Scramble is on to tell satellite's landing site

9/21/2011

CAPE CANAVERAL, Fla. -- NASA scientists are doing their best to tell us where a plummeting six-ton satellite will fall later this week. If they're off a little bit, it could mean the difference between hitting Florida or landing on New York, Iran, or India.

Pinpointing where and when hurtling space debris will strike is an imprecise science. For now, scientists predict the earliest it will hit is tomorrow U.S. time, the latest Saturday. The strike zone covers most of Earth.

Not that citizens need to take cover. The satellite will break into pieces, and NASA put the chances that somebody on Earth will get hurt at 1 in 3,200. But any one person's odds of being struck have been estimated at 1 in 21 trillion. As far as anyone knows, falling debris has never injured anyone. Nor has significant property damage been reported. That's because most of Earth is covered in water and has vast regions of empty land.

NASA doesn't want you to pick up anything that falls. The space agency says there are no toxic chemicals present, but there could be sharp edges. Also, it's government property. It's against the law to keep it as a souvenir or sell it on eBay. NASA's advice is to tell the police.

The 20-year-old research satellite is expected to break into more than 100 pieces as it enters the atmosphere, most of it burning up. Twenty-six of the heaviest metal parts are expected to reach Earth, the biggest chunk weighing about 300 pounds. The debris could be scattered over an area about 500 miles long.

Jonathan McDowell isn't worried. He is in the potential strike zone, along with most of the world's 7 billion citizens. He is with the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

"There's stuff that's heavy that falls out of the sky almost every year," he said. So far this year, he noted, two massive Russian rocket stages have taken the plunge.

As for the odds of the satellite hitting someone, "it's a small chance. We take much bigger chances all the time in our lives," he said. "So I'm not putting my tin helmet on or hiding under a rock."

All told, 1,200 pounds of wreckage is expected to smack down -- the heaviest pieces made of titanium, stainless steel, or beryllium. That represents just one-tenth the mass of the satellite, which stretches 35 feet long and 15 feet in diameter.

The strike zone straddles all points between latitudes 57 degrees north and 57 degrees south. That's as far north as Edmonton, Alberta, Canada, and Aberdeen, Scotland, and as far south as Cape Horn, the southernmost tip of South America. Every continent but Antarctica is in the cross hairs.

Back when the Upper Atmosphere Research Satellite was launched to study the ozone layer in 1991, NASA didn't always pay attention to the "what goes up must come down" rule. Nowadays, satellites must be designed to burn up on re-entering the atmosphere or have enough fuel to be steered into a watery grave or up into a higher, long-term orbit.

Predicting where the satellite will strike is a bit like predicting weather several days out, said NASA orbital debris scientist Mark Matney.

Experts expect to have a good idea by tomorrow about when and where UARS might fall, Mr. Matney said. They won't be able to pinpoint the exact time, but they should be able to narrow it to a few hours.

Given the spacecraft's orbital speed of 17,500 mph, or 5 miles per second, a prediction that is off by just a few minutes could mean a 1,000-mile error. It probably won't be clear where it fell until afterward, he said.

If it happens in darkness, it should be visible.