Russia's defence ministry says a failed probe designed to travel to a moon of Mars has crashed, showering debris over the southern Pacific, according to news reports.

The ministry said the fragments fell Sunday 1,250 kilometres west of Chile's Wellington Island.

The Phobos-Ground was one of the heaviest and most toxic pieces of space junk ever to crash to Earth, but space officials and experts said the risks posed by its crash were minimal as the probe's toxic rocket fuel and most of the craft's structure were to burn up in the atmosphere anyway.

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

"The resulting risk isn't significant," said Prof. Heiner Klinkrad, head of the European Space Agency's Space Debris Office that is monitoring the probe's descent.

20 to 30 fragments were expected to survive re-entry

Roscosmos had predicted that only between 20 and 30 fragments of the probe with a total weight of up to 200 kilograms would survive the re-entry and plummet to Earth.

Klinkrad agreed with that assessment, adding that about 100 tonnes of space junk fall on Earth every year. "This is 200 kilograms out of these 100 tonnes," he said.

Thousands of pieces of derelict space vehicles orbit Earth, occasionally posing danger to astronauts and satellites in orbit, but as far as is known, no one has ever been hurt by falling space debris.

The Phobos-Ground weighed 13.5 tonnes, and that included a load of 11 tonnes of highly toxic rocket fuel intended for the long journey to the Martian moon of Phobos. It has been left unused as the probe got stuck in orbit around Earth shortly after its Nov. 9 launch.

Roscosmos said earlier all of the fuel would burn up on re-entry, a forecast Klinkrad said was supported by calculations done by NASA and the ESA. He said the craft's tanks were made of aluminum alloy that has a very low melting temperature, and they would be bursting at an altitude of more than 100 kilometres.

The space era has seen far larger spacecraft to crash. NASA's Skylab space station that went down in 1979 weighed 77 tonnes and Russia's Mir space station that deorbited in 2001 weighed about 130 metric tonnes. Their descent fuelled fears around the world, but the wreckage of both fell far away from populated areas.

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.

Bob McDonald, the host of CBC's Quirks and Quarks, said scientists were disappointed that the mission failed.

"No one has ever gone to the moon of another planet and then brought soil back to Earth, so scientists were really looking forward to it. The craft was also carrying a Chinese probe that was piggy-backing to go on to Mars along the way, so there's a lot of disappointment today."

The predecessor to Phobos-Ground, Mars-96, which was built by the same Moscow-based NPO Lavochkin company, also suffered an engine failure and crashed shortly after its launch in 1996. Its crash drew strong international fears because of some 200 grams of plutonium onboard.

The craft eventually showered its fragments over the Chile-Bolivia border in the Andes Mountains, and the pieces were never recovered.

The worst ever radiation spill from a derelict space vehicle came in January 1978 when the nuclear-powered Cosmos 954 satellite crashed over northwestern Canada. The Soviets claimed the craft completely burned up on re-entry, but a massive recovery effort by Canadian authorities recovered a dozen fragments, most of which were radioactive.

The Phobos-Ground also contains a tiny quantity of the radioactive metal Cobalt-57 in one of its instruments, but Roscosmos said it poses no threat of radioactive contamination.

With files from the CBC