Russia's space agency said its doomed Phobos-Grunt mission will fall out of Earth's orbit Sunday or Monday, over the Indian Ocean, but poses no big risks.

Roscosmos claimed all the fuel will burn in the atmosphere and pose no danger. However, the space agency said the precise time and place of the crash can only be determined a few hours before its actually plunge.

The Phobos-Grunt, at14.6 tons, will be one of the heaviest spacecraft ever to plummet to Earth, reports said. But only between 20 and 30 fragments of the Phobos probe with a total weight of up to 200 kilograms will survive the re-entry, Roscosmos said.

The ambitious mission has been turned into one of the heaviest, most toxic pieces of space junk ever, experts said.

NPO Lavochkin, which built the Mars probe, said that based on the design, the craft's fuel tanks are made of aluminum alloy which should melt early on re-entry, backing up official assurances that the fuel would burn up on its way down.

Some experts pointed out, however, that part of the fuel might have frozen in the cold of space and could survive the fiery descent, which could be a threat if it spills over populated areas.

The probe was successfully launched Nov. 9 and entered a preliminary orbit where its engines were supposed to fire to set it on its path to Mars but never did. Attempts to fix the glitch by Russian and European Space Agency experts failed.

The $170-million Phobos-Ground mission, Russia's most expensive and the most ambitious space endeavor since Soviet times, was intended to land on the Martian moon Phobos, collect soil samples and fly them back to Earth, giving scientists precious materials that could shed more light on the genesis of the solar system.