What are the risks from Phobos probe's downfall?
By Paul Marks The hurt must be running deep within Russia’s space agency, Roscosmos. Just as NASA’s car-sized Mars Science Laboratory successfully made its final course correction for the Red Planet this week, Roscosmos was preparing for a much grimmer task: tracking its minibus-sized Phobos-Grunt probe, which is set to meet an untimely end in Earth’s atmosphere over the next few days. The mission’s aim was to place a lander on the Martian moon Phobos and return soil (“grunt” in Russian) samples to Earth in a small capsule. Additionally, it would have launched a small Chinese Mars orbiter. But after Phobos-Grunt launched on 8 November, its launch rocket’s upper stage failed to fire, marooning the probe in Earth orbit. It was doomed from then on. New Scientist looks at the risks of the probe’s fiery fall to Earth. It’s dead in orbit. Why can’t it stay there? Its parking orbit was elliptical, reaching as high as 347 kilometres and dipping down as low as 207 kilometres. That means it regularly encountered the entrails of the atmosphere – the edge of what is known as the molecular regime. The friction from air molecules there caused aerodynamic drag, reducing the spacecraft’s velocity and slowly but surely degrading its orbital altitude. As of early Friday, Phobos-Grunt’s orbit was circularising due to this drag at an altitude of about 160 kilometres. It will experience tremendous incandescent heating and break up when it gets to 75 kilometres, says Hugh Lewis, a space debris specialist at the University of Southampton in the UK. When will it re-enter the atmosphere? Predictions vary because atmospheric effects change the craft’s orbit regularly. It could start tumbling anytime and plummet quickly. Predictions of the timing of Phobos-Grunt’s re-entry have been changing over the last few days, but as of Friday, the US government’s not-for-profit Aerospace Corporation predicted it would occur on Sunday, 15 January at 1752 GMT. Where might debris fall? The spacecraft’s orbital inclination means it will re-enter somewhere between a latitude of 51 degrees north and south, says Richard Crowther of the UK Space Agency. But where is anyone’s guess because the margin of error remains high until a few hours before re-entry. On Friday, the Aerospace Corporation predicted re-entry over South America, but the uncertainty on that prediction is large. Like the re-entries of the US UARS satellite in September 2011 and the German ROSAT telescope in October, the chances are high that debris will fall over the oceans rather than on land – simply because the ocean covers a lot more of the Earth’s surface. Old rocket bodies regularly fall out of orbit. Why do we care about Phobos-Grunt’s re-entry? Rocket bodies are usually “passivated”, or depleted of fuel and battery power before they re-enter. This is to ensure that they don’t vent gas in orbit and move unexpectedly, and that their fuel tanks don’t reach the ground full of volatile, toxic fuel. Phobos-Grunt, however, is fully fuelled with toxic hypergolic chemicals – unsymmetrical dimethylhydrazine and dinitrogen tetroxide (N2O4), which combust on contact in a rocket nozzle without needing an ignition source. Phobos-Grunt has lost contact with controllers and its fuel cannot be vented into space. Will parts of it survive to reach the surface? Roscosmos expects about 200 kilograms to reach the ground, and the UK Space Agency predicts that this will be scattered in a debris field up to 200 kilometres long and tens of kilometres wide. “It’s a relatively big spacecraft at 13 tonnes, and all its propellant is still onboard,” says Lewis. “But many of its parts are made of material – like its aluminium fuel tanks – with a very low melting point. So it’s likely they wouldn’t survive the heating of re-entry. The propellants will probably be dispersed at a high level.” Why did the Pentagon once shoot down a satellite rather than let it re-enter? The reason was likely twofold. Clearly the feat demonstrated its anti-missile and anti-satellite technology. But the spacecraft, the USA-193 spy satellite, also had half a tonne of toxic hydrazine fuel in tough titanium tanks that might have reached the ground, says Crowther. Titanium has a melting point of 1668 °C (aluminium, of which Phobos-Grunt’s tanks are made, melts at just 660 °C). So blowing the defunct spy satellite up just before its re-entry over the Pacific in 2008 ensured no debris from the explosion remained to hit the ground. What caused the mission to fail in the first place? There’s been a lot of finger pointing, but it’s all heat and no light so far. Some say the craft’s systems were never fully tested before launch owing to a crunched schedule. This week, the slightly absurd suggestion of sabotage was voiced too. The UK Space Agency thinks it knows the what but not the why behind the failure: “We believe the upper stage rocket didn’t fire owing to a fault with the onboard scheduling for the autonomous firing system,