NASA's Swift space telescope is expected to crash back to Earth in the next few days.
The Neil Gehrels Swift Observatory will reenter Earth's atmosphere in an uncontrolled fashion, after a robotic rescue mission failed to boost the scope to a higher orbit.
Swift launched to Earth orbit back in November 2004. Over the next two decades, it studied gamma-ray bursts — the most powerful explosions in the universe — and other cosmic objects and events using a trio of telescopes: a Burst Alert Telescope (BAT), an X-Ray Telescope (XRT), and its Ultraviolet/Optical Telescope (UVOT).
On Oct. 20, NASA reported that Swift's orbit had decayed to 171 miles (275 kilometers) above Earth and that the roughly 3,300-pound (1,500-kilogram) observatory was no longer able to compensate for the effects of the increased drag. It entered safe mode, followed by Swift's operations team at Pennsylvania State University sending a command to the spacecraft that effectively ended its operations ahead of its destructive and fiery nose-dive.
It's unclear at the moment exactly when and where Swift will plunge to Earth. But researchers definitely have their eyes peeled.
A surge of reentries
In physics fashion, for the most part, what goes up must come down — with more space hardware to follow.
Thanks to the growing surge in low Earth orbit constellations and technology refresh cycles, there are 10 times more objects reentering per year than there were just five years ago, said Darren McKnight, a senior technical fellow at LeoLabs, a company that keeps a watchful eye on what's going on above our planet.
"It is unfortunate that Swift was not 'reorbited,' but from a reentry hazard perspective it is a small portion of the stuff coming back in. The rub on this, of course, is that, while many people bemoan the reentry of more objects, I am much more upset about the rapid rate of abandoning massive objects above 400 miles [644 km]," McKnight told Space.com.
Swift will likely disintegrate sufficiently upon reentry to not pose a ground casualty hazard, McKnight said. However, the internal workings of the observatory's X-ray and optical telescope might survive, McKnight said.
Where in the world?
Similar in view is Dean Sladen, a quality manager and aerospace engineer at the U.K. precision-components supplier Accu Components.
"Once Swift falls to about 120 kilometers [75 miles], the end usually comes within a few minutes. The air in front of it heats up so much that the satellite will start to glow. Solar panels and antennas are typically the first to tear off, and at around 78 kilometers [48 miles], roughly eight times the height of a passing airliner, the main body breaks apart. Most of it will melt and vaporize," said Sladen.
While it isn't yet known exactly where any surviving pieces will land, Sladen noted that Swift's orbit only passes over a band within about 21 degrees of the equator.
"That band is mostly ocean, though it also crosses populated parts of Africa, South and Southeast Asia and Latin America," he said.
Surviving leftovers
As for which parts are most likely to survive and reach ocean or ground, Sladen pointed out that most of Swift's body is aluminum, wiring and electronics, which melt away early in the reentry process. What reaches Earth is usually small, dense and heat-resistant, such as titanium, steel and glass.
"The most likely survivors are its six heavy reaction wheels, the spinning flywheels it used to aim itself, along with tougher telescope parts such as lenses and mirrors. Its mask of around 52,000 tiny lead tiles will almost certainly melt. Only a handful of small, scorched fragments are expected to make it down — nothing recognizable as a telescope," Sladen said.
Sladen said that, at the current time, it appears that no detailed risk assessment has been published for Swift's fall.
"However, this does not mean NASA has not done the work," said Sladen.
"NASA's own orbital debris requirements state that the risk of human casualty from surviving debris shall not exceed one in 10,000, so it is highly likely this work has been completed but not published," he told Space.com. "Personally, I believe it should be published, as NASA has done with previous satellites. The risk is almost certainly very small, but it would give people a clear, factual picture rather than guesswork."
Design for demise
Sladen added that there's far more focus on how a satellite's life ends these days than there was in the past.
"Many now carry enough propulsion to steer themselves into a remote stretch of ocean, or are built from materials designed to burn up completely, an approach called 'design for demise.' NASA and the European Space Agency limit the chance of anyone being hurt by a reentry to one in 10,000, and if a satellite can't meet that, it's expected to come down in a controlled way," said Sladen.
So, should people be worried about having a nasty run-in with Swift leftovers?
Sladen doesn't think so. NASA has said that harm to anyone on the ground is unlikely, and LeoLabs also describes the risk as very small. Most of the spacecraft will burn up, and any surviving pieces will most likely fall into the sea, he said.
"The real concern is the long-term trend. With more satellites being launched than ever, the number of uncontrolled reentries is growing, which is why designing satellites to come down safely matters more and more," Sladen concluded.
