An out-of-control US spy satellite could hit Earth in late February or early March, US government officials say.
The satellite, which has lost power, could contain hazardous materials, and it is unknown where on the planet it might come down, the officials, who spoke anonymously because the information was classified as secret, said.
It was not clear how long ago the satellite lost power, or under what circumstances.
"Appropriate government agencies are monitoring the situation," said Gordon Johndroe, a spokesman for the National Security Council, when asked about the situation.
"Numerous satellites over the years have come out of orbit and fallen harmlessly. We are looking at potential options to mitigate any possible damage this satellite may cause."
He would not comment on whether it was possible for the satellite to perhaps be shot down by a missile, saying it would be inappropriate to discuss specifics at this time.
A senior government official said politicians and other nations were being kept apprised of the situation. Such an uncontrolled re-entry could risk exposure of US secrets, said John Pike, a defence and intelligence expert.
Spy satellites are typically disposed of through a controlled re-entry into the ocean so no-one else can access the spacecraft, he says.
Mr Pike said it was unlikely the threat from the satellite could be eliminated by shooting it down with a missile, because that would create debris that would then re-enter the atmosphere and burn up or hit the ground.
Mr Pike, director of the defence research group GlobalSecurity.org, estimated that the spacecraft weighed about 20,000lbs and was the size of a small bus. He said the satellite would create 10 times less debris than the Columbia space shuttle crash in 2003.
Agencies