Update on SWIFT rescue mission

Back last Tuesday, the story broke that the reaction wheel attitude control system on the Link spacecraft sent up to rescue the Swift gamma ray observatory had apparently failed and the Link satellite was spinning out of control. Two of the satellite’s three reaction wheels, used for pointing, had stopped working.

Things didn't look good for the little spacecraft.

... Link was rotating on multiple axes, rendering it unable to maintain a reliable communications link with the ground and complicating efforts to arrest the spin. Two of the satellite’s three reaction wheels, used for pointing, also stopped working. Through sporadic radio contact, engineers discovered a problem with some of the spacecraft’s cold gas thrusters used for finer attitude control.

It's worth pointing out that Link had been working fine, and was out of communication for a while, something that typically happens for many satellites. 

Katalyst’s ground team, working from a control center near Denver, hurried to find a fix and recover the Link satellite. The good news was, the spacecraft’s other systems remained healthy, including its power supply, three xenon-fueled electric thrusters, and the rendezvous and robotics hardware the Link satellite needs to capture Swift.

Remember that, as pointed out last week, Link's electric thrusters are low powered for this sort of task which means they'll need to run them for a long period of time. As of Friday (the 31st), the spacecraft had cut its spin rate in half, from about 9 degrees per second to approximately 4 degrees per second. Naturally, they hope to reduce it much more so they can do the task Link was developed for. 

“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),” Ghonhee Lee, CEO of Katalyst said. “We do know the states of the reaction wheels and the thrusters and things like that. So our GNC team—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.” 

The Katalyst engineering team hopes to be able to stop the errant motions entirely and to recover the two reaction control wheels that seem to be completely dead. It would help if they knew exactly what was wrong. They don't really know why the wheels stopped working properly - whether it was a component problem of some sort in the Link satellite or even a collision with space junk of some kind. There are cameras on the Link satellite that can be steered around to look for evidence of what happened. Once the data links are running closer to capacity again. 

It's being considered that the automatic fault detection - a fancy version of turn it off and back on again - could be at least part of the cause.

Whatever the cause, Katalyst’s mission control team lost contact with Link for more than 24 hours after it spun out of control. The satellite then automatically reset itself, just as it was supposed to do.

“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.

While busy and this setback ensures they're not quite "there" yet, the Katalyst team is working to complete the rescue mission they contracted for.

“We’re very committed to pursuing this mission,” Lee said. “We’re working side by side with NASA. I just came from a technical working meeting with their controls team, where we’re reviewing the algorithms. It seems like there’s nothing that is preventing us from reestablishing three-axis control of the spacecraft with the remaining reaction wheel, with the remaining thrusters, as well as the electric propulsion.

“It’s not the original mode that we expected to control the spacecraft with, but it seems like we will have sufficient stability and controllability of the spacecraft,” Lee said. “With this, we believe we’ll be able to do the rendezvous with Swift. We believe that we’ll be able to go and do things like the inspection of Swift within a close approach of a few dozen meters. And as it stands right now, we have not made a formal assessment, but we believe that a capture of Swift, an attempted capture of Swift, is very much in the cards.”

A plume of plasma exhaust coming from one of the Link satellite’s xenon-fueled electric engines. Credit: Katalyst Space Technologies