The Russian military says the doomed Phobos-Grunt spacecraft may reenter the Earth's atmosphere on Jan. 15.
The $165-million mission launched on Nov. 9, 2011, to return samples from the Martian moon Phobos.
The spacecraft never left Earth orbit.
20 to 30 fragments weighing up to 200 kilograms (440 pounds) are expected to fall to Earth.
Fragments of Russia's stranded Mars probe Phobos-Grunt could fall to Earth on Jan. 15, the spokesman of Russia's military space forces told Russian news agencies on Wednesday.
"As of Wednesday morning, the fragments of Phobos-Grunt are expected to fall January 15, 2012. The final date could change due to external factors," said spokesman Alexei Zolotukhin, quoted by the Interfax news agency.
In an embarrassing setback, the $165-million probe designed to travel to the Mars moon of Phobos and bring back soil samples, blasted off on Nov. 9, 2011, but failed to leave the Earth's orbit.
The military space forces' monitoring center had earlier predicted in November that the probe, which is gradually descending and slowing down, would fall to Earth in January or February.
Space forces spokesman Zolotukhin said the probe is now circling at an altitude of between 184 kilometers (114 miles) and 224 kilometers (139 miles) above Earth.
The Russian space agency said in December that it expected the 13.5-tonne probe to fall to Earth between Jan. 6 and 19, but that it would only be possible to predict the exact time and place a few days in advance.
It said that 20 to 30 fragments weighing a total of no more than 200 kilograms (440 pounds) were expected to fall to Earth, with the spacecraft's highly toxic fuel burning up on entering the Earth's atmosphere.
The ambitious and high-stakes project aimed to revive Russia's interplanetary program, which has not seen a successful mission since the fall of the Soviet Union, and prepare the way for a manned mission to Mars.
Russia has experienced a series of serious space failures in the past year.
An unmanned Progress supply ship bound for the International Space Station crashed into Siberia in August last year after its launch by a Soyuz rocket, forcing the rockets' temporary grounding.
Russia also lost three navigation satellites, an advanced military satellite and a telecommunications satellite.
In the latest setback, a fragment of a Russian communications satellite crashed into a Siberian village in December after it failed to reach orbit due to the failure of its Soyuz rocket.