A defunct man-made satellite the size of a bus is plunging toward Earth, and will likely arrive, breaking up on reentry, by the end of the week.

NASA officials say it's not likely to threaten people on the ground, but they are monitoring the 20-year-old satellite's plunge closely.

The Upper Atmospheric Research Satellite was launched in 1991 and deactivated in 2005 after completing its mission, according to SPACE.com. The vehicle weighs 6.5 tons, but is expected to burn up in the atmosphere.

Donald J. Kessler, a retired NASA scientist, finds concern about falling debris "a little ridiculous," the Los Angeles Times reports.

 

In an interview with The Times, he said that about one piece of space debris, or space junk if you prefer, falls out of orbit daily. The public generally doesn't hear about such events because the debris usually burns up in Earth's atmosphere. Kessler said it's almost impossible to predict where a piece of space debris that does not burn up in the atmosphere might fall.

"These things make it around the Earth once every 90 minutes," he said. "It could enter anywhere on that path, so you can't predict where it will be."

ABC News has a glimpse of the satellite (or its space shuttle courier, at least), back at the moment of launch in 1991, when it still had years of life and work ahead of it.