An artist's concept of the Upper Atmosphere Research Satellite (UARS) satellite in space. The 6 1/2-ton satellite was deployed from space shuttle Discovery in 1991 and decommissioned in December 2005.
For the latest news about NASA's UARS spacecraft fall, visit: Falling NASA Satellite: Complete Coverage of UARS Spacecraft's Fiery Demise.
A decommissioned NASA satellite is expected to plummet to Earth today (Sept. 23), and agency officials are monitoring the dead spacecraft closely to try to narrow down when and where the debris will fall.
According to NASA, the Upper Atmosphere Research Satellite, or UARS, will make its fiery descent through the atmosphere some time this afternoon or early evening (Eastern Daylight Time), but while it is still too soon to tell where pieces of the defunct satellite will land, scientists have been able to rule out North America from the potential impact zone.
"Re-entry is possible sometime during the afternoon or early evening of Sept. 23, Eastern Daylight Time. The satellite will not be passing over North America during that time period," NASA officials said in a statement late Thursday. "It is still too early to predict the time and location of re-entry with any more certainty, but predictions will become more refined in the next 24 hours." [Photos of NASA's Huge Falling Satellite UARS]
The bus-size, 6.5-ton UARS spacecraft is one of the largest NASA satellites to plunge back to Earth uncontrolled in more than 30 years.
"The most massive NASA satellite to re-enter uncontrolled since Skylab was the Pegasus 2 satellite in November 1979," Nick Johnson, chief scientist of NASA's Orbital Debris Program Office at the Johnson Space Center in Houston, told SPACE.com in an e-mail last week. "It had a mass of 10.5 metric tons, almost twice that of UARS."
Skylab, the first American space station, fell to Earth in 1979, and debris from the outpost plunged into the Indian Ocean and onto parts of Australia. The Pegasus 2 satellite was launched in 1965 to study micrometeoroids in low-Earth orbit. The satellite remained in orbit for 14 years before splashing down into the mid-Atlantic Ocean on Nov. 3, 1979.[6 Biggest Spacecraft to Fall Uncontrolled From Space]
Despite the fact that UARS will fall to Earth uncontrolled, NASA officials have stated that the chance of debris landing in populated regions of the planet is extremely remote.
"There's always a concern," said Mark Matney, a scientist at the Orbital Debris Program Office. "But, populated areas are a small fraction of the Earth's surface. Much of the Earth's surface has either no people or very few people. We believe that the risk is very modest."
Scientists at the Orbital Debris Program Office at the agency's Johnson Space Center in Houston calculated the odds that anyone anywhere in the world will be hit by UARS debris at 1-in-3,200. But, the chance that you personally will get hit is much smaller, somewhere on the order of 1-in-several trillion, NASA officials said. [How You Can Track the Falling Satellite]
Agency officials have estimated that roughly 26 large pieces of the satellite will survive the intense journey through Earth's atmosphere. Approximately 1,170 pounds (532 kilograms) of material is expected to actually reach the ground, but even though the spacecraft is expected to re-enter over a 500-mile (804-kilometer) path, these pieces will likely fall over water or uninhabited areas of the planet, Matney said. [Where Could UARS Satellite Debris Fall?]
The $750 million climate satellite measures 35 feet (10.7 meters) long and 15 feet (4.5 m) wide. It was launched in 1991 aboard the space shuttle Discovery, and studied the ozone layer and chemical makeup of Earth's upper atmosphere before it was decommissioned in December 2005.
Originally, NASA anticipated that the UARS satellite would fall to Earth sometime between late September and early October, but heightened solar activity last week increased the spacecraft's drag, pulling it down to Earth sooner than expected. You can track the UARS satellite fall using several skywatching tools, including the Satellite AR Android app created by the company Analytical Graphics Inc.
Editor's note: If you snap a photo or observe the re-entry of NASA's UARS satellite and want to share it with SPACE.com for a story or gallery, contact managing editor Tariq Malik at: firstname.lastname@example.org.