The unmanned Phobos-Grunt probe, seen in a Nov. 2 photo, was supposed to fly to Phobos, a moon of Mars, and bring samples of its soil back to Earth. (Russian Roscosmoc space agency/Associated Press)A Russian space probe became stuck in orbit Wednesday after an equipment failure, raising fears it could come crashing down and spill tons of highly toxic fuel on Earth unless engineers can steer it back to its flight path.

The Phobos-Grunt spacecraft was headed for one of Mars' two moons when it developed technical problems.

U.S. space and Defence Department officials are tracking it.

Officials at NASA in Washington figure it will be at least a week, maybe more, before the errant space probe falls back to Earth, if it does. The Russians are trying to get it back on course.

One independent U.S. expert on the Russian space program said the spacecraft could become the most dangerous manmade object ever to hit the planet. But those at the U.S. space agency and other space debris experts are far less worried. They believe the fuel will probably explode harmlessly in Earth's upper atmosphere.

NASA chief debris scientist Nicholas Johnson says the spacecraft's orbit is already starting to degrade slightly.

"From the orbits we're seeing from the U.S. Space Surveillance Network, it's going to be a couple weeks before it comes in," Johnson said Wednesday afternoon. "It's not going to be that immediate."

The unmanned $170 million Phobos-Grunt craft was successfully launched by a Zenit-2 booster rocket just after midnight Moscow time Wednesday (3:16 p.m. ET Tuesday) from the Baikonur cosmodrome in Kazakhstan. It separated from the booster about 11 minutes later and was supposed to fire its engines twice to set out on its path toward Mars, but never did.

Federal Space Agency chief Vladimir Popovkin said neither of the two engine burns worked, probably because the craft's orientation system failed.

Spacecraft could circle Earth for weeks before crash

He said engineers have three days to reset and fix the spacecraft's computer program before its batteries die — but the space agency later said the probe's orbit and its power sources could allow it to circle the Earth for about two weeks.

Russia news agencies cited space experts who offered widely varying estimates of how long the craft could stay in orbit before crashing down — from five days to one month.

The Zenit-2SB rocket carrying the Phobos-Grunt spacecraft blasted off successfully early Wednesday. But after the probe separated from the rocket, its engines failed to fire (Oleg Urusov, Pool/Associated Press)James Oberg, a NASA veteran who has written books on the Russian space program and who now works as a space consultant, said it's still possible to regain control over the probe.

"This is not an impossible challenge," Oberg said in an email to The Associated Press. "Nothing irreversibly bad has happened, the full propellant load is still available, and short-term 'stay healthy' manoeuvrs can be performed" like deploying the craft's solar panels to boost its power.

He warned, however, that if controllers failed to bring the Phobos-Grunt back to life, the tons of highly toxic fuel it carries would turn it into the most dangerous spacecraft ever to fall from orbit.

"About seven tons of nitrogen teroxide and hydrazine, which could freeze before ultimately entering, will make it the most toxic falling satellite ever," he said. "What was billed as the heaviest interplanetary probe ever may become one of the heaviest space derelicts to ever fall back to Earth out of control."

Oberg said such a crash could cause significantly more damage than the Russian Mars-96 that crashed in the Andes Mountains or the American USA 193 spy satellite that was shot down by a U.S. Navy missile in 2008 to prevent it from splashing its toxic fuel.

The Russian rescue effort Wednesday was being hampered by a limited earth-to-space communications network that already forced flight controllers to ask people in South America to help find the spacecraft. Amateur astronomers were the first to spot the trouble when they detected the craft was stuck in an Earth orbit.

The Phobos-Grunt spacecraft is 13.2 tonnes, with fuel accounting for a large share of its weight. It was manufactured by the Moscow-based NPO Lavochkin, which specializes in interplanetary vehicles.

Data Russia shared with NASA shows that about 11 tonnes of the spacecraft is fuel, Johnson said. The key is whether that fuel remains in liquid form or freezes. If it's liquid it would harmlessly blow up about 80 kilometres above ground, Johnson said. But if frozen, it could fall to Earth posing more of a hazard.

Most U.S. space debris experts believe it will likely stay liquid.

"We've had much larger objects than this come down and not have a problem," said William Ailor of the Aerospace Corp.'s Center for Orbital and Reentry Debris Studies.