A U.S. spy satellite, reportedly the size of a school bus, will re-enter the atmosphere and plunge to Earth some time before April.

However, where it will land, nobody knows.

"They're usually controlled," Sarah Poirier, staff astronomer with the Ontario Science Centre in Toronto, told CTV.ca of such celestial descents.

"You might have an object of this size falling back into Earth every three or four weeks, but normally they can control where it comes down," she said.

"For spy satellites in particular, they want to control where it lands so it doesn't end up in some foreign country where their secrets can come out," Poirier said.

"This is a bit of an anomaly," as the satellite is out of control, she said. The object lost power and U.S. authorities lost the ability to communicate with the satellite.

Such crashes happen more often than one might think, Poirier said. There are up to 3,000 satellites and another 10,000 pieces of space junk floating around up there, waiting to return to Earth.

Satellites of all types come equipped with small rockets and rocket fuel. Because of the atmosphere's drag effect or the need to avoid space junk, scientists must periodically adjust a satellite's orbit, she said.

During controlled re-entries, scientists try to aim the satellite so that it can land in an ocean. For example, NASA guided the Compton Gamma Ray Observatory to land in the Pacific Ocean in 2002.

The U.S. can track this satellite's path as it descends. It will have a good idea of where the satellite will hit about a day before it happens, she said.

"They can then notify the local authorities," Poirier said.

Danger is minimal

The now-defunct satire magazine National Lampoon once made fun of satellite crash fears with a faux news story that NASA, the U.S. space agency, had decided to give hardhats to every resident of Wyoming -- something it claimed to do every so often for a deserving state.

That article was published about the time of the 1979 Skylab crash, the U.S.'s first space station. The uncontrolled re-entry of that 78-tonne object left a trail of debris over western Australia and the Indian Ocean.

The risk to people from even an uncontrolled re-entry should still be minimal, she said, adding she's never heard of anyone being injured or killed by falling space debris.

That's not to say it couldn't happen if a person is in the wrong place at the wrong time.

Poirier said pieces of debris about the size of a cafeteria tray could survive re-entry and strike Earth.

Think about an object that size moving at least 320 kilometres per hour, or the speed of a dive-bombing peregrine falcon.

"It could do some serious damage," she said.

Another issue is the satellite's rocket fuel, a substance known as hydrazine.

Poirier said that substance has toxic effects on the liver, kidneys and central nervous system. "But the engineers are saying the fuel tanks should break up and burn up before they hit the ground," she said.

In Canada's most famous case of toxic satellite debris, Cosmos 954, a nuclear-powered satellite belonging to the then-Soviet Union, broke up over the North.

The satellite's debris landed on Jan. 24, 1978, hitting mainly near Great Slave Lake in the Northwest Territories. However, fragments were found in Alberta and Saskatchewan -- an area of about 124,000 square kilometres.

Efforts to recover the debris lasted until October and cost $15 million and only picked up a tiny fraction of the material. The Soviets eventually paid about half that bill.

Profiting from space junk

Cleaning up space junk costs big bucks. But if a piece of this satellite were to land in your backyard, it could put some jingle in your jeans.

"There's a crazy market out there for meteorites or pieces of asteroids that crash down," Poirier said.

"I'm sure there's a market on eBay for this stuff. I think this stuff can potentially be valuable if you do find it.

"And certainly if you find a piece of a government spy satellite and you report it, I'm sure they're going to be all over you," Poirier said.

Did she mean a visit from the "Men in Black"?

"Yeah, exactly - and waving their pen in front of your eyes!" she joked, referring to the 1997 movie starring Will Smith and Tommy Lee Jones as alien-hunting government agents.