NASA says debris from a decommissioned satellite is expected to re-enter the Earth's atmosphere sometime Friday, but it is unclear where the expected debris will land.

The space agency said Thursday it has ruled out North America as a crash site for the 5.4 ton satellite, which is about the size of a city bus. It is expected to burn brightly enough to be spotted during daylight hours and break apart into about 26 pieces as it re-enters the atmosphere.

NASA says the risk to public safety or property is “extremely small.” The agency says there is a one-in-3,200 chance that satellite debris would injure someone. NASA says the debris is most likely to crash into one of the world's oceans, since water makes up most of the Earth's surface.

But NASA officials warn that people who think they have found a piece of the satellite should not touch it.

The prospect of space junk falling from the sky has generated considerable interest among U.S. media and users of social media.

The space agency says debris from the Upper Atmosphere Research Satellite could be scattered along a path more than 800 kilometers long.

NASA launched UARS in 1991 on a three-year mission to study ozone levels in the atmosphere. NASA took UARS out of service in 2005.

Since then, the satellite was placed into lower orbit to avoid any collision with the International Space Station.

The satellite is the largest piece of space debris to fall to Earth since NASA's Skylab space station crashed in western Australia 32 years ago. No one was hurt.