Crippled Russian probe set to hit Earth within days

Posted by Emma Woollacott

The stricken Russian spacecraft, Phobos-Grunt, is likely to crash to Earth on Sunday or Monday, say scientists tracking its path.

It's being monitored intensively by ESA, NASA and the Russian space agency Roscosmos, who say they're expecting it to come down safely. There's been serious concern that the spacecraft's fuel tanks could survive reentry intact, braking up and releasing toxic chemicals on impact.

However, scientists believe that the relatively flimsy construction of its aluminum fuel tanks means they're likely to rupture well before this point.

"Analyses by Roscosmos and NASA indicate that the fuel tanks, filled with unsymmetrical dimethylhydrazine – referred to as UDMH – will burst above 100 km altitude, release the propellant and largely demise thereafter," says says Professor Heiner Klinkrad, head of ESA's Space Debris Office.

"This, combined with a relatively low dry mass of just 2.5 tonnes, means that Phobos–Grunt is not considered to be a high-risk reentry object. Roscosmos expects that at most, some 20 to 30 fragments may reach Earth's surface, with a total mass of less than 200 kg."

Phobos–Grunt was launched on 8 November 2011 into an initial Earth orbit of 206 x 341 km. The but injection into an Earth-escape trajectory to Mars failed. It was declared lost by the Russian space agency, Roscosmos, on 13 December, and has been spinning out of control since.

In recent days, the head or Roscosmos Vladimir Popovkin has hinted that sabotage could have been to blame. He told told the Izvestia newspaper: "I don’t want to make any accusations, but today there is powerful equipment to influence spacecrafts, and the possibility of their use should not be ruled out."

Whatever the cause of the failure, Phobos-Grunt is likely to be on its way back very soon. Its erratic flight path means it could come down anywhere between 51.4 degrees north or south of the equator - a region that includes dozens of major cities, including New York and London.

However, say scientists, the odds are that it will hit water rather than land, and it's quite possible that nobody will even see it fall.