Cause of Lucy solar array deployment problem identified

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WASHINGTON — Engineers have identified the likely reason one of two solar arrays on NASA’s Lucy asteroid mission failed to latch in place after launch, but NASA is still studying whether to fix the problem.

At a Jan. 25 meeting NASA’s Small Bodies Assessment Group, Hal Levison, principal investigator for Lucy at the Southwest Research Institute, expressed confidence that, regardless if the solar array is fully deployed or not, the issue will not affect the spacecraft’s ability to carry out its mission to study several Trojan asteroids leading and following Jupiter in its orbit around the sun.

Shortly after its Oct. 16 launch, deployed two large circular solar arrays, each 7.3 meters in diameter. The arrays are designed to unfurl like fans and latch into place. While one array, called -Y, did completely unfurl and latch, the other, +Y, did not latch into place.

“People have been working day and night since the launch to try and figure out what’s going on, and I think we understand it,” Levison said. The +Y array, rather than unfurling a full 360 degrees, instead went 347 degrees. In that configuration, the spacecraft is still generating more than 90% of its expected power. “Power is not an issue for the spacecraft, nor will it be through the entire mission if we have to fly it like it is.”

The arrays unfurl when a motor pulls on a lanyard, swinging one end of the array around and into place. Levison said that the most likely reason the array did not latch is that, for some reason, there was a loss of tension in the lanyard during deployment. That caused it to fall off a spool and wrap around the motor shaft. About 75 centimeters of lanyard remains to be pulled in.

“It matches the data really well, so we have really high confidence this is true,” he said. One possible cause of the loss of tension, he added, is a “dynamic interaction” between the two arrays during the deployment.

Mission managers are considering two alternatives. One is to turn the motors back on and try to bring in the remaining lanyard segment so that the array can lock into place. “We’re almost there, so I think if we can pull a little harder, we might be able to get it to latch,” he said. The motor can pull harder, he said, but engineers want to assess the risks of doing so before making another attempt.

The other option is to keep the array as is. While the array can generate enough power without being fully deployed, Levison said engineers want to study its integrity in that configuration during main engine burns. “The analysis so far is looking good. We should be able to do at least some of the main engine burns we’re planning.”

There’s no rush to decide whether to redeploy the array or leave it as is. “We have plenty of time because we’re not scheduled to fire the main engine for a while,” he said. “We’re taking our time to carefully go through our options.”

That assessment matches the most recent NASA update about the mission, published Jan. 12. It stated that the current plan for the mission “supports a latch attempt in the late April timeframe” but that engineers were still studying leaving the array in its current unlatched condition.

Levison added that all other aspects of the spacecraft, including its instruments, were working well. “Except for this problem, the spacecraft is really kicking butt,” he said. “The instruments and the spacecraft are all behaving nominally.”