Most of it will burn up in the atmosphere, but 20 to 30 chunks of charred debris, weighing about 450 lbs., could make it to the surface, said Roscosmos. Just where it might crash will not be clear until just hours before it actually happens.
Phobos-Grunt was launched toward Mars in November, but radio contact was lost and it never got beyond low Earth orbit.
The world's space agencies agreed that any one person's chances of getting hit by debris are tiny -- something like 1 in 20 trillion, based on the spacecraft's orbit and the amount of debris that might survive re-entry. . The chances that of the 7 billion people on Earth, one of them, somewhere, could be hit are more like 1 in 3,000.
If this sounds familiar, it should. Spent satellites fall from orbit all the time, though most burn up completely before anything reaches Earth's surface. There have been a few recent -- but harmless -- exceptions: NASA's UARS satellite sent debris crashing into the Pacific in September, and the German ROSAT space telescope scattered debris in the Indian Ocean in October.
So the worst damage was to Russian pride. Roscosmos chief, Vladimir Popovkin, went so far as to suggest that someone had sabotaged the probe.
"It would not be desirable to accuse anybody, but today there are very powerful means of influence for space vehicles which cannot be excluded," he said in an interview with the Russian daily Izvestia, translated by ABC News. He gave no specifics, and sources say the U.S. government, mildly offended, stopped helping the Russians track their errant probe in its final days.
More likely, said space analysts, it was the Russians' own fault.
"Certainly the quality control was lacking," said Charles Vick, who follows Russian space efforts for GlobalSecurity.org, "and testing the spacecraft ... was never done due to lack of funds."
Phobos-Grunt (Phobos is one of Mars' two moons; Grunt is Russian for ground) had an ambitious mission -- to orbit Mars, land on Phobos, scoop up a soil sample, and bring it home for study. Astronauts have brought back moon rocks, and an American probe returned minute samples from the tail of a comet in 2006, but Mars has been seen as the next destination in space.
Phobos, only 15 miles across, may be an asteroid that was captured by the gravity of Mars eons ago. Scientists would very much like to know what it is made of.
NASA's Opportunity rover, which landed on Mars in 2004, is still working, and a new, larger one, called Curiosity, is on the way there. In the half century since the space age began, Russia has tried and failed 19 times to reach Mars.
"Truly a travesty for the exploration of space," said Vick. "A loss for all concerned."
ABC News' Gina Sunseri contributed to this report.