A Russian spacecraft designed to enhance the nation’s faded space glory in a mission to one of Mars’s moons has turned into one of the heaviest, most toxic pieces of space junk ever.

It’s something that is coming down because of an accident...

It will come crashing down to earth in just a few days.

The Russian space agency Roscosmos’s latest forecast has the unmanned Phobos-Ground probe falling out of earth’s orbit tomorrow or Monday, with the median time placing it over the Indian Ocean just north of Madagascar.

It said the precise time and place of its uncontrolled plunge can only be determined later, and unless someone actually spots fiery streaks in the sky, no one may ever know where any surviving pieces end up.

Space experts agree it is unlikely to pose big risks.

At 13.2 tonnes, the Phobos-Ground is one of the heaviest spacecraft ever to plummet to earth, considerably larger than the two defunct satellites which fell to earth last autumn and landed in the water.

It is cylindrical and about the size of a van.

Roscosmos predicted that only between 20 and 30 fragments of the Phobos probe with a total weight of up to 200kg will survive the re-entry and plummet to earth.

It is the third satellite to crash out of the sky in under five months.

An old Nasa atmospheric research satellite came tumbling down in September, and a German science satellite followed suit in October. But both were well past their prime.

Russia’s Phobos-Ground probe is still a mere baby. It was launched in November, and a glitch left it stranded in orbit around earth instead of bound for Mars to collect soil samples.

“What’s different about this re-entry is that it’s not a re-entry of an old, inert satellite that just was expected for years. It’s something that is coming down because of an accident... for me, that puts it in a different category,” said Jonathan McDowell, of the Harvard-Smithsonian Centre for Astrophysics in Cambridge, Massachusetts.

Another striking difference is the 11 tonnes of highly toxic rocket fuel on board Phobos-Ground, accounting for the bulk of its weight for the long journey to the Martian moon of Phobos.

This makes it potentially the most toxic spacecraft to fall ever.

Roscosmos insists all the fuel will burn in the atmosphere and pose no danger, and some experts in Russia and the West share that forecast.

And if it is any consolation, both of the two previous uncontrolled satellites harmlessly showered fragments over water.

“The fuel indeed poses lethal danger in case of close contact, but I haven’t heard of a single case of any civilians poisoned by rocket fuel from all the derelict satellites and failed rockets throughout the space era,” said Igor Lissov, an independent Moscow-based space observer. “The objective reality is that it burns on re-entry. There is no reason to panic”.

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

Such fears prompted the United States to shoot down its USA-193 spy satellite with a Navy missile in 2008. Some botched Russian rocket launches in the past have showered fragments over populated areas in Siberia and neighbouring Kazakhstan.

In the latest such mishap, fragments of a Russian satellite which failed to enter a designated orbit after its launch last month came down around Novosibirsk, the third-largest Russian city with a population of about 1.5 million, damaging some houses but injuring no one.

The fragments of the Meridian satellite, however, fell from a lower altitude at a far slower speed than the Phobos-Ground debris will have on re-entry.

Engineers from the Moscow-based company NPO Lavochkin, which built the Mars probe, said in an article giving a detailed description of the design that its fuel tanks are made of aluminium alloy.

That means they should melt early on re-entry, backing up official assurances that the fuel would burn up on its way down.

The 170 million dollar Phobos-Ground mission was Russia’s most expensive and the most ambitious space endeavour since Soviet times.

The spacecraft was intended to land on the crater-dented, potato-shaped Martian moon, 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.

The probe was successfully launched on November 9 and entered a preliminary orbit where its engines were supposed to fire to set it on its path to Mars.

They never did, and attempts to fix the glitch by Russian and European Space Agency experts failed.

Russia’s space chief has acknowledged the Phobos-Ground mission was ill-prepared and risks of its failure were high, but said that Roscosmos had to give it the go-ahead so as not to miss the limited earth-to-Mars launch window.

Phobos-Ground marked Russia’s first planned foray beyond earth’s orbit since a botched 1996 robotic mission to Mars.

That probe, designed by the same Lavochkin company, crashed shortly after launch due to an engine failure. The firm also built two other Phobos-bound probes that failed in 1988.