NASA handout photo of the UARS satellite being deployed. REUTERS

An archaic NASA satellite the size of a bus is set to descend into the Earth's atmosphere this weekend, according to officials at NASA.

The six and a half-ton UARS spacecraft is scheduled to reach the surface of the planet sometime on Friday, 23 September, according to the latest projections.

The 20-year-old Upper Atmospheric Research Satellite was deactivated in 2005 after completing its mission but is expected to break apart on re-entry, with NASA speculating that there is a one in 3,200 chance that some of those parts could hit someone.

"Re-entry is expected Sept. 23, plus or minus a day," NASA officials said in a statement Sunday.

Predicting exactly where the satellite will touch down is an imprecise art, with a small discrepancy in the timing of the re-entry translating into thousands of miles of difference in the crash site.

At the present time, NASA predicts the satellite will crash somewhere between 57 degrees north latitude and 57 degrees south latitude, the Washington Post reports - putting the crash site anywhere between northern Newfoundland and the southernmost tip of South America.

However, Donald J. Kessler, a retired NASA scientist, is baffled by the amount of concern the story has provoked, the Los Angeles Times reports.

In an interview with the paper, he said that about one piece of space debris falls out of orbit every day.

Such events usually go unreported because the debris usually burns up in Earth's atmosphere. He added that it's almost impossible to predict where a piece of space debris that does not burn up in the atmosphere might fall.

"These things make it around the Earth once every 90 minutes," he said. "It could enter anywhere on that path, so you can't predict where it will be."