Update.
Wednesday, Sep. 21, 2011
RE-ENTRY ALERT: UARS, a NASA satellite the size of a small bus, will re-enter the atmosphere later this week. Best estimates place the re-entry time during the late hours of Sept. 23rd over a still-unknown region of Earth. "It is still too early to predict the time and location of re-entry," says NASA. "Predictions will become more refined over the next two days."
The disintegration of UARS is expected to produce a fireball that could be visible even in broad daylight. Not all of the spacecraft will burn up in the atmosphere, however; according to a NASA risk assessment, as many as 26 potentially hazardous pieces of debris could be scattered along a ground track some 500 miles long. The same report puts the odds of a human casualty at 1 in 3200.
On Sept. 15th, astrophotographer Theirry Legault video-recorded the doomed satellite during one of its last passes over France:
"The satellite appears to be tumbling, perhaps because a collision with satellite debris a few years ago," notes Legault. "The variations in brightness are rapid and easily visible to the human eye." (Other observers have reported UARS flashes almost as bright as Venus.)