A veteran observatory comes down
NASA's Neil Gehrels Swift Observatory is expected to drop back into Earth's atmosphere in the next few days, according to Space.com. The reentry will be uncontrolled. A robotic rescue mission that was meant to push the telescope into a higher orbit did not succeed.
Swift went into orbit in November 2004. Over the following two decades it studied gamma-ray bursts, described as the most powerful explosions in the universe, along with other cosmic objects and events. It carried three instruments: the Burst Alert Telescope, the X-Ray Telescope and the Ultraviolet/Optical Telescope.
Neither the timing nor the location of the reentry has been pinned down, Space.com reported, though researchers are watching closely.
How the final weeks unfolded
Drag had pulled the roughly 1,500-kilogram (3,300-pound) spacecraft down to 275 kilometers (171 miles) by the time NASA gave an update on Oct. 20. The agency said the observatory could no longer counteract that drag. Swift entered safe mode. The operations team at Pennsylvania State University then sent a command that effectively ended its work before the fiery descent.
What experts expect during reentry
Dean Sladen, an aerospace engineer and quality manager at the U.K. components supplier Accu Components, described the likely sequence to Space.com. Once Swift sinks to about 120 kilometers (75 miles), he said, the end typically arrives within minutes. The air ahead of the spacecraft becomes hot enough to make it glow, and solar panels and antennas tend to tear away first. At around 78 kilometers (48 miles), the main body breaks apart, and most of it melts and vaporizes.
Sladen said most of Swift is aluminum, wiring and electronics, which burn away early. What reaches the surface is usually small, dense and heat-resistant, made of materials such as titanium, steel and glass. He identified the likeliest survivors as the six heavy reaction wheels used to aim the telescope, plus tougher optical parts such as lenses and mirrors. He said the observatory's mask of about 52,000 small lead tiles will almost certainly melt. In his view, only a handful of scorched fragments will reach the surface, and nothing will look like a telescope.
Darren McKnight, a senior technical fellow at LeoLabs, told Space.com that Swift will probably break up enough to avoid posing a casualty hazard. He added that the inner workings of the X-ray and optical telescopes might survive.
Where debris could land and how risky it is
Swift's orbit keeps it within about 21 degrees of the equator, Sladen noted. That band is mostly ocean, but it also crosses populated areas of Africa, South and Southeast Asia and Latin America. Sladen said any pieces that survive will most likely fall into the sea.
Sladen said he had not seen a detailed risk assessment published for Swift. He said that does not mean the work was skipped, since NASA's debris rules set a ceiling of one in 10,000 on the odds that surviving fragments hurt someone. He argued the agency should release the analysis, as it has for earlier satellites, so the public has facts instead of guesswork. NASA has said harm to people on the ground is unlikely, according to the report, and LeoLabs also describes the risk as very small.
Sladen said that satellites are now often designed with their ending in mind. Some carry enough propulsion to steer toward a remote stretch of ocean, while others use materials meant to burn up entirely, an approach known as design for demise. Spacecraft that cannot meet the one-in-10,000 limit are expected to come down in a controlled way, he said.
A growing trend
McKnight said the number of objects reentering each year is now 10 times higher than it was five years ago, driven by growing constellations in low Earth orbit and faster technology refresh cycles. He said Swift is only a small part of what is coming back. He added that he is more troubled by the abandonment of massive objects above 400 miles (644 kilometers).
Sladen made a similar point, saying the real concern is the long-term trend. As more satellites launch, uncontrolled reentries will become more common, which makes safe-descent design increasingly important, he said.
