A space satellite the size of a bus fell to Earth overnight, but NASA says it is unsure where it landed after entering the atmosphere over the vast Pacific Ocean.
The U.S. Air Force's Joint Space Operations Center and NASA say the unused six-ton satellite fell between 11:23 p.m. EDT and 1:09 a.m. EDT.
The 20-year-old Upper Atmosphere Research Satellite had been orbiting 145 kilometres above Earth, a NASA statement said.
The agency was predicting the vessel would pass over Canada, Africa and Australia, as well as three oceans, after reentering the atmosphere.
Although NASA announced on Twitter that the satellite "penetrated the atmosphere over the Pacific Ocean," the precise time and location of the crash is not yet known.
Eyewitness accounts and unconfirmed sightings were being reported around the world early Saturday.
Eyewitness testimony and video posted on Twitter claimed pieces of the 550-kilogram satellite were spotted falling near Okotoks, a town south of Calgary. Similar sightings were reported in the skies over Texas and Hawaii.
While a NASA spokesman told The Associated Press that it was possible some of the debris landed in Alberta, the RCMP dismissed one such claim Saturday afternoon.
RCMP Sgt. Patrick Webb said the video being circulated online was likely a hoax and that authorities had heard nothing about falling debris in the area.
NASA said some 26 pieces of the satellite were expected to reenter Earth's atmosphere in the uncontrolled crash. None of the pieces were expected to be larger than 135 kilograms.
The Upper Atmosphere Research Satellite was launched aboard the space shuttle Discovery in 1991, on a mission to study the ozone layer. But NASA decommissioned the UARS in 2005, after the satellite used the last of its fuel to descend into a lower orbit. That set the stage for its uncontrolled return.
It is the largest NASA spacecraft to land on Earth in an uncontrolled crash in more than 30 years.
The 75-ton Skylab space station and the 10-ton Pegasus 2 satellite both crashed in 1979.
NASA had said the chance of someone being hurt in the crash were low: about 1 in 3,200. But -- and this is an important 'but' -- the chance any one person would be hit was even lower: 1 in 21 trillion.
In fact, the only confirmed incident involving space junk striking a person was in 1997, when Lottie Williams of Tulsa, Okla., was grazed in the shoulder by a small bit of debris from a discarded piece of a Delta rocket.
There are roughly 22,000 pieces of satellites, rockets, and other junk orbiting the Earth right now, and NASA is trying to watch each one of them through its Orbital Debris Program Office.
When UARS was build, there were no rules about satellite construction. But these days, satellites must be designed to disintegrate upon re-entry or have enough fuel to be steered into the ocean.
NASA estimates that medium-sized junk falls back to Earth once a week. Debris the size of this UARS satellite comes down about once a year, though this is the biggest NASA satellite to fall in three decades.
With files from The Associated Press and The Canadian Press
