麻豆传媒

The hurt must be running deep within Russia鈥檚 space agency, Roscosmos. Just as NASA鈥檚 car-sized Mars Science Laboratory successfully made 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鈥檚 atmosphere over the next few days.

The mission鈥檚 aim was to place a lander on the Martian moon Phobos and return soil (鈥済runt鈥� 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鈥檚 upper stage failed to fire, marooning the probe in Earth orbit. It was doomed from then on. 麻豆传媒 looks at the risks of the probe鈥檚 fiery fall to Earth.

It鈥檚 dead in orbit. Why can鈥檛 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 鈥� the edge of what is known as the molecular regime. The friction from air molecules there caused aerodynamic drag, reducing the spacecraft鈥檚 velocity and slowly but surely degrading its orbital altitude.

As of early Friday, 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 , 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鈥檚 orbit regularly. It could start tumbling anytime and plummet quickly. over the last few days, but as of Friday, the US government鈥檚 not-for-profit Aerospace Corporation it would occur on Sunday, 15 January at 1752 GMT.

Where might debris fall?

The spacecraft鈥檚 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鈥檚 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 鈥� simply because the ocean covers a lot more of the Earth鈥檚 surface.

. Why do we care about Phobos-Grunt鈥檚 re-entry?

Rocket bodies are usually 鈥減assivated鈥�, or depleted of fuel and battery power before they re-enter. This is to ensure that they don鈥檛 vent gas in orbit and move unexpectedly, and that their fuel tanks don鈥檛 reach the ground full of volatile, toxic fuel.

Phobos-Grunt, however, is fully fuelled with toxic hypergolic chemicals 鈥� 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.

鈥淚t鈥檚 a relatively big spacecraft at 13 tonnes, and all its propellant is still onboard,鈥� says Lewis. 鈥淏ut many of its parts are made of material 鈥� like its aluminium fuel tanks 鈥� with a very low melting point. So it鈥檚 likely they wouldn鈥檛 survive the heating of re-entry. The propellants will probably be dispersed at a high level.鈥�

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, , 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聽掳C (aluminium, of which Phobos-Grunt鈥檚 tanks are made, melts at just 660聽掳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鈥檚 been a lot of finger pointing, but it鈥檚 all heat and no light so far. Some say the craft鈥檚 systems were never fully tested before launch .

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: 鈥淲e believe the upper stage rocket didn鈥檛 fire owing to a fault with the onboard scheduling for the autonomous firing system,鈥� says Crowther.