earth will not be very hot. Heating stops 20 miles up, and it cools after that.” Any surviving wreckage belongs to Nasa, and it is against the law to keep or sell even the smallest piece.
There space said sharp edges could be dangerous and warned people not to pick up pieces if they find them, urging them to contact local law enforcement authorities instead. [via The Telegraph (UK) and NASA]