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.

A Thousand Pounds to Survive Reentry?

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

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.

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.

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.