Phobos-Grunt - Mishap sequence Image caption Russian scientists were hoping Phobos-Grunt would end a dreadful run of failure for Russia at Mars 9 Nov: The probe launches successfully on its Zenit rocket from the Baikonur Cosmodrome

It is 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

Russian space agency officials say neither burn on the big cruise stage took place

The probe remains in a low-Earth orbit while the anomaly is investigated by engineers

After two weeks, contact is still impossible; but the probe maintains its orbit

Eventually, it would fall back to Earth. Roscosmos says perhaps between December and February What would Mars probe failure mean for Russian space?

The Russian space agency has conceded there is now little chance of reviving its Mars mission, Phobos-Grunt.

The probe has been stuck circling the Earth since its launch on 9 November, unable to fire the engine that would take it on to the Red Planet.

Engineers have tried in vain to contact the spacecraft, and Roscosmos deputy head Vitaliy Davydov said the situation now looked very grim.

"One should be a realist," he was quoted as saying by RIA Novosti.

"If we've been unable to establish communication with [Phobos-Grunt] for such a long time, there are few chances that we shall fulfil the expedition now," were his comments reported by the Russian news agency at a press conference in mission control centre at Korolev on the outskirts of Moscow.

"If we establish contact [with the probe] and begin to understand what's wrong with it, then we shall be able to draw some conclusions," Davydov said.

Later, another Russian news agency, Interfax, quoted Davydov as saying that Phobos-Grunt might fall from orbit anytime between late December 2011 and February 2012.

"It is an interesting question how [the probe] will behave. There is fuel on board. If there is an explosion, it is one thing, but if it simply starts falling apart with no explosion, then it is another thing," Interfax reported the deputy head as saying.

The spacecraft weighed some 13 tonnes at launch - double the mass of Nasa's recently re-entered UARS satellite.

What is more, most of the 13 tonnes is made up by the propellants unsymmetrical dimethylhydrazine (UDMH) and dinitrogen tetroxide (DTO), both of which are toxic.

If the Phobos-Grunt mission is truly lost, then professional and amateur groups will be modelling its orbit in an attempt to determine precisely where and when it might come down.

As with UARS, much of the spacecraft will burn up in the atmosphere; but any parts made of high-temperature metals, such as titanium or stainless steel, stand a chance of making it all the way to the surface.

Indeed, it is the fuel tanks that often survive the fall because their spherical shapes enable them to spin up and dissipate heat more easily.

However, the probability is that any debris would hit the ocean, given that more than 70% of the Earth's surface is covered by water. This was the case with UARS and the German Rosat X-Ray telescope that returned to Earth last month.

To date, Phobos-Grunt has been maintaining its orbit, but trackers will be monitoring the spacecraft closely to catch any change in its behaviour.

The probe was built to land on the Martian moon Phobos and scoop up rock for return to Earth. Such a venture would yield fascinating new insights into the origin of the 27km-wide moon and the planet it circles.

The mission was also notable because China's first Mars satellite, Yinghuo-1, was launched piggy-back on the main Russian spacecraft.

Jonathan.Amos-INTERNET@bbc.co.uk