Russia's space agency (Roscosmos) says its unsuccessful Mars probe will fall back to Earth next month. The unmanned Phobos-Grunt spacecraft became stranded in orbit in November.

The spacecraft's mass at launch was some 13 tonnes, most of which was the propellants unsymmetrical dimethylhydrazine (UDMH) and dinitrogen tetroxide (DTO). Roscosmos says it expects only about 200kg to make it all the way through.

Current Roscosmos estimates for the timing of the fall are between 6 and 19 January, but this window will be narrowed nearer the event. With more than 70% of the Earth's surface covered by water, the chances are that any fragments that do survive the fiery re-entry will end up in the ocean.

The agency says it expects the toxic fuel on board to burn up on re-entry, but 20-30 fragments of the spacecraft will survive to the surface. These are extremely unpleasant substances and the Russian authorities will be hoping they are destroyed during the descent.

Phobos-Grunt is currently moving around the Earth at an altitude that varies between 201km (perigee) and 275km (apogee).

Phobos-Grunt was built to land on the larger of Mars' two moons, Phobos, to scoop up rock and bring it back to Earth.

http://www.bbc.co.uk/news/science-environment-16221975