One of NASA’s hardest-working space telescopes has gone quiet. The Neil Gehrels Swift Observatory, which spent nearly 22 years chasing the most powerful explosions in the universe, has ended its science mission. The spacecraft made its final observation on October 20 and is expected to re-enter Earth’s atmosphere in the coming weeks.

NASA announced the end of the mission on Friday, and SpaceNews reports that re-entry is expected in November. Here is what happened, and why astronomers are calling Swift’s legacy a scientific bonanza.

Why Swift had to stop

Every satellite in low Earth orbit feels a little drag from the thin upper atmosphere, and that drag slowly pulls it lower. According to NASA’s Swift blog, recent solar activity made the effect much stronger for Swift, which began sinking rapidly.

NASA had looked at a rescue. A high-risk, fast-turnaround commercial servicing mission was meant to boost the observatory to a higher orbit, but it ended in August without making an attempt. To buy time for that mission, the Swift team had paused science observations to minimize drag. Once the boost was off the table, the team restarted observations, and Swift went on to study hundreds of targets, from the remains of exploded stars to active supermassive black holes in distant galaxies and comets in our own solar system.

Pointing the telescopes again increased drag, and the spacecraft sank faster. On October 20, Swift reached an altitude of 171 miles (275 kilometers). At that height it could no longer compensate for the drag and entered safe mode. The operations team at Penn State then sent a command that effectively ended Swift’s operations to prepare it for re-entry.

A two-year mission that lasted more than ten times as long

Swift launched on November 20, 2004, and began work the following month. It was designed for a two-year primary mission. “Swift delivered an incredible, lasting return on investment for the nation, designed for a two-year primary mission but productive for more than 10 times as long,” said Shawn Domagal-Goldman, director of the Astrophysics Division at NASA Headquarters, in NASA’s end-of-mission announcement.

The observatory was built to solve a specific problem. Gamma-ray bursts flare up and fade so quickly that big telescopes could rarely be pointed at them in time. Swift was designed to detect a burst, pinpoint its location, alert other observatories, and swing its own X-ray and optical/ultraviolet telescopes onto the spot before the afterglow faded. According to principal investigator S. Bradley Cenko of NASA’s Goddard Space Flight Center, that usually took Swift about a minute.

In 2018, NASA renamed the mission in honor of Neil Gehrels, its first principal investigator, following his death.

The numbers behind the legacy

NASA’s tally of Swift’s career is impressive:

  • More than 2,000 gamma-ray bursts observed
  • More than 1,400 supernova explosions studied
  • About 7,000 published scientific papers that used Swift data
  • More than 1.5 million images from its Ultraviolet/Optical Telescope
  • A public archive of roughly 12 terabytes, spanning nearly 22 years

Swift also witnessed stars being shredded by black holes, monitored active galaxies, carried out an all-sky X-ray survey and studied asteroids and comets.

The 0.03-second breakthrough

One of Swift’s signature achievements came early. Short gamma-ray bursts reach peak brightness in less than two seconds, and before Swift no one had been able to locate them fast enough to catch their afterglows. In May 2005, Swift turned toward a burst called GRB 050509B, which lasted just 0.03 seconds, quickly enough to detect 11 X-ray photons. It became the first short burst with a detected afterglow.

Those observations supported long-standing theories that short gamma-ray bursts likely come from the mergers of two neutron stars, collapsed objects with about the mass of the Sun packed into something the size of a city.

A flexible multitool for astronomers

Speed was not Swift’s only strength. Its planning system let astronomers request “target-of-opportunity” observations that could be commanded from the ground in as little as a few minutes, and it could schedule up to 75 independent targets a day. That flexibility turned Swift into a go-to tool for anyone studying fast-changing events in the sky.

What comes next

NASA says it will keep looking for ways to respond quickly to cosmic events without Swift, using existing missions to help fill the gap and exploring options ranging from fast-to-orbit commercial capabilities to new missions. Meanwhile, Swift’s open archive will remain available to researchers everywhere.

Swift’s final weeks in orbit will end with a fiery plunge, but its data will keep producing discoveries for decades. For a mission that was supposed to last two years, that is quite a run.

Image: Swift Observatory rendering back alpha (SVS13926) by NASA/Goddard Space Flight Center/Chris Smith (KBRwyle) via Wikimedia Commons, Public Domain Mark 1.0