By Alissa de Carbonnel
MOSCOW (Reuters) - Russia's failed Mars probe Phobos-Grunt is expected to plummet back to Earth on Sunday, sending space officials scrambling to predict where it will hit in the countdown to re-entry.
Space agency Roskosmos says debris from its doomed 14-ton spacecraft, which includes 11 tons of toxic rocket fuel, will fall to Earth between 1841 and 2105 GMT (1:41 and 4:05 p.m. EST).
Due to constant changes in the upper atmosphere, which is strongly influenced by solar activity, the exact time and place of the satellite's return is unknown.
The crash site could be anywhere along an elliptical orbit over a broad swathe of the globe, from a latitude of 51.4 degrees north - roughly as far north as London - to 51.4 degrees south, on the same latitude as the heel of Argentina.
The $165-million spacecraft, designed to retrieve soil samples from the Martian moon Phobos, was meant to be Russia's first successful interplanetary mission in over two decades.
But it became stuck in orbit after a botched launch on November 8, and has since been slowly losing altitude due to gravity's pull.
Experts say the falling space junk poses little risk. The probe's aluminum fuel tank is expected to burn up high in the atmosphere.
"If anyone gets to see it, it will be a fabulous show. I don't think there has been an explosion of such a large volume of fuel in space history," Igor Marinin, editor of the space journal Novosti Kosmonavtiki, told Reuters.
Some 20 to 30 small pieces of debris with a total weight of 200 kg (440 lbs) could hit Earth, Roskosmos said, adding that a tiny radioactive cargo of Cobalt-57 was too small to cause harm.
One component likely to survive re-entry is a small return capsule specifically designed to crash-land back on Earth in 2014, mission scientist Alexander Zakharov said.
"This is the capsule that was meant to bring back samples from Phobos, it's disappointing," Zakharov said. "We're hoping Roskosmos will approve a new craft to accomplish this mission."
Phobos-Grunt was one of five botched launches last year that marred celebrations of the 50th anniversary of Yuri Gagarin's pioneering first human space flight and hurt Moscow's pride.
In an apparent attempt to deflect blame, Russia's space agency chief hinted foreign sabotage might be the reason.
"I don't want to blame anyone, but there are very powerful means to interfere with spacecraft today whose use cannot be ruled out," Vladimir Popovkin told the daily Izvestia.
Stargazers worldwide are watching for reentry, including the Inter-Agency Space Debris Coordinating Committee, an offshoot of the United Nations Committee on the Peaceful Uses of Outer Space.
Under a U.N. space convention, Russia could be liable to pay compensation for any harm caused by bits of falling spacecraft.
In 1981, the Soviet Union paid Canada $3 million for the cost of cleaning up radioactive debris scattered in the crash of a Soviet nuclear-powered reconnaissance satellite, Kosmos 954.
With most of the planet's surface covered by water, Russia's errant space probe is likely to splash into the ocean.
When NASA's defunct Upper Atmosphere Research Satellite fell out of orbit in September, it showered debris into the Pacific Ocean. Germany's Rosat X-ray telescope re-entered a month later over the Bay of Bengal.
(Reporting By Alissa de Carbonnel; editing by Andrew Roche)