What went up is going to come down — and soon.

A failed Russian Mars probe stranded above Earth since November will plunge to its doom this weekend, likely on Sunday.

Experts expect the 11 tons of fuel on board to explode high in the atmosphere as friction eats through the craft’s aluminum tanks.

But predicting when and where space objects will fall is tricky. Solar flares and other “space weather” expand and contract Earth’s atmosphere, altering the drag on falling objects.

That means the spacecraft, called Phobos-Grunt, could plummet back to Earth over North America, South America, Europe, Asia or even Australia.

“It’s not possible to say where the thing is going to fall down,” Heiner Klinkrad, head of the orbital debris office at the European Space Agency in Darmstadt, Germany, said in an interview Friday.

Roscosmos, the Russian space agency, expects 500 pounds of the nearly 15-ton craft to survive reentry, with the rest incinerating. The agency’s latest prediction shows it crashing into the Atlantic Ocean just off the coast of South America.

But that prediction can change — and most likely will.

As the craft drops lower and lower, its path will become more certain. By late Saturday, Klinkrad said, the ESA will begin crossing out large areas of the world that will not be hit.

The ESA’s latest predictions peg the plunge for Sunday morning, Eastern time. But reentry could occur as early as Saturday night or as late as Monday morning, Klinkrad said.

The U.S. Strategic Command, the wing of the military tasked with tracking space debris, is also eyeing Sunday and Monday for the uncontrolled reentry.

“Unfortunately, we are not 100 percent definite until we don’t see it anymore” on radar screens, said Rodney Ellison, a spokesman for Strategic Command at Offutt Air Force Base in Omaha.

A joint statement issued Friday by NASA and the Federal Emergency Management Agency said the agencies are “closely monitoring the situation and are taking all necessary precautions and preparations to be ready to support our federal, state and local partners in the unlikely event that there are impacts to the United States.”

The statement urged anyone who finds space debris to contact local law enforcement.

Shortly after the Nov. 8 launch of Phobos-Grunt, a booster rocket failed to fire, stranding the probe. Repeated efforts by Roscomos and the ESA to control the craft were unsuccessful. The craft has been slowly dropping, and by Friday, it was just 115 miles high and descending more rapidly as the wispy upper atmosphere tugged on it.

Klinkrad said he and his staff will be “working day and night” to refine the craft’s reentry point. If Phobos-Grunt appears headed for land, the ESA will issue bulletins to its member states informing them of the risk. But odds are high that any debris will plow harmlessly into the ocean.

“In 50 years of modern spaceflight, we have not had one serious injury due to reentering space objects,” Klinkrad said.

A few U.S. scientists find themselves in the awkward position of hoping the hardest part of the craft will hit land — but not hurt anybody, of course. A cone-shaped capsule aboard Phobos-Grunt was designed to survive a fiery reentry. It holds a hardened, hockey-puck-size disk that the Planetary Society filled with hardy microbes and tiny animals called tardigrades, or water bears, to see if the critters might survive the rigors of a 34-month trip to Mars and back.

“If the probe should come in over land, it’s possible the return capsule could survive and be recovered,” said David Warmflash, a scientist who helped design the Living Interplanetary Flight Experiment, often called the LIFE experiment.The impending crash adds to concerns about the possible dangers of space debris. Last year, about 70 tons of space junk plummeted to Earth, including NASA’s school bus-size UARS satellite. During the UARS death watch in September, NASA estimated a 1-in-3,200 chance that the satellite could land on somebody. It didn’t. Instead, UARS broke up over the northern Pacific Ocean.

The problem of space junk has drawn increasing international attention. In 2007, the United Nations Committee on the Peaceful Uses of Outer Space adopted new guidelines for space-faring countries to minimize the dangers of space junk.

NASA now designs its satellites with thrusters that can point defunct craft toward the ocean or boost them into orbits so high they will never come back down, said agency spokeswoman Beth Dickey.

The Phobos-Grunt mission was supposed to be a technological triumph for Russia. The $163 million craft was to travel to one of the moons of Mars, collect a soil sample (“Grunt” is Russian for “ground”) and return the sample to Earth in a hardened capsule.

Such a return of a sample from Mars or its moons has never been attempted. It would have been a watershed moment for Russia.

Instead, Phobos-Grunt has become a searing embarrassment. The high-profile blunder has spurred Russian space officials to offer increasingly bizarre theories to explain the problem. In an interview published in the Russian newspaper Izvestia on Tuesday, the director of the Russian space agency, Vladimir Popovkin, implied that an anti-satellite weapon may have damaged Phobos-Grunt.

“We don’t want to accuse anybody, but there are very powerful devices that can influence spacecraft now,” Popovkin said. “The possibility they were used cannot be ruled out.”

Other Russian space experts said the cause of the failure was likely much more Earth-bound.

“My strong belief is there are systemic problems in the industry which build spacecraft,” said Anatoly Zak, a Russian space historian and journalist who publishes the Web site RussianSpaceWeb.com. “Before the launch, I said the chances of mission success were nearly zero.”