NASA UARS satellite
The seven-ton Upper Atmosphere Research Satellite (UARS), which was deployed in 1991, is expected to re-enter the Earth's atmosphere. NASA

NASA's six-ton Upper Atmosphere Research Satellite, or UARS, has been slowly tumbling from orbit since its mission ended in 2005, and is now expected to re-enter the Earth's atmosphere and break apart on Friday, leaving scientists to try and find out where the debris will fall.

According to the space agency, while most of the debris will incinerate as it hurtles through the air, there is still 26 pieces which weigh a total of about 500kg and are expected to survive all the way and fall somewhere on Earth.

While NASA has pointed out that satellites as large as UARS re-enter Earth's atmosphere about once a year and there have been as of yet, no reports of any deaths or injuries to people from falling debris, experts are for now unable to make an exact prediction about when and where UARS will impact, but have managed to rule North America out of the potential impact zone.

"Re-entry is possible sometime during the afternoon or early evening of Sept. 23, Eastern Daylight Time. The satellite will not be passing over North America during that time period," NASA officials said in a statement late Thursday. "It is still too early to predict the time and location of re-entry with any more certainty, but predictions will become more refined in the next 24 hours," a statement reads.

Along with NASA, The Aerospace Corporation, which operates the Centre for Orbital and Reentry Debris Studies, has also been monitoring the satellite's re-entry into the Earth's atmosphere.

Dr. William Ailor, the director of the centre, backed the space agency's previous reassuring comments by also playing down fears of any potential injury arising from falling debris, by saying that in 50 years of objects placed into space and subsequently returning to Earth "only one person [has been] brushed on the shoulder by a very light piece of debris".

"And so, we all think that the probability of getting hit by something is very low, but the evidence says that that's the case," Ailor stated confidently.

"It's not zero for something like this, but it's very low that somebody would get hit."

"As far as the sky falling, I don't think so," he added.

Originally, NASA first believed that the UARS satellite would fall to Earth sometime between late September and early October, but heightened solar activity last week increased the spacecraft' course, pulling it down to Earth sooner than expected.