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Fireball picture: Hayabusa reenters the atmosphere.

The sample capsule from Japan's Hayabusa spacecraft became a fireball as it fell to Earth last June.

Photograph by Takashi Ozaki, Yomiuri Simbun/AP

Traci Watson

for National Geographic News

Published September 21, 2011

It may be doomed, but the NASA satellite that's about to crash-land on Earth isn't going out quietly. (Also see "Space Debris: Five Unexpected Objects That Fell to Earth.")

To scientists' surprise, the six-ton Upper Atmosphere Research Satellite, or UARS, has picked up speed and is now expected to plummet through the atmosphere Friday.

Only two weeks ago government scientists projected that the satellite could return to Earth as late as the first days of October.

"The spacecraft is coming in a little faster than we'd originally anticipated," said NASA orbital debris scientist Mark Matney. As a result, "it's coming in sooner rather than on the later side."

The satellite's speed is due to a recent spike in the amount of ultraviolet rays being emitted by the sun, Matney said. (See "As Sun Storms Ramp Up, Electric Grid Braces for Impact.")

The radiation increase caused Earth's atmosphere to expand, which increased drag on the satellite, causing it to fall faster.

(See "Solar Megastorm Could Cripple Satellites for a Decade.")

A Thousand Pounds to Survive Reentry?

Experts predict that most of the UARS spacecraft will burn up in Earth's atmosphere.

(Related: "Hayabusa Spacecraft Returns With Fiery Show.")

But more than 1,100 pounds (500 kilograms) of debris will probably survive the fiery plunge and slam down to Earth.

The biggest piece to reach the surface intact will most likely be a 300-pound (150-kilogram) piece of the spacecraft's frame.

However, it's still too early to know where the satellite's components will land, Matney said.

The only tip scientists can give for now about the location of the "debris footprint" is that it will be somewhere between 57 degrees north latitude and 57 degrees south latitude—an area encompassing most of Earth's populated land.

(Also see "Space Station to Fall to Earth—Find Out How and Where.")

Odds of Debris Hitting You: 1 in 3,200

UARS, which collected data on Earth's atmosphere from 1991 to 2005, was designed well before scientists started to worry about space debris.

(See "Space Junk Cleanup Needed, NASA Experts Warn.")

During the satellite's development, its creators focused on how to deploy it safely, not how it would end its days, said the University of Michigan's Wilbert Skinner, one of the lead scientists for UARS.

"I don't think we took any time at all wondering what its ultimate fate would be," Skinner said, adding that, at the time, such concerns were "background noise."

Though the satellite's death plunge will be dramatic—and will create a visible light show for nearby observers—it is unlikely to endanger anyone on the ground.

The odds that debris from UARS will strike a human are 1 in 3,200, NASA says.

"There's no need to panic—no need to put on a hard hat," Matney said. "The probability [of harm] is quite low, certainly compared to other daily risks that we're quite comfortable with."

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