For two decades NASA's Neil Gehrels Swift Observatory has answered whenever the high-energy universe called. Now its science work is ending, just as its capabilities might be most valuable. The spacecraft is falling uncontrolled and will burn up in Earth's atmosphere within days.
Launched in 2004, the telescope was built to study gamma-ray bursts, among the most powerful explosions ever seen, which often faded before slower telescopes could respond. Its name reflects its ability to swing into action the moment a flash appeared.
Image: NASA — https://science.nasa.gov
A mission that outlasted its plan
The mission was meant to last two years. It has run for almost twenty-two. Over time the satellite lost roughly half its original altitude, as tiny collisions with atmospheric molecules slowed it bit by bit. Recent years of intense solar activity sped that up, ruling out the 2030s that engineers had hoped for. An attempt to boost its orbit failed, and the descent became unavoidable.
A unique view of the sky
Swift carried three instruments: a Burst Alert Telescope that watched gamma rays across up to half the visible sky with enough resolution to pinpoint sources, and two more that could be pointed quickly, one for X-rays and one for visible and ultraviolet light. Together they gave a multi-wavelength view of the same patch of sky in short order, a pairing no other observatory has matched.
"It's such a unique telescope," says Wen-fai Fong of Northwestern University, who is not part of the mission team. "It has all these views of the universe in one observatory, and it can rapidly point any of these instruments."
Discoveries from the first month
The scientific record is broad. Within a month of launch, in December 2004, Swift had observed a gamma-ray burst that helped establish that a highly magnetized neutron star could trigger such an event. Its observations also challenged the classic split between long bursts from exploding stars and short bursts from merging neutron stars, turning up events that fit neither category cleanly. "The more data you get, the more confused you get," says Lynn Cominsky of Sonoma State University.
Image: NASA — https://science.nasa.gov/missions/swift/nasas-swift-mission-ends/
All of Swift's data was made public almost immediately, a practice that Fong credits with inspiring other observatory teams to do the same.
Beyond gamma rays
As its reputation grew, other astronomers asked to use the nimble observatory for objects that change across the sky. In its later years it observed five to ten opportunistic targets on most days, adding up to hundreds or thousands a year. Its most important partnerships were with observatories hunting neutrinos and gravitational waves. In 2017 Swift took part in two back-to-back discoveries, and since then no attempt to find light linked to a neutrino or gravitational-wave signal has succeeded.
What remains
Current and future observatories will cover part of Swift's work, but not all of it. During the six months of pause this year, while preparing the boost attempt, researchers already felt its absence. Judith Racusin of NASA Goddard, who worked with Swift in its early years, says the loss will be felt. What endures is the archive of data that scientists will keep mining long after operations end.
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