Artist's conception of UARS. Credit: NASA

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 the possibility of 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 Research 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. Based on the spacecraft's construction, NASA estimated that 26 pieces totaling half a metric ton would survive the fiery breakup of reentry, with a 1-in-3,200 chance of striking someone somewhere on Earth. 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, the German ROSAT spacecraft followed suit, plunging back to Earth over the Bay of Bengal. ROSAT was smaller than UARS but had some sturdier components; as a result the German Aerospace Center stated that up to 30 pieces of the satellite, with a total mass of 1.7 metric tons, could make it back to Earth. The estimated odds of debris hitting someone this time around were a bit higher, at 1 in 2,000. Again the fall proved benign; no ROSAT debris has been recovered.

The closely timed reentries of UARS and ROSAT is a mere coincidence, as it turns out. But neither was an exceptionally rare event. Pieces of debris, whether defunct spacecraft, rocket bodies or other mission by-products, are falling from space all the time—we just don't usually notice them, talk about them, or run scary television-news segments about them. "Reentries are very, very routine," says Nicholas Johnson of NASA's Johnson Space Center in Houston, the agency's chief scientist for orbital debris.

The circumstances surrounding UARS and ROSAT simply attracted more attention to the reentries than Earthbound debris usually gets. Part of the reason that the UARS satellite's fall turned so many heads was that the bus-size spacecraft was relatively large, and NASA held a news conference as UARS was nearing its final descent. "UARS was the biggest NASA satellite to reenter uncontrolled in over 30 years," Johnson says. "So for us it was a noteworthy event." And ROSAT's relatively high debris risk was an attention-getter for that satellite, despite its pedestrian size.

Artist's conception of ROSAT. Credit: DLR

Humankind has survived decades of spacecraft and orbital debris falling to Earth—including pieces larger than either UARS or ROSAT—without significant incident, thanks to the fact that most debris is small and most of Earth is ocean or sparsely populated land. And Johnson notes that the rate of rocket launches is lower now than it was at the height of the cold war. "I think we need to put things in perspective," he says. "UARS was a little bit larger than normal, but the international community has a satellite of that mass coming in about once a year." Objects comparable in mass to ROSAT come back even more frequently. Large pieces of debris from various spent rockets also commonly fall from orbit and land on Earth, including a number of fragments of Delta 2 rockets from GPS satellite launches. A piece of orbital debris reenters the atmosphere more or less daily.

Johnson and his colleagues keep a list of all NASA objects in orbit, including an estimate of when reentering objects will make the fiery plunge into the atmosphere. "We have nothing large coming in for quite a while," Johnson says. "The biggest thing we have is an Atlas Centaur upper stage next year." That rocket stage weighs in at about 2.2 metric tons, about the same as ROSAT. Also in 2012, we will witness the notable, if likely uneventful, return of the Explorer 8 satellite after more than 50 years in space. NASA launched the 40-kilogram probe in 1960 to study the ionosphere, the region of charged particles in Earth's upper atmosphere.

Not only are reentries by satellites and debris fairly commonplace, they are actively courted. NASA has a rule on the books stating that dead spacecraft be moved into a safe "graveyard" orbit or into an accelerated atmospheric demise within 25 years of decommissioning. The longer a spacecraft stays in orbit, after all, the more likely it is to collide with something else, yielding a shower of dangerous fragments that can in turn produce further collisions. (Just such a smash-up occurred in 2009, when a dead Russian satellite collided with an intact Iridium communications satellite.) "We did lower the orbit of UARS with all remaining propellant that was onboard," Johnson says, even though the satellite predated the 25-year rule.

NASA is not too keen to have dangerous debris raining down on the planet if it can be avoided, however. So the space agency attempts to "design for demise" when building spacecraft. "One of the things we do now in the early design phase is we look at the materials and we look at the potential for survivability," Johnson says. "And if it looks like some of these things would survive reentry, we go look and see if we can't replace them." NASA guidelines, enacted after the deployment of UARS, now strive to prevent uncontrolled reentries that pose as much of a risk as UARS did.

International Space Station. Credit: NASA

With very large spacecraft, such as the Hubble Space Telescope or the International Space Station, an uncontrolled reentry is not at all palatable, so the plan is to use thrusters to drive those craft into the ocean—far from inhabited land—when their time comes. "We've always thought about bringing it back in a controlled manner," Johnson says of Hubble, which is about twice as massive as UARS was. "We put a unit on board that would allow a propulsion unit to be installed at a later date. NASA is committed to at some point, prior to its natural reentry, going up and attaching a propulsion system."

The space station, too, will need some thrust from attached vehicles to return it to Earth safely. "ISS will come in, probably in one piece, over the ocean," Johnson says. "It may take more than one [vehicle] because ISS is so massive. We're confident that we can do it—we just hope it won't be for a while."