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.
This story was updated at 11:50 a.m. ET.
NASA's defunct UARS climate satellite fell back to Earth overnight, though agency officials are still trying to pinpoint exactly where and when.
The 6.5-ton Upper Atmosphere Research Satellite, or UARS, plunged through the atmosphere sometime between 11:23 p.m. EDT (0323 Sept. 24 GMT) on Friday (Sept. 23), and 1:09 a.m. EDT (0509 GMT) Saturday (Sept. 24), NASA officials said.
"The Joint Space Operations Center at Vandenberg Air Force Base in California said the satellite entered the atmosphere over the North Pacific Ocean, off the west coast of the United States," according to a NASA statement. "The precise re-entry time and location of any debris impacts are still being determined."
The spacecraft was the largest NASA satellite to fall uncontrolled from space in 32 years. So far, there is no evidence that anyone was harmed by falling UARS debris.
"NASA is not aware of any reports of injury or property damage," officials wrote in a statement.
The doomed satellite plummeted through Earth's atmosphere hours later than scientists had originally predicted, but despite its uncontrolled re-entry, agency officials maintained throughout that the risk to public safety was very remote. [Photos of NASA's Huge Falling Satellite UARS]
"The satellite was passing eastward over Canada and Africa as well as vast portions of the Pacific, Atlantic and Indian oceans during that period," NASA officials said in their latest update. "The precise re-entry time and location are not yet known with certainty."
NASA and the U.S. Air Force had been closely monitoring the UARS satellite, and increased solar activity during the week caused the dead spacecraft to plummet faster than was originally expected. As a result, agency officials projected that UARS would fall to Earth sometime this afternoon, but the satellite's tumbling, combined with more stable solar activity, slowed the spacecraft's rate of descent today. [Complete Coverage of NASA's Falling Satellite UARS]
NASA officials were unable to narrow down exactly when and where the satellite would fall ahead of time, but according to the agency's latest update, UARS was flying over Canada, Africa and parts of the Pacific, Atlantic and Indian Oceans during their projected window of re-entry.
NASA estimated that roughly 26 large pieces of the spacecraft would survive the re-entry process to be strewn over a 500-mile (804-kilometer) path. The largest piece of debris is expected to be approximately 300 pounds (136 kg), but it is still unconfirmed whether debris fell over land or water.
Finders aren't keepers
NASA has advised that when it comes to satellite remnants, it's not a case of finder's keepers. In fact, the agency urged anyone who comes across what they think is a piece of the UARS satellite, however unlikely, to leave it where it is and contact local authorities.
"Just remember, 'not UARS — or rather, yours — to keep,'" said space artifacts expert Robert Pearlman, editor of the website collectSPACE.com and a frequent SPACE.com contributor. "Should the public come across debris they believe to be from the downed satellite, they should alert the authorities, or the authorities may come after them."
Even if it is in fragments, the UARS satellite is still property of NASA and the U.S. government, Pearlman added, which makes the debris off limits for anyone thinking of keeping a piece as a souvenir.
"Regardless of where it falls, in or outside the borders of the nation, the U.S. has first right of refusal on any remnants, as either enforced by U.S. law or a United Nations treaty," Pearlman said.
But apart from the legal aspect, NASA's advice to not touch any potential pieces of satellite debris is also for safety reasons.
"Though UARS has no harmful chemicals or radiological material aboard, the nature of it being ripped apart during reentry may have formed sharp, jagged edges," Pearlman said. "NASA doesn't want to see anyone inadvertently harmed while trying to collect a souvenir."
Spectacular light show
As UARS' orbit decayed, some folks hoped the spacecraft's fiery demise would bring about a brilliant light show. At least one pair of lucky skywatchers in Florida were able to catch a glimpse of the doomed satellite as it circled the Earth before its final descent.
Thomas Marxo Jr. told SPACE.com that he and his wife spotted the UARS satellite Friday evening as it cruised overhead to the east of South Florida. Marxo had researched the time when the spacecraft was slated to make an orbital pass, but because of cloudy skies, was about to give up when he saw the dying satellite streak across the sky.
"It was very bright and traveling at a greater speed then any of the other satellites that I have observed," Marxo said in an email. "I feel as if I have observed a part of history that I can tell the grandkids about someday."
The $750 million UARS satellite is the largest NASA satellite to fall uncontrolled from space since 1979. Skylab, the first American space station, plummeted to Earth in 1979, and debris from the complex plunged into the Indian Ocean and onto parts of Australia.
Also in 1979, NASA's Pegasus 2 satellite, which was almost twice the mass of UARS, made an uncontrolled splashdown in the mid-Atlantic Ocean. Pegasus 2 was launched in 1965 to study micrometeoroids in low-Earth orbit. [6 Biggest Spacecraft to Fall Uncontrolled From Space]
UARS was launched in 1991 aboard the space shuttle Discovery to study the ozone layer and the chemical makeup of Earth's upper atmosphere. The satellite measured 35 feet (10.7 meters) long and 15 feet (4.5 m) wide. NASA decommissioned the satellite in December 2005.
Sustainability in space
Despite UARS' uncontrolled re-entry, NASA maintained that the chance of any pieces of the satellite falling on a densely populated area was extremely remote. To date, there have been no reported injuries or casualties from re-entering satellites or spacecraft.
In 1997, however, Lottie Williams of Tulsa, Okla., reported that she was struck on the shoulder by falling space junk while out walking. The debris was later confirmed to have been part of a fuel tank from a Delta 2 rocket, and other pieces of the booster were recovered several hundred miles away in Texas.
Still, orbital debris experts are hoping the fall of the UARS satellite will highlight the need to promote sustainability in space.
"This is not an uncommon event; space debris is re-entering out atmosphere all the time," said William Ailor, director of the Center for Orbital and Reentry Debris Studies at The Aerospace Corporation in California. "We are trying to learn everything that we can about orbital and re-entry debris so that we can protect space missions and human interests on the ground."