200 Miles Above Earth, a Refrigerator-size Satellite En Route to Rescue NASA's $500 Million Swift Gamma-ray Observatory

A week ago, more than 200 miles above Earth, a refrigerator-size satellite en route to rescue NASA's $500 million Swift gamma-ray observatory suddenly spun out of control. The spacecraft, named Link, was rotating on multiple axes, leaving it unable to maintain reliable communications with the ground and complicating efforts to stop the spin. Two of its three reaction wheels, used for pointing, also failed.

Through sporadic radio contact, engineers discovered a problem with some of the satellite's cold gas thrusters, which are used for finer attitude control. Link is built, owned, and operated by Katalyst Space Technologies, a satellite servicing startup that won a $30 million contract from NASA to rendezvous with Swift, grab it, and boost its orbit before the observatory succumbs to aerodynamic drag and burns up in the atmosphere. The clock is ticking: in a few months, Swift will be too low for Katalyst to complete the rescue.

This marks the first time NASA has contracted with a commercial company to service one of its satellites, and the agency gave Katalyst less than a year to assemble the mission. Link launched July 3 to begin its pursuit of Swift, and the mission proceeded mostly as planned until last Saturday. “When this happened, it was during one of the passes without comms, Ghonhee Lee, CEO of Katalyst, told Ars.

“We were, immediately prior, in a very stable configuration. Katalyst's ground team, working from a control center near Denver, hurried to find a fix and recover the satellite. The good news: Link's other systems remained healthy, including its power supply, three xenon-fueled electric thrusters, and the rendezvous and robotics hardware needed to capture Swift.

The first step involves using the satellite's plasma engines to gradually slow the spin. These engines are primarily designed for orbit-raising, but they can also be used for attitude control because they vector their thrust with a two-axis gimbal. Link's electric propulsion system is efficient but low in thrust, meaning it takes time to generate enough impulse to regain full control of the satellite's pointing.

“We've been able to use the thrusters to point in the opposite direction of the rotation rate and affect the attitude control that way, which has been highly effective, Lee said. As of Friday, the spacecraft had cut its spin rate in half, from about 9 degrees per second to approximately 4 degrees per second. Officials hope to reduce it further, allowing Katalyst to improve communications with the spacecraft.

“Once we are in a more stable configuration, with much more high-bandwidth comms, we'll be able to use that to downlink all the rich data about the condition of the spacecraft, Lee said. “We do know the states of the reaction wheels and the thrusters and things like that. guidance, navigation, and control team—has been very hard at work with NASA, essentially remapping all of the control algorithms to be able to have an updated controller ready to go for when we are able to stabilize the spacecraft.

Katalyst may eventually recover the two inoperable reaction wheels, but the team's priority is restoring control by uplinking a new algorithm to manage orientation through a combination of the one remaining reaction wheel and thrusters. Engineers aren't sure yet what caused the emergency last weekend; it could have been an internal problem or a collision with space junk. Lee said two cameras on the satellite will look for signs of damage after ground teams stabilize it.

Whatever the cause, Katalyst's mission control lost contact with Link for more than 24 hours after it spun out of control. The satellite then automatically reset itself, as designed. “This was built-in fault protection logic saying, 'Hey, if I haven't heard from anybody in 24 hours, there must be something wrong.

I'm going to toggle the power, turn it on and off again, Lee said. “It's just built in. However, that shutdown mode is ungraceful.

It basically pulls the plug on everything, and that creates some downstream effects for sensitive systems such as the reaction wheels. “Basically, there was a big thermal spike that came about as a result of this that over-temperatured the upstream electronic circuits that control the reaction wheels, which ultimately led them to being inoperable, Lee said. The problem with the cold gas thrusters appears to be a “separate issue, he added.”””””””””