Look out below! A defunct satellite the size of a school bus is falling back to Earth. NASA's Upper Atmosphere Research Satellite, or UARS, is expected to reenter the atmosphere at the end of September. And pieces of the satellite will probably survive the fiery breakup.

"We looked at the hundreds of different components on UARS, and we believe that we know what's going to survive. We just don't know where.”

Nick Johnson of NASA's Orbital Debris Program.

“And we believe it's going to be 26 different components that will hit the surface of the Earth somewhere, with a total mass a little over 500 kilograms."

UARS studied the atmosphere from 1991 to 2005. It's 35 feet long and about 13,000 pounds, which is large by space junk standards. But debris falls from space all the time, and big objects the size of UARS come down about once a year. Since Earth is mostly water and sparsely populated land, chances are UARS won't cause any harm.

But “we just will not know precisely where it's going to come down until it's come down." So keep watching the skies.

—John Matson

[The above text is a transcript of this podcast.]