The biggest satellite to fall to Earth in decades will come crashing down in the new few days. And while odds are not good that a person will be hit -- officials can't rule that out.

NASA's Upper Atmosphere Research Satellite, or UARS, will likely crash into Earth on or around Friday, Sept. 23. The satellite is the size of a bus, and it's the biggest to make an uncontrolled re-entry into Earth's atmosphere in decades, NASA officials say

Odds are just 1-in-3,200 that falling satellite debris will hit a person on the ground, according to NASA and experts, but the possibility does exist. However, odds that you will get hit by debris are just 1-in-21 trillion, NASA said, since some 7 billion inhabit the Earth.

NASA expects at least 26 large pieces of the defunct, bus-size Upper Atmospheric Research Satellite (UARS) to fall to the ground around Sept. 23, plus or minus a day. The 6 1/2-ton NASA climate probe was launched in 1991 and shut down in 2005 after completing its mission.

Now, it's falling to Earth and NASA says the window of reaching the ground has been narrowed to three days.

Re-entry is expected Sept. 23, plus or minus a day, NASA said in a statement. The re-entry of UARS is advancing because of a sharp increase in solar activity since the beginning of this week.

A NASA spokesman told SPACE.com that the reason the UARS is expected to fall early in its re-entry window is due to a sharp rise in solar activity. Solar impact from the sun can yield an extra pull on satellites, NASA officials have said.

The debris is expected to fall in an area of Earth that's 500 miles long, NASA said, though the agency can't be certain. Forecasts suggest the debris will land somewhere between the latitudes of northern Canada and southern South America. But because the majority of the Earth's surface is covered in water, an ocean landing is most likely, experts have said.

Look at how much of Earth is covered with water, Victoria Samson, the Washington Office Director of the Secure World Foundation, told SPACE.com this week. There's a really good chance it's going to go straight into the ocean.

But NASA officials have told ABC News they won't know where the satellite will hit until two hours before it enters the Earth's atmosphere, moving at a speed of some five miles per second.

NASA scientists say the 26 different components identified as falling to the ground weigh in total mass more than 1,200 pounds. Many satellites have re-entered the atmosphere, landing on Earth over the years, yet to date nobody has been injured from falling debris said experts.

NASA Space NASA NASA