The Russian space agency, Roscosmos, which has given up hope of recovering the spacecraft, is now concentrating on figuring out what will happen to the pieces that fall to Earth.
The agency expects about 20 to 30 fragments totaling about 440 pounds (200 kilograms) to reach the ground, with very low risk to people.
"Due to its predominantly low-melting construction materials, Phobos-Grunt will largely burn up during its reentry, so the corresponding risk to the population on ground is low," said Heiner Klinkrad, head of the space-debris office at the European Space Agency's space operations center in Darmstadt, Germany.
Phobos-Grunt is also carrying 11 tons of toxic fuel propellants, all of which should be consumed by the massive fireball at reentry.
"Roscosmos does not expect any environmental contamination—neither for an impact on ground nor at sea," Kilnkrad noted.
Predicting Mars Probe's Reentry Is Tricky
Phobos-Grunt was meant to go to Phobos—one of the red planet's two moons—where it would have scooped up soil samples and sent them back to Earth in 2014.
But the Russian probe was doomed soon after reaching orbit on November 8, 2011, when the spacecraft failed to fire its rockets and got stuck in Earth's orbit.
As the probe circles about 120 miles (200 kilometers) above Earth, Phobos-Grunt's orbit is rapidly lowering due to drag from Earth's outer atmosphere, Klinkrad said.
Predicting the probe's precise reentry time and location have been tricky, since it will depend on how the probe behaves in its final days and hours in orbit.
"When the 14-ton spacecraft completes its last orbits, at altitudes below 120 kilometers [75 miles], aerodynamic forces can break off parts of the appendices, like solar arrays," Klinkrad said.
In addition, "short-term changes in solar and geomagnetic activity can [have] a strong influence on the ambient atmospheric air density and affect the orbit lifetime," he said.
(Related: "Pictures: Five 'Cursed' Mars Missions.")
Meanwhile, before it meets its demise, Phobos-Grunt can be spotted in the night sky without any optical aid whatsoever—if you know what to look for.
"Under favorable conditions, Phobos-Grunt is readily visible to the unaided-eye as a bright, rapidly moving, starlike object," said veteran satellite tracker Ted Molczan of Toronto, Canada.
"Observers have noted a pronounced orange hue, which results from the gold-colored thermal blanket that covers most of its surface."
But the real sky show will occur when the dead probe burns up in the atmosphere.
As it sinks into the denser layers of Earth's atmosphere, the probe will heat up and begin to glow brightly, forming a long plasma tail and resembling "a surreal-looking comet," Molczan said.
"Eventually, the combination of extreme heat and rapid deceleration will cause it to fragment into many pieces that will spread out along the path of descent," he said.
"The debris trail will move rapidly across the sky, visible for perhaps one to two minutes, assuming [a viewer has a] reasonably unobstructed view of the sky."
(Related pictures: "Space Debris—Five Unexpected Objects That Fell to Earth.")
If any pieces do make it to the ground, the European Space Agency's Klinkrad warns people not to take them as souvenirs.
"If someone finds a fragment, it should be turned in to national authorities, because they remain the property of Russia."