MOSCOW (AP) - A Russian spacecraft on its way to Mars with 12 tons of toxic fuel is stuck circling the wrong planet: ours. And it could come crashing back to Earth in a couple weeks if engineers can't coax it back on track.

Space experts were hopeful Wednesday that the space probe's silent engines can be fired to send it off to Mars. If not, it will plummet to Earth. But most U.S. space debris experts think the fuel on board would explode harmlessly in the upper atmosphere and never reach the ground.

The launch mishap was the latest in a series of recent Russian failures that have raised concerns about the condition of the country's space industries.

The unmanned $170 million Phobos-Ground craft successfully got into orbit, propelled off the ground by a Zenit-2 booster rocket just after midnight Moscow time Wednesday from the Baikonur cosmodrome in Kazakhstan. After separating from its booster, 11 minutes later, it was supposed to fire its engines twice and head to Mars.

Neither engine fired. So the spacecraft couldn't leave Earth's orbit, flying between 129 and 212 miles above Earth. And that orbit is already deteriorating, according to American satellite tracking.

The Federal Space Agency said the probe's orbit and its power sources could allow it to circle the Earth for about two weeks. That jibes with calculations made by NASA.

"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," NASA chief debris scientist Nicholas Johnson said Wednesday afternoon. "It's not going to be that immediate."

The craft was aiming to get ground samples from Phobos, one of Mars' two moons, and return them in a daring expedition hailed by eager scientists, who said it may include bits of Mars that may have been trapped on its moon.

Amateur astronomers were the first to spot the trouble when they detected the craft was stuck in an Earth orbit.