By Mike Wall, SPACE.com London and New York are in the huge area where Russian spacecraft might land, but it is most likely to ditch in the sea
A defunct Russian spacecraft is due to re-enter the atmosphere sometime after midday (GMT) on Sunday, say scientists who are watching its orbit closely.
They cannot predict precisely where it will hit the Earth, but say the most likely scenario is that it will splash down in the ocean. However, most of the world's major cities, including London and New York, are potentially in the firing line.
At a briefing at the Science Media Centre in London on Thursday, Richard Crowther, chief engineer at the UK Space Agency, said he was not unduly concerned by the return of the Phobos-Grunt spacecraft.
"It certainly doesn't keep me awake at night worrying about the possibility of space debris coming through my roof," he said. "If you look at the Earth from space, it's mostly water. If you look at the land masses, most of the populations are concentrated around the coasts or in certain regions.
"It's no surprise that we don't see these re-entries occurring, even though they occur on a daily basis, because it happens mostly in the oceans or at night-time or under clouds."
The spacecraft's current flightpath will bring it down somewhere between 51.4° north and south of the equator – south of the latitude of Watford in the UK and Calgary in Canada, and north of the Falkland Islands.
It is the largest planetary rocket ever built by the Russian space agency, Roscosmos, and was intended to land on the Martian moon Phobos and bring samples back to Earth. The mission also included a Chinese-built orbiter and containers of bacteria to test their survival in space.
But the spacecraft's rocket boosters failed to ignite after it had been launched into a parking orbit around the Earth in November. Despite repeated attempts to contact it from the ground, Phobos-Grunt remained stuck in orbit and the Russian authorities decided to abandon the mission.
"What went wrong? That's still under investigation by the Russian authorities but, clearly, the upper stage didn't fire," said Crowther, who is also head of the UK delegations to the Inter-Agency Space Debris Co-ordination Committee and the United Nations Committee on Peaceful Uses of Outer Space. "Because of the setup, there was no opportunity to communicate with the vehicle and try and fire it from the ground."
Any parts that do not burn up during re-entry will most likely land in the sea. To predict exactly where, scientists need to know when the probe starts to make its final descent into the Earth's atmosphere. "Because we're going through the very tenuous rarefied parts of the atmosphere, you can see orders of magnitude changes in density occurring over a short period of time, unpredictably," said Crowther. "That will dictate the lifetime of the orbit. Even on the last orbit, which takes 90 minutes to complete, the uncertainty is still plus or minus nine minutes."
The probe will be travelling at 8km/s during this final orbit of the Earth, so the uncertainty in pinpointing the exact location of the landing is 4,300km.
Last week, officials at the Russian space agency said that up to 30 fragments, weighing a total of 200kg, might reach the Earth's surface. The 10 tonnes of fuel and oxidiser are likely to burn up on re-entry or be vented into space as the probe's orbit decays.
The parts most likely to survive include the cone-shaped sample-return capsule, which is protected with a heat shield – though even that is uncertain. "[It] is designed to return to atmosphere and survive re-entry on its own. If it comes in on this complex with things going on around it, it's not going to see clean air and it's very likely that it will also be destroyed," said Crowther.
If any parts of the probe cause damage to people or property on the ground, which Crowther said was highly unlikely, any costs would have to be met by the Russian government, according to United Nations conventions. If any parts of the probe landed in the UK, the UK government would be obliged to pay for its return to Russia.
There are no confirmed injuries from man-made space debris falling to Earth in the past and no record of significant property damage from a falling satellite.
Crowther said that the return of satellites and space probes to Earth would be discussed at a forthcoming UN meeting on space debris. Space engineers believe the design of objects being launched into Earth orbit needs to take account of re-entry. "Up until now we've been designing our spacecraft to be as robust as possible to survive what is a very hazardous environment in space," said Crowther. "We need to consider the design of our spacecraft so that they do break up and disintegrate and don't pose a hazard on the ground should they come back in an uncontrolled manner."
In September a Nasa vehicle called the Upper Atmosphere Research Satellite (UARS) splashed down in the Pacific Ocean, despite speculation that it could hit an inhabited region.