NASA takes back control of the CAPSTONE lunar orbiter

NASA has regained control of its CAPSTONE spacecraft after the lunar orbiter lost power and communications and spent weeks off in safe mode while tumbling through a vacuum.

Early last month, shortly after the craft performed a course correction maneuver en route to orbiting the Moon, ground control realized the cubesat was now spinning at a speed that its reaction wheels embedded could not control. He had lost complete attitude control in three axes.

Communications-wise, the bird remained silent for 24 hours until telemetry finally reached Earth. By then, engineers knew the spacecraft was spinning in space, had lost or lost power, and was repeatedly restarting. The spinning probe was still on course for the Moon but had to be restored to normal operation.

On September 8, an operational emergency was declared and teams from aerospace companies Advanced Space (which developed and managed the probe) and Terran Orbital (which built the thing) racked their brains to develop a fix. Given the poor signal condition from the spacecraft, teams used NASA’s Deep Space Network to gather information and communicate with the craft.

Soon the boffins, primarily those in 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 restore it to proper working order. It was later determined that the bird was then running in safe mode and spinning stably. This gave the team hope that the microwave-sized satellite could be fully recovered.

“The rapid response allowed mission operators to quickly reconfigure the spacecraft’s operational state to stabilize the situation while recovery plans could be further evaluated,” Advanced Space said mid-last month.

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

Since then, instructions to set the orbiter to the right were successfully downloaded and executed last week, which allowed the probe to point its solar arrays at the Sun and its radio antennae at Earth, handing control back to the ground on the driver’s seat. With reliable power and communications, more commands could be sent as needed and data received to complete the mission.

“Initial telemetry and observational data after the recovery attempt indicate a successful recovery of the system which has now regained three-axis attitude control,” Advanced Space confirmed in a statement ahead of the weekend.

“The updated spacecraft attitude oriented the spacecraft’s solar arrays toward the Sun and implemented an orientation for the downlink antennas that significantly improves data downlink performance compared to pre-recovery attitude.”

Crews believe the loss of attitude control stemmed from a partially open valve on one of CAPSTONE’s eight thrusters. Each time the propulsion system was pressurized, the faulty valve transmitted some thrust and unexpectedly jerked the spacecraft out of position. A workaround is in the works.

“Over the next few days, the status of the spacecraft will be monitored as the team works to assess further modifications to the spacecraft’s operating procedures so that upcoming critical events can be conducted in the presence event of a valve that remains partially open. In parallel, the mission will work to design possible fixes for this valve-related issue to further reduce the risk of future propulsion operations,” Advanced Space said.

CAPSTONE launched in June and is flying toward the Moon where it will enter a near-rectilinear halo orbit (NRHO). The orbit will mean that the small satellite 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 should reach the Moon and be inserted into an NHRO on November 13. ®

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