Published online 29 January 2008 | Nature | doi:10.1038/news.2008.536

News: Briefing

Watch out for falling satellites

With no one at the wheel, should we be worried about the large US spy satellite now headed for a crash landing?

What is happening?

An out-of-control US spy satellite will crash to Earth in the coming months, government officials say. The satellite is large enough that remnants are likely to survive atmospheric re-entry and strike the Earth, sometime in late February or early March, says Gordon Johndroe, a spokesman for the National Security Council.

Is that normal?

A titanium motor casing that fell from the sky in 2001.NASA Orbital Debris Program Office

“This is relatively routine in that satellites de-orbit all the time,” says Johndroe. Pieces of uncontrolled debris heavier than two tonnes — mostly discarded rocket stages — crash to Earth as often as once every three weeks, says Jonathan McDowell, an astronomer and launch observer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

Many discarded pieces retain some power, so that controllers on Earth can guide them to a point far from human habitation, usually using a final dive into an ocean. In 2001, Russian space officials broke up the old Mir space station in this way over the South Pacific. That's not the case for this US one, however.

“Obviously, we want to take a look at the potential for it to land in a populated area,” says Johndroe.

What are the chances of it crashing through my roof?

Exceedingly slim, says McDowell. Remember that some 70% of the Earth is water, and most lands are void of people. “There is no reason for people to get alarmed about it,” he says.

According to the NASA Orbital Debris Program Office, there have been no confirmed instances of serious property damage or injury caused by crashing debris in 40 years.

What is this satellite?

Since it is a spy satellite, this isn't public information. But it is likely to be USA 193, which, according to news reports, was launched at the end of 2006 for the National Reconnaissance Office (NRO) and which was lost to ground-based controllers upon reaching orbit. At the time, amateur sky watchers noticed that its orbit was slightly 'off' and wasn’t being corrected. Based on the slight losses in altitude, they had predicted an eventual crash this spring, McDowell says.

If the dying satellite is indeed USA 193, its specifications can be better defined. USA 193 was launched on a Boeing Delta 2 rocket, which limits the size of the satellite to between two and four tonnes and five or six metres in length — about the size of a minivan, McDowell says.

This satellite is now at an altitude of 250 kilometres and is falling a kilometre per day. It orbits in the mid-latitudes, between 58 degrees north and south. Where it will fall within that range is impossible to know now, so McDowell says the chance of landing in the United States, for example, is 2%, based on area. Better estimates should be available a day before landing.

Is a 'minivan-sized' satellite a big one?

Nowhere near as big as a satellite that fell uncontrollably in 1979: the more-than 70-tonne US Skylab I crashed through the atmosphere and scattered debris across the Indian Ocean and Western Australia.

What will happen to this one on entry?

The atmosphere will tear up the satellite. Aluminium parts and outer layers will burn. Twisted chunks of heavier pieces, like the fuel tank, could survive. McDowell says that the satellite could contain as much as a tonne of hydrazine, a highly toxic propellant which could be hazardous to be near but not dangerous when dispersed in the atmosphere.

ADVERTISEMENT

Have bits of satellites survived re-entry intact before?

Yes. The NASA Orbital Debris Program Office hosts some pictures of pretty big chunks, including: a 250-kilogram steel fuel tank and a 30-kilogram titanium tank of a launch vehicle that both landed in Texas in 1997, and a 70-kilogram titanium rocket motor casing that landed in Saudi Arabia in 2001. One noteworthy ill-fated satellite actually crash-landed through the roof of a workshop supporting its launch in 2006, having fallen from a botched take-off (see 'Plucky satellite is laid to rest').

How many things are there that could potentially fall out of orbit?

The NASA Orbital Debris Program Office says there are some 11,000 objects bigger than 10 centimetres tracked by the US Space Surveillance Network. Of course, the biggest man-made object in the sky, by far, is the International Space Station. Nearly 45 metres long and more than 200 tonnes heavy, the concern about the ISS is in protecting it from debris, rather than worrying about it becoming debris itself.

If it lands in the middle of nowhere, does it matter?

While the impact is likely to be a nonevent, the falling satellite does serve as a reminder that the NRO — the United States' "eyes and ears in space" — is having a bad time of things. In 2005, the NRO cancelled a next-generation reconnaissance technology project, called Future Imagery Architecture, after Boeing had spent more than $4 billion on it, according to the New York Times.

“The NRO has had a bad decade,” says Jeffrey Lewis, a space policy analyst at the New America Foundation in Washington, DC. “We knew this satellite has been dead for a long time. But it reminds us of how much trouble they’ve had.” 

Commenting is now closed.