An equipment failure on a Russian space probe has raised concerns that it could come crashing back to Earth in a couple of weeks unless engineers can put it on its designated path out of orbit.
The spacecraft was headed for one of Mars' two moons when it developed technical problems early Wednesday, Moscow time.
NASA and Defense Department officials are tracking it. If the Phobos-Grunt probe's problem can't be fixed, it will be a couple of weeks before it falls back to Earth, NASA officials said in Washington.
The Russians are trying to get the spacecraft back on course.
NBC News space analyst James Oberg said Phobos-Grunt (Russian for "Phobos-Soil") could become the most dangerous manmade object ever to hit the planet. But experts at the U.S. space agency and other space debris experts are 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 Phobos-Grunt craft was successfully launched by a Zenit-2 booster rocket just after midnight Moscow time (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.
The $170 million mission is aimed at getting ground samples from Phobos, one of Mars' two moons, and bringing them back to Earth.
Latest in a string of failures
Wednesday's mishap was the latest in a series of recent Russian failures that have raised concerns about the condition of the country's space industries.
Federal Space Agency chief Vladimir Popovkin said neither of the two engine burns worked, probably because the craft's orientation system failed. 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 that the probe's orbit and its power sources could allow it to circle the Earth for about two weeks.
Russian 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.
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' maneuvers 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-Ground back to life, the tons of highly toxic fuel it carries could turn it into the most dangerous spacecraft ever to fall from orbit.
"About seven tons of nitrogen tetroxide 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 probe, which crashed in the Andes Mountains in 1996; or the USA-193 spy satellite that was shot down by a U.S. Navy missile in 2008 to prevent it from splashing its toxic fuel.
Later Wednesday, Oberg said he was "growing more confident as we realize that the vehicle is healthy; it didn't blow up."
"They have a chance of doing a Hubble repair, an Apollo 13, 'snatching victory out of jaws of defeat' kind of thing," he said.
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.
Phobos-Grunt was Russia's first interplanetary mission since Mars-96, which failed when the probe crashed shortly after the launch due to an engine failure.
The spacecraft is 13.2 metric tons (14.6 English tons), 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 that Russia shared with NASA shows that about 11 metric tons 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 50 miles (80 kilometers) 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 Re-entry Debris Studies.
Misfortune on the way to Mars
NPO Lavochkin also designed the craft for Russia's botched 1996 launch and the two probes sent to Phobos in 1988, which also failed. One was lost a few months after the launch due to an operator's mistake, and contact was lost with its twin when it was orbiting Mars.
The Russian space agency responded to the failures by promising to establish its own quality inspection teams at rocket factories to tighten production oversight.
In contrast with the failures that dogged Soviet and Russian efforts to explore Mars, several NASA landers and rovers, including Spirit and Opportunity, have successfully studied the Red Planet.
If Russian space experts manage to fix the Phobos-Grunt, it should reach Mars orbit in September 2012 and land on Phobos in February 2013. The return vehicle is expected to carry up to 200 grams (7 ounces) of ground samples from Phobos back to Earth in August 2014.
Phobos-Grunt's flight plan is arguably the most challenging unmanned interplanetary mission ever. It requires a long series of precision maneuvers for the probe to reach the potato-shaped moon measuring just 20 kilometers (over 12 miles) in diameter, land on its cratered surface, scrape it for samples and fly back.
Scientists hoped that studies of Phobos' surface could help solve the mystery of its origin and shed more light on the genesis of the solar system. Some believe Phobos is an asteroid captured by Mars' gravity, while others think it's debris from when Mars collided with another celestial object.
China contributed to the mission by adding a mini-satellite that is to be released when the craft enters an orbit around Mars on its way to Phobos. The 115-kilogram (250-pound) satellite, Yinghuo-1, was due to become the first Chinese spacecraft to explore Mars, studying the planet during two years in orbit.
"If this had worked it would be a fantastic mission," said Cornell University astronomer Steve Squyres, who has worked on several successful and failed Mars probes. "It is a reminder, if we needed one, that space exploration is hard and Mars missions are tricky."
NASA has its own Mars mission ahead: A car-sized rover called Curiosity is set to launch Nov. 25 from Florida and touch down on the Martian surface next summer.
AP science writer Seth Borenstein contributed from Washington. This report also was supplemented by msnbc.com.