Smithsonian.com

NASA's Upper Atmosphere Research Satellite, currently descending towards earth. Photo courtesy NASA

Earlier this week, NASA estimated that the falling Upper Atmosphere Research Satellite (UARS) would not land in North America, and that it would make landfall Friday morning. Now, the six-ton satellite’s destination and ETA are less certain: latest reports indicate it will come down sometime late Friday night or early Saturday morning EST, and that pieces of it could indeed land in the U.S.

Worrywarts across the country are fretting about the news, but it’s really not necessary, says Paul Ceruzzi, curator of Aerospace Electronics and Computing at the Air and Space Museum. “Debris coming down to earth is not the problem at all, because the atmosphere really protects us. We’re bombarded by meteors all the time,” he says. “The odds of some person, somewhere, getting hit by this satellite are 1 in 3,200. The odds of you getting hit is 1 in 22 trillion.”

The satellite, originally launched in 1991 as a tool to research the upper atmosphere, has been defunct for some time. “They were left with this satellite that had outlived its useful life. After they pushed it into a lower orbit, it ran out of fuel, so they lost the capability of doing anything else with it, and it’s been that way for several years,” Ceruzzi says.

“What’s happened now is something called the 11-year sunspot cycle,” he says. Currently, the increased level of sunspots has heated up the upper atmosphere, creating more drag on the satellite and gradually bringing it down.

Predicting the exact location where the satellite pieces will fall has been difficult, because the speed of descent has varied widely, and the earth rotates underneath the spacecraft as it orbits. Today’s NASA revisions came after scientists tracking UARS realized it was descending more slowly than expected.

“Because it’s now coming down tonight or tomorrow morning, it could land in the United States, which it couldn’t have done if it had landed this afternoon,” says Ceruzzi. The estimated 26 pieces that survive re-entry will shower down along a path that could extend to some 500 miles long. “It could be anywhere, really, between 57 degrees North and South latitude. It’s a very, very broad area,” he says.

Modeling the speed at which the object is falling is complicated by the satellite’s complex shape. “If it were a sphere, it would be a very straightforward calculation of drag. But because it’s an irregular rectangle, and it’s tumbling. It’s not coming in straight, it’s tumbling around, nobody knows which way it’ll be oriented,” Ceruzzi says. “Just a little bit of velocity here and there can make all the difference in the world.”

Ceruzzi notes that other, much larger satellites have repeatedly made impact with the earth, and have never caused an injury. And although the odds of the satellite injuring a person are very small, he notes that the problem of space debris is growing, and already affects our use of the earth’s orbit.

“It’s just like polluting a river, or Lake Erie or something. If it’s polluted, you can’t use it for fishing, or drinking, or recreation,” he says. “There are lanes of orbit in space that are filling up with debris, and you really can’t use them. If you put up a satellite in that lane, it’ll get hit by some debris, which will destroy the satellite. If a person is in space flight, they would get killed by it.”

Clouds of orbiting space debris have been caused by accidental satellite explosions, military maneuvers, and even unexpected collisions. This space junk, says Ceruzzi, is the real concern, rather than individual satellites such as UARS coming down to earth. “Things in lower orbit do eventually come back to earth, it sometimes takes a long time. But things in higher orbit can stay up there for hundreds or thousands of years,” he says. “These will continue causing trouble unless you go up there and somehow clean them out.”