By Kathryn Westcott
This is not Armageddon: stand down Bruce Willis, we don't need you!
An out-of-control US spy satellite - possibly the size of small bus - is believed to be plummeting out of its orbit and is expected to crash somewhere on the planet within weeks. But space experts don't believe the rogue satellite poses much of a threat to humankind.
The world was put on alert when Skylab plunged to Earth in 1979
Some sections of the media, however, have seized the opportunity to sound the alert, recalling the Hollywood blockbuster, Armageddon, in which an asteroid the size of Texas heads towards Earth. In the film, a team led by Willis is sent to try to nuke the rock from the inside.
In reality, a spy satellite heading uncontrollably towards Earth is not an uncommon event, says Dr Ruediger Jehn, a space debris analyst at the European Space Agency (Esa). He says that satellites come out of orbit and fall back to Earth harmlessly on average once a year.
Normally, when US spy satellites reach the end of their lives, they are disposed of through a controlled re-entry and dumped in the Pacific Ocean, so that no-one can learn their secrets.
But, Dr Jehn says older satellites are often more difficult to de-orbit properly.
"When they re-enter they usually burn up in the atmosphere because a lot of heat has developed and there is a lot of friction," he says.
"Only heat-resistant or very heavy objects will survive. There is a risk in this case that something will hit the ground, but given that the Earth is so big, the probability in this case that someone will be hit is really remote."
US defence officials have released few details of the satellite because of its sensitive nature. Such spacecraft are used for reconnaissance and information gathering. No-one knows for certain how big it is but experts say it is probably a few tonnes.
White House officials told journalists on Saturday that a large US spying satellite, whose engine had failed, was falling from orbit.
Officials say they have no idea where it might land but that they are keeping other countries abreast of the situation.
John Pike, a space expert and director of GlobalSecurity.org in Virginia, told the BBC that "by now the satellite is probably not much more than 150km to 200km up, and it's falling by almost 1km a day." This means that it is expected to hit the top of the Earth's atmosphere by the end of February or early March.
"It could come down anywhere and it's very difficult to predict," says Esa's Dr Jehn.
"About 24 hours before it hits the ground, we might get a better idea. You can predict ground track very well - that's the line over the Earth where the satellite is overflying. Maybe an hour or so beforehand experts might know roughly where the satellite will land - but it is only rough because the line is something like 10,000km long."
In 1979, the world was given warning of a large spacecraft falling uncontrollably from the sky.
The 77.5-tonne US space laboratory, Skylab I, eventually plunged to Earth, scattering debris across the southern Indian Ocean and sparsely populated Western Australia.
Days beforehand, police across India's 22 states were put on full alert.
There has been a lot of speculation that the craft contains a high quantity of hydrazine rocket fuel.
"Usually, the hydrazine is consumed but I understand there could have been a malfunction, which means there is more than usual left on board," says Dr Jehn.
"This could reduce the risk of it crashing into the Earth. When the velocity of the satellite is reduced during entry into the denser layers of the atmosphere, the satellite will get very, very hot. The hydrazine will probably cause it to explode and it will be broken up into many, many pieces."
Dr Jehn said that some 10% of the craft could reach the ground, with the rest forming tiny particles in the Earth's atmosphere.
He said those pieces would have the same impact as a plane that explodes in the sky scattering debris on the ground.
Experts believe that the most likely scenario is that the pieces will drop into the sea somewhere, given that almost three-quarters of the Earth's surface is covered by water.
There is the possibility, however, that they could land in Russia or China.
"If souvenir hunters come up with bits and pieces of the satellite, I'm sure that the Russians would pay a pretty penny for them," says John Pike.
But Dr Jehn, says that while Russia and China would be clearly interested, he wasn't sure how many military secrets they could glean from the charred metal.
"Not much hardware will survive that you can usefully use," he says. "The Russians used to put explosive materials on board their own spy satellites, which would be triggered when they had come to the end of their lives. They would burn up and take their secrets with them. This is probably what will happen here."