There’s a nearly 6-ton big-as-a-bus and pretty much dead climate satellite—the largest in three decades—about to smack uncontrolled into planet Earth. There’s also a chance, albeit a ridiculously small one, that someone could get hit by one of its several dozens pieces made up of titanium, aluminum, steel and beryllium—the biggest weighs close to 300 pounds—if they live somewhere in the potential debris drop zone.
Nothing like that’s actually happened before, but NASA says there’s still a 1-in-3,200 chance debris from the Upper Atmosphere Research Satellite, or UARS, could hit a person somewhere on the planet when it falls back to Earth sometime Friday, September 23. Not great odds for UARS, but not exactly “pfft, your fear of flying’s completely irrational!” either. The chances you’ll get hit, by contrast, are another matter entirely: try somewhere in the neighborhood of 1-in-trillions, says NASA. Contrast with the chances you’ll die in a plane crash, around 1 in 2,000,000.
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“There’s always a concern,” says Mark Matney (via Space), a scientist with NASA’s Orbital Debris Program Office. “But, populated areas are a small fraction of the Earth’s surface. Much of the Earth’s surface has either no people or very few people. We believe that the risk is very modest.”
So you’re probably safe, but at 1-in-3,200, someone else might not be. You’ll probably never die in a plane crash, but some people do. Just because it probably won’t be you doesn’t make those odds any less unsettling.
But wait, can’t we track our movements in cars or on foot down to the last step? Snap pictures of buildings, vehicles and even ourselves from space? Shouldn’t we be able to tell where this thing’s going to splash down? Why can’t we just pilot the thing to crash, as they say, “in the drink”? After all, most of the planet’s water, isn’t it?
Well, there’s the part about it being dead, which is why they’re calling its entry “uncontrolled.” It’s fairly old as satellites go, deployed back in September 1991, and thus it went up before rules were in place requiring space objects like this to meet certain standards, such as controlled re-entry systems, mitigating re-entry crashdown risks to the Earth’s human populace.
But there’s also the part where, as an object’s orbit decays, we’re simply unable to predict in fine detail where it’ll wind up—call it space debris’ “cone of uncertainty,” except this one stretches latitudinally from northern Canada all the way to southern South America. So to answer the question posed in the post’s title: they won’t. Not in any sort of actionable detail, anyway.
Space junk expert Brian Weeden tells NPR you can blame factors like: observational errors, the shifting density of the Earth’s atmosphere, basic weather patterns, and the irregularity, shape-wise, of the satellite itself. “And all of that makes it a very complex issue and one that’s very, very hard to predict accurately,” says Weeden.
Of course, if you’re lucky enough to safely witness a piece of this thing sparking across the sky, or even bump into a piece of it on the ground after it’s come down, let this warning serve for all: leave it alone.
“If you find something you think may be a piece of UARS, do not touch it,” says NASA. “Contact a local law enforcement official for assistance.”