Nasa has said debris from its defunct six-ton satellite which is heading to Earth could land in the United States.
It had been thought the country was out of the zone where up to 26 pieces, weighing a total of half a ton, would come down.
But there is now a slim chance that debris which survives the fiery re-entry will fall over America in the next few hours.
Scientists are unable to pinpoint the time and place where the Upper Atmosphere Research Satellite , or Uars, will return to Earth.
This is due to the satellite's unpredictable tumbles and changes in the thickness of the atmosphere, which is partly down to a powerful solar flare.
The US space agency said: "The satellite's orientation apparently has changed and that is now slowing its descent.
"There is a low probability any debris that survives re-entry will land in the United States but the possibility cannot be discounted because of this changing rate of descent."
Satellites as large as Uars re-enter Earth's atmosphere about once a year and Nasa said there have been no reports of any deaths or injuries to people from falling debris.
But space expert Doug Millard at London's Science Museum thinks this one is worth watching.
He said: "This is one of the largest satellites up there.
"It's about the size of a double-decker bus. Most satellites when they come down, they are smaller, they burn up and no one notices. Because of the size it's a little more significant."
Nasa has said the odds of a piece of the Uars debris striking a person is about one in 3,200.
The agency insists most of the 20-year-old probe will burn up in the atmosphere.
The probe is being tracked by radar stations and experts around the world.
Flight Lieutenant Mike Farrington and his team at RAF Fylingdales have been monitoring the 35ft long satellite since its launch.
He said the tumbling probe is causing tracking difficulty because of its shape, size and speed.
"There's a great deal of uncertainty over where it will re-enter. Due to the irregular size and shape of this object it's impossible to say," Fl Lt Farrington said.
"These things move at about 4.6 miles (7.5km) per second in space and something moving at that speed is very difficult to predict.
"We've got some world-class analysts here who've been working round the clock.
"But it's impossible for anyone using any of the resources anywhere across the globe to actually predict when and where this object will land."
Launched in 1991, the satellite has been monitoring chemicals in the atmosphere, slowly losing altitude since completing its mission in 2005.
This satellite has caused a scare already. In 2010, it forced the International Space Station into a collision-avoiding manoeuvre.
Nasa said, in 50 years of space travel, no one has ever been hurt by falling debris.