Image caption Phobos-Grunt imaged from the ground on 29 Nov, at an altitude of 230km and at a range of 274km (ralfvandeburgh.startje.be)

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 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.

Current Roscosmos estimates for the timing of the fall are between 6 and 19 January, but this window will be narrowed nearer the event.

Professional and amateur groups around the world will also be modelling the decay in the orbit in an attempt to determine precisely where and when Phobos-Grunt might come down.

Phobos-Grunt - Mishap sequence 8 Nov (GMT): The probe launched successfully on its Zenit rocket from the Baikonur Cosmodrome

It was dropped off 11 minutes later in an elliptical orbit some 345km above the Earth

Two firings from the probe's hydrazine-fuelled cruise stage were planned over South America

The first, lasting 11.5 minutes, should have raised the orbit of Phobos-Grunt to 4,000km

A second burn, four hours into the mission, was to have sent the probe on a path to Mars

But Russian engineers later confirmed that neither burn took place

Controllers have had only limited radio contact with the probe; mission recovery was not possible

Re-entry is now expected in early January; 20-30 fragments are expected to survive descent

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

The maximum latitudes are 51 degrees North and South, encompassing London (UK) in the Northern Hemisphere and Punta Arenas (Chile) in the Southern Hemisphere.

The spacecraft's mass at launch was some 13 tonnes, most of which was the propellants unsymmetrical dimethylhydrazine (UDMH) and dinitrogen tetroxide (DTO).

These are extremely unpleasant substances and the Russian authorities will be hoping they are destroyed during the descent.

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.

Roscosmos says it expects only about 200kg to make it all the way through.

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

Such a venture would have yielded fascinating new insights into the origin of the 27km-wide object and the planet it circles.

The mission was notable also because China's first Mars satellite, Yinghuo-1, was launched piggy-back on the main Russian spacecraft. It will come back into the atmosphere with the Russian probe.