Two well-publicized satellite falls a month apart got me wondering: Is this the new normal? After all, there is plenty of junk in orbit, and it can’t stay up there forever. And NASA, along with many other space agencies, now requires that satellites tumble back to Earth sooner rather than later once their useful lifetimes have ended so as to limit collisions in orbit. So how often are we going to be hearing about inbound satellites—and worrying about the ever so slim chance that they might kill us? A call to NASA’s top orbital debris scientist clarified the issue and reassured me that we are not now witnessing the leading edge of a debris storm.
But first a brief recap. In September, NASA’s defunct Upper Atmosphere Re-search Satellite, or UARS, came streaking back into Earth’s atmosphere. UARS returned to Earth uncontrolled, meaning that NASA and the U.S. military could only guess where the pieces might land. Ultimately UARS did the world a favor and plunked itself down in the remote South Pacific, out of harm’s way. Just a month later Germany’s smaller ROSAT spacecraft followed suit, diving back to Earth over the Bay of Bengal. Again the fall proved benign.
Neither event, as it turns out, was all that rare. Pieces of space junk, whether derelict spacecraft, rocket bodies or other mission by-products, fall from orbit more or less daily. UARS turned so many heads because it “was the biggest NASA satellite to reenter uncontrolled in over 30 years,” says Nicholas Johnson, NASA’s chief scientist for orbital debris at the Johnson Space Center in Houston. And ROSAT came with a relatively high debris risk because of its construction.
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But UARS-size objects belonging to other space-faring agencies fall from orbit roughly once a year; ROSAT-size junk is even more common. Humankind has survived decades of reentries without significant incident, thanks to the fact that most of Earth is ocean or sparsely populated land. “Reentries are very, very routine,” Johnson says. And rules enacted since the launch of UARS are helping to ensure our safety. Engineers now “design for demise” when building spacecraft, swapping out materials that survive reentry.
Johnson and his colleagues keep a list of all NASA objects in orbit, including an estimate of when those objects will make the fiery plunge into the atmosphere. Two huge ones on the books are the Hubble Space Telescope and the International Space Station. The plan with both is to use thrusters to drive the craft into the ocean when their time comes.