One of the heaviest pieces of space junk ever to fall back to Earth is set to come crashing down on Sunday or Monday.

Russia's Phobos-Grunt probe was launched on Nov. 9, 2011, but suffered a technical failure and never left orbit.

Roughly the size of a small bus and weighing 13.2 tonnes, the probe that was supposed to get to the Martian moon Phobos is a unique piece of returning space debris.

While most probes falling to Earth are derelict and at the end of their functional lives, Phobos-Grunt — which translates as Phobos-Ground — is relatively new and carries a full complement of rocket fuel.

The fuel, which accounts for 11 tonnes of the spacecraft’s weight, is highly toxic, but the Russian space agency expects the fuel to burn up when the probe enters Earth’s atmosphere on Sunday or Monday.

Experts still don’t have a precise idea where the remnants of the satellite will land.

Huge changes

"Re-entry times are estimates," said Michel Doyon, manager of flight operations for the Canadian Space Agency.

"Slight changes in variables will have huge changes. Even up to a few hours before, it's hard to estimate."

Variables affecting trajectory estimates include the density of the atmosphere, the velocity that the object is falling at and if the object is spinning or stationary, he said.

All these factors contribute to the overall drag coefficient of the object.

"When things are falling, these variables — mainly the atmospheric density — affect the drag," he said.

Doyon said that current estimates have the fragments from Phobos-Grunt landing in one of four locations: the middle of the Pacific Ocean, the south of the Atlantic Ocean, near northern Africa in the region of Morocco or somewhere in China.

Low chance of falling on someone

He also said that if parts of the probe do reach Earth, the likelihood of them striking or injuring someone is extremely low.

"Without knowing really where it’s going to fall, three chances out of four have it landing in water," he said. "Chances are fairly low of hitting someone."

When dealing with space debris, the bigger risks aren’t from objects entering Earth’s atmosphere from space, but instead from objects colliding there, Doyon said.

"The main concern right now is the objects that aren’t falling," he said. "Right now there are approximately 20,000 objects over 10 centimetres travelling around the Earth. Each of these objects is tracked and catalogued."

The U.S. Defence Department's Joint Space Operations Centre in Vandenburg, Calif., tracks the objects and notifies countries that own them if a collision is likely to happen. Doyon said once the countries are alerted of a potential collision, they take steps to avoid it and alter orbits of objects such as satellites.

Assessing the threat

"We analyze the data and assess the threat," he said. "Once we assess the data, we decide if it’s best to move the satellite."

The number of possible collisions has risen greatly over the past few years because the amount of space debris has increased.

The collision between a U.S. iridium communications satellite and a defunct Russian satellite in 2009 and the Chinese destruction of a weather satellite in 2007 have created an additional 5,000 to 6,000 pieces of debris.

"Those two collisions caused the majority of the issues," Doyon said.

With files from The Associated Press