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NASA regains control of CAPSTONE lunar orbiter after a tumble

Pinging something missing on a network is frustrating enough, imagine if it's out in space

NASA is back in control of its CAPSTONE spacecraft after the lunar orbiter lost power and communications and spent weeks powered down in safe mode while tumbling through the void.

Early last month, soon after the craft completed a trajectory correction maneuver on its way to orbit the Moon, ground control realized the cubesat was now spinning at a rate that its on-board reaction wheels couldn't get in check. It had lost full three-axis attitude control.

Communications-wise, the bird went silent for 24 hours until some telemetry finally got through to Earth. At that point, engineers knew the spacecraft was spinning through space, had lost or was losing power, and was repeatedly rebooting itself. The twirling probe was still pretty much on course for the Moon but had to be brought back to normal working order.

On September 8, an operational emergency was declared, and teams at aerospace outfits Advanced Space (which developed and manage the probe) and Terran Orbital (who built the thing) racked their brains to develop a fix. Given the feeble state of the signals from the spacecraft, the teams used NASA's Deep Space Network to gather information and communicate with the craft.

Soon the boffins, primarily those at Terran Orbital, were able to figure out and send a message to CAPSTONE – short for Cislunar Autonomous Positioning System Technology Operations and Navigation Experiment – to bring it back into a good operating state. It was next determined the bird was then running in safe mode and spinning in a stable manner. That gave the team hope the microwave-oven-size satellite could be fully recovered.

"The rapid response enabled mission operators to quickly reconfigure the operational state of the spacecraft to stabilize the situation while recovery plans could be further evaluated," Advanced Space said in the middle of last month.

"Based on this exceptional effort and using the limited data available, the operations teams have determined that the spacecraft is in safe mode and appears to have successfully been placed in a stable state. The vehicle is in a rotating orientation that provides partial illumination of the solar panels and results in weak transmission signals from the spacecraft low gain antennas."

Since then, instructions to set the orbiter right were successfully uploaded and last week executed, resulting in the probe pointing its solar panels at the Sun and its radio antennas at Earth, putting ground control back in the driver's seat. With reliable power and comms, more commands could be sent as needed, and data received, to complete the mission.

"Initial telemetry and observation data after the recovery attempt points to a successful recovery of the system which has now regained three-axis attitude control," Advanced Space confirmed in a statement before the weekend.

"The updated spacecraft attitude has oriented the spacecraft solar arrays to the Sun and implemented an orientation for the downlink antennas which significantly improves data downlink performance as compared to the pre-recovery attitude."

The teams believe the loss of attitude control stemmed from a partially open valve on one of CAPSTONE's eight thrusters. Every time the propulsion system was pressurized, the flawed valve would impart some thrust and unexpectedly jolt the spacecraft out of position. A workaround is being prepared.

"Over the coming days, the spacecraft status will be monitored while the team works to evaluate subsequent changes to the spacecraft operating procedures so that upcoming critical events can be conducted in the possible presence of a valve that remains partially open. In parallel, the mission team will work to design possible fixes for this valve related issue to further reduce the risk of future propulsive operations," Advanced Space said.

CAPSTONE was launched in June, and is flying to the Moon where it will enter a near-rectilinear halo orbit (NRHO). The orbit will mean the little sat will travel along a slightly more curved path around the Moon compared to more traditional elliptical lunar orbiters. CAPSTONE is examining whether NRHOs are stable enough to place the proposed Lunar Gateway, a space station for astronauts, around the Moon in the future.

The mission hasn't gone smoothly so far, clearly. Even before the thruster anomaly, the bird lost communication with Earth due to a software bug in its flight software in July. If all goes according to plan, CAPSTONE is expected to reach the Moon and be inserted into a NHRO on November 13. ®

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