RESOLVING THE LEDGER
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NASA's Neil Gehrels Swift Observatory is back to studying the universe after spending months in a low-drag configuration while awaiting an orbital rescue mission that ultimately fell short.
Mission controllers restarted two of Swift's three science instruments — its X-ray Telescope (XRT) and Ultraviolet/Optical Telescope (UVOT) — on Aug. 26. Both instruments had been shut down in February as part of an effort to minimize atmospheric drag and keep the aging spacecraft in orbit long enough for another spacecraft to reach and boost it to a higher altitude, according to an Aug. 28 statement from NASA.
Swift's third instrument, the Burst Alert Telescope (BAT), remains offline. Operations of the BAT were halted in April to conserve power, allowing controllers to position Swift's solar panels in a way that further reduced drag from its interaction with Earth's atmosphere . NASA says the mission team hopes to resume data collection with the instrument within the next few weeks.
"Over the last 21 years, Swift has been NASA's multitool for studying the cosmos ," NASA officials said in the statement. "It observes a wide range of light, rapidly points at short-lived outbursts, and sends alerts to other facilities in space and on the ground to help coordinate follow-up observations."
The return to science follows NASA's decision to abandon plans to boost Swift's orbit using LINK, a robotic servicing spacecraft built by the Arizona company Katalyst Space. LINK launched July 3 aboard a Northrop Grumman Pegasus XL rocket with the ambitious goal of rendezvousing with Swift, grabbing the observatory and raising its orbit.
However, the rescue mission encountered trouble in late July when LINK developed attitude-control problems that caused it to spin and disrupted communications. Two of the spacecraft's three reaction wheels were found to be inoperable, while its cold-gas thruster system also experienced some loss of functionality.
Although engineers managed to reduce LINK's spin and regain attitude control, NASA and Katalyst announced Aug. 19 that the spacecraft would no longer attempt to capture and boost Swift. Instead, LINK is expected to rendezvous with Swift and perform proximity operations, providing an opportunity to demonstrate technologies that could be used for future in-space satellite servicing missions — leaving Swift without the orbital boost intended to extend its life and its future increasingly uncertain.
Launched in 2004, Swift was originally designed for a two-year mission studying gamma-ray bursts , the most powerful explosions in the universe. More than two decades later, the observatory continues to detect and rapidly follow up on fleeting cosmic events in gamma-ray, X-ray , ultraviolet and visible light, while alerting other observatories so they can turn their attention to the same targets.
Unlike some spacecraft in low Earth orbit , Swift doesn't have a propulsion system capable of counteracting atmospheric drag. Increased solar activity has heated and expanded Earth's upper atmosphere, increasing the drag on Swift and causing its orbit to decay faster than previously expected.
NASA's low-drag operations had been expected to keep Swift above an altitude of about 185 miles (300 kilometers) until October. Below that threshold, operating the observatory becomes increasingly difficult and its descent accelerates, NASA officials said in the statement.
Now that science operations have resumed — and the planned orbital boost is off the table — NASA expects Swift to reach that critical altitude within the next one to two months. Without intervention, the observatory is likely to crash back to Earth later this year, mostly burning up as it reenters the atmosphere, according to the space agency.
For now, the veteran space telescope is making the most of the time it has left — and once again scanning the universe.
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