MOSCOW » A Russian space probe, which was designed to boost the nation’s pride on a bold mission to a moon of Mars but got stuck in Earth orbit instead, came down in flames Sunday, showering its fragments into the Pacific Ocean south of Chile’s coast.

The fragments of the Phobos-Ground landed in water 775 miles west of Wellington Island off Chile’s southern coast, the Russian military Air and Space Defense Forces said in a statement carried by the country’s news agencies.

The military space tracking facilities were monitoring the probe’s crash, its spokesman Col. Alexei Zolotukhin said.

The $170 million craft 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 because the toxic rocket fuel on board and most of the craft’s structure would burn up in the atmosphere high above the ground anyway.

The Phobos-Ground was designed to travel to one of Mars’ twin moons, Phobos, land on it, collect soil samples and fly them back to Earth in 2014 in one of the most daunting interplanetary missions ever. It got stranded in Earth’s orbit after its Nov. 9 launch, and efforts by Russian and European Space Agency experts to bring it back to life failed.

Prof. Heiner Klinkrad, Head of The European Space Agency’s Space Debris Office that was monitoring the probe’s descent, said the craft didn’t pose any significant risks.

"This one is way, way down in the ranking," he said in a telephone interview from his office in Berlin, adding that booster rockets contain more solid segments that may survive fiery re-entries.

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.

Russia’s space agency Roscosmos predicted that only between 20 and 30 fragments of the Phobos probe with a total weight of up to 200 kilograms (440 pounds) would survive the re-entry and plummet to Earth.

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

The Phobos-Ground weighed 14.9 tons, and that included a load of 12 tons of highly toxic rocket fuel intended for the long journey to the Martian moon of Phobos and left unused as the probe got stranded in orbit around Earth.

Roscosmos said that all of the fuel will 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 are made of aluminum alloy that has a very low melting temperature, and they will burst at an altitude of more than 60 miles.

The space era has seen far larger spacecraft crash. NASA’s Skylab space station that went down in 1979 weighed 85 tons and Russia’s Mir space station that de-orbited in 2001 weighed about 143 tons. Their descent fueled fears around the world, but the wreckage of both fell far away from populated areas.

The Phobos-Ground was Russia’s most expensive and the most ambitious space mission since Soviet times. Its mission to the crater-dented, potato-shaped Martian moon was to give scientists precious materials that could shed more light on the genesis of the solar system.

Russia’s space chief has acknowledged the Phobos-Ground mission was ill-prepared, but said that Roscosmos had to give it the go-ahead so as not to miss the limited Earth-to-Mars launch window.

Its predecessor, Mars-96, which was built by the same Moscow-based NPO Lavochkin company, experienced an engine failure and crashed shortly after its launch in 1996. Its crash drew strong international fears because of around 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 contained a tiny quantity of the radioactive metal Cobalt-57 in one of its instruments, but Roscosmos said it poses no threat of radioactive contamination.

The spacecraft also carried a small cylinder with a collection of microbes as part of an experiment by the Pasadena, California-based Planetary Society that designed to explore whether they can survive interplanetary travel. The cylinder is attached to a capsule that was supposed to deliver Phobos ground samples back to Earth.

Igor Marinin, the editor of Russia’s Novosti Kosmonavtiki magazine, said on Russia’s NTV television that it would likely be destroyed.