Watch the skies this Friday as a defunct NASA satellite is expected make a fiery re-entry to the Earth's atmosphere.

Scientists at NASA are predicting the 6.5 ton Upper Atmosphere Research Satellite (UARS) will hit Earth on Friday, although it could happen a day earlier or later.

The 20-year-old satellite was originally expected to re-enter the Earth's atmosphere either late September or early October but the re-entry has advanced due to a sharp increase in solar activity since the beginning of this week.

It's a little bit unpredictable, and as a result, it's coming in a little faster than we originally anticipated. said Mark Mateny of the Orbital Debris Program at Johnson Space Center in Houston, Texas.

Most of the 6.5-ton UARS' parts will burn up in Earth's atmosphere when it finally falls from orbit later this week.

Though NASA said the chances of the wreckages of the defunct satellite falling to the Earth is very remote, it has warned that the uncontrolled falling of the debris may cause risk to billions of people.

Pieces of various sizes -- the largest probably 300 pounds, or the weight of a refrigerator -- could land across an area of 500 miles. That means pieces could fall over most of the world's six inhabited continents as well as its three largest oceans.

We believe there will be 26 different components that will hit the surface of the Earth somewhere with a total mass of a little over 500 kilograms [1,100 lbs], NASA's orbital debris program chief scientist Nick Johnson said. The largest piece that we think is going to come back is part of the structure of UARS and it is going to have a mass of just in excess of 150 kilograms, so a little more than 300 pounds.

Johnson also pointed out there has been no incident of anyone in the world being injured by falling debris in the 54-year history of the space age.

The odds of any one individual among the Earth's 7 billion inhabitants being struck by the satellite are about one in 21 trillion.

UARS was a $750 million mission deployed from the shuttle Discovery in 1991 to study the Earth's atmosphere and its interactions with the Sun. It measured important ozone depletion related to climate change. According to NASA, readings from UARS gave evidence that Chlorine in the atmosphere is at the root of the polar ozone hole.