A space satellite the size of a bus was expected to re-enter Earth's atmosphere overnight Friday, but experts say it's hard to pinpoint where the debris will land.

According to a statement issued by NASA on Friday evening, the 20-year-old Upper Atmosphere Research Satellite was orbiting 145 kilometres above Earth.

The agency was predicting the vessel would re-enter the atmosphere between 11 p.m. ET night and 3 a.m. Saturday morning, passing over Canada, Africa and Australia, as well as three oceans during that time.

When it falls, the satellite will break up into hundreds of pieces and scatter, with the debris field expected to span 800 kilometres. But exactly where it could land is still a mystery.

Randy Attwood, the managing editor of Space Quarterly, said it would likely fall over an ocean.

"The Earth is a very, very big place and there will be some pieces coming down that will survive re-entry," Attwood said.

NASA predicts that about 25 large chunks -- the largest about 135 kilograms -- will survive. Attwood thinks there is only an astronomically-small chance any of us will be hit.

"I wouldn't really worry about being hit. No one has ever been killed or even hurt by a re-entering satellite," he said.

In fact, the only confirmed incident was in 1997, when Lottie Williams of Tulsa, Okla., was grazed in the shoulder by a small bit of debris from a discarded piece of a Delta rocket.

NASA said the odds that someone will be struck by a piece of the satellite are very low: about 1 in 3,200. But -- and this is an important 'but' -- the chance you will be hit is much lower: 1 in 21 trillion.

The Upper Atmosphere Research Satellite was launched in 1991 aboard the space shuttle Discovery to study the ozone layer. But NASA decommissioned it in 2005, when it used up the last of the satellite's fuel to put it into a lower orbit. That set the stage for its uncontrolled return.

"Over time, its orbit has decreased because of drag," Attwood explains.

"Even though we think it's a vacuum up there in space, there are always air molecules, and it has been smashing into these air molecules, slowing down and so sometime later today it will hit some of the more dense parts of the atmosphere and begin to re-enter and break up."

If you are lucky to stumble upon what you think is a piece of the satellite, NASA warns you not to touch it. There will be no hazardous chemicals on it, but people can get hurt by sharp edges, the space agency said.

There are roughly 22,000 pieces of satellites, rockets, and other junk orbiting the Earth right now, and NASA is trying to watch each one of them through its Orbital Debris Program Office.

When UARS was build, there were no rules about satellite construction. But these days, satellites must be designed to disintegrate upon re-entry or have enough fuel to be steered into the ocean.

NASA estimates that medium-sized junk falls back to Earth once a week. Debris the size of this UARS satellite comes down about once a year, though this is the biggest NASA satellite to fall in three decades.

With files from The Associated Press