The 6 1/2-ton Upper Atmosphere Research Satellite (UARS) was expected to fall to Earth sometime this afternoon (Sept. 23), but changes in the school bus-size satellite's motion may push it to early Saturday, according to NASA's latest observations of the spacecraft.
"The satellite's orientation or configuration apparently has changed, and that is now slowing its descent," NASA officials wrote in a status update Friday morning. "There is a low probability any debris that survives re-entry will land in the United States, but the possibility cannot be discounted because of this changing rate of descent." [Complete coverage of NASA's falling satellite]
NASA expects about 26 large pieces of the UARS spacecraft to survive re-entry through Earth's atmosphere and reach the planet's surface. The biggest piece should weigh about 300 pounds. The spacecraft is the largest NASA satellite to fall from space uncontrolled since 1979. [6 Biggest Spacecraft to Fall Uncontrolled From Space]
NASA officials have said the the chances that a piece of UARS debris hits and injures one of the nearly 7 million people on the planet are about 1 in 3,200. However, the personal odds of you being struck by UARS satellite debris are actually about 1 in several trillion, NASA officials have said.
As of 10:30 a.m. EDT (1430 GMT) Friday, the UARS satellite was flying in an orbit of about 100 miles by 105 miles (160 kilometers by 170 km), and dropping. NASA launched the UARS satellite in 1991 to study Earth's ozone layer and upper atmosphere. The satellite was decommissioned in 2005.
"Re-entry is expected late Friday, Sept. 23, or early Saturday, Sept. 24, Eastern Daylight Time," NASA officials wrote. "Solar activity is no longer the major factor in the satellite's rate of descent."
The sun has had an extremely active week, one that has included several solar flares. High solar activity can cause the Earth's atmosphere to heat and expand, which can increase drag on a low-flying satellite like UARS, making it fall faster.
Late Wednesday (Sept. 21), NASA predicted that the UARS satellite would not be over North America when it finally plunged down to the Earth's surface. That scenario has changed now that the 20-year-old satellite's descent has slowed, the agency said.
But where the UARS spacecraft will fall still remains anyone's guess. NASA orbital debris experts have said the satellite could fall anywhere between the latitudes of Northern Canada and Southern South America, a region of Earth that encompasses much of the planet.
"It is still too early to predict the time and location of re-entry with any certainty, but predictions will become more refined in the next 12 to 18 hours," NASA officials wrote in the latest update.
"It is still too early to predict the time and location of re-entry with any certainty," NASA said in a statement.
The Aerospace Corp., which tracks space debris, estimates the strike will happen sometime between about 6 p.m. and 4 a.m. EDT, which would make a huge difference in where the debris might wind up. Those late-night, early morning passes show the satellite flying over parts of the United States.
Any surviving wreckage is expected to be limited to a 500-mile swath.
The Upper Atmosphere Research Satellite, or UARS, will be the biggest NASA spacecraft to crash back to Earth, uncontrolled, since the post-Apollo 75-ton Skylab space station and the more than 10-ton Pegasus 2 satellite, both in 1979.
Russia's 135-ton Mir space station slammed through the atmosphere in 2001, but it was a controlled dive into the Pacific.
Some 26 pieces of the UARS satellite — representing 1,200 pounds of heavy metal — are expected to rain down somewhere. The biggest surviving chunk should be no more than 300 pounds.
Earthlings can take comfort in the fact that no one has ever been hurt by falling space junk — to anyone's knowledge — and there has been no serious property damage. NASA put the chances that somebody somewhere on Earth would get hurt at 1-in-3,200. But any one person's odds of being struck were estimated at 1-in-22 trillion, given there are 7 billion people on the planet.
"Keep in mind that we have bits of debris re-entering the atmosphere every single day," Matney said in brief remarks broadcast on NASA TV.
In any case, finders definitely aren't keepers.
Any surviving wreckage belongs to NASA, and it is against the law to keep or sell even the smallest piece. There are no toxic chemicals on board, but sharp edges could be dangerous, so the space agency is warning the public to keep hands off and call police.
The $740 million UARS was launched in 1991 from space shuttle Discovery to study the atmosphere and the ozone layer. At the time, the rules weren't as firm for safe satellite disposal; now a spacecraft must be built to burn up upon re-entry or have a motor to propel it into a much higher, long-term orbit.
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