Phew, NASA’s CAPSTONE is no Longer Tumbling in Space

Engineers with the trouble-plagued CAPSTONE mission to the Moon have made progress in stabilizing the spacecraft. A month ago, the microwave-oven-sized CAPSTONE (Cislunar Autonomous Positioning System Technology Operations and Navigation Experiment) began tumbling and lost its orientation in space. But now, after weeks of painstaking and patient troubleshooting, team members successfully executed an operation to stop the spacecraft’s spin. NASA says this clears a major hurdle in returning the spacecraft to normal operations.

Animation of the CAPSTONE mission in orbit of the Moon. Credits: NASA/Daniel Rutter

The mission for small satellite is to conduct tests to make sure the unique lunar orbit for NASA’s future Lunar Gateway is actually stable.  It launched on June 28, 2022 from New Zealand on Rocket Lab’s Electron rocket. In early July, after conducting a course correction burn, the spacecraft lost contact with the Deep Space Network; however, communications were reestablished after about 24 hours.

But on September 8th, at the end of another course correction maneuver, the spacecraft’s attitude started to deviate. CAPSTONE’s reaction wheels were unable to counter the spacecraft’s oscillations, and the vehicle entered into an uncontrolled spin. With its antenna no longer pointed at Earth, communications were lost again.

Even though engineers were soon able to re-establish a weak communications link, data indicated the spacecraft’s solar arrays weren’t producing enough energy to charge the batteries, which was causing the spacecraft to reset frequently from lack of power. Most worrying was that without energy to run the onboard heaters, the thrusters needed to stop the tumble could freeze. But the mission team was able to put CAPSTONE into safe mode, which allowed the solar panels to focus on supplying power to heat the spacecraft. Then the team could focus attempting to solve the tumbling problem. They were able to determine the problem arose from a thruster valve that was partially stuck open.

CAPSTONE mission timeline. Credit: Advanced Space and Terran Orbital.

In reviewing the telemetry data, one encouraging bit of data was that CAPSTONE was on course for eventually reaching its desired orbital trajectory. The unusual lunar orbit, called a near rectilinear halo orbit (NRHO), is an elongated polar orbit that brings a spacecraft within 1600 km (1,000 miles) of one lunar pole on its near pass and 70,000 km (43,500 miles) from the other pole every seven days. Since the orbit uses a balance point in the gravities of Earth and the Moon, it is theorized that spacecraft in this type of orbit require less propulsion capability for spacecraft flying to and from the Moon’s surface than other circular orbits and requires minimal energy to maintain. This is CAPSTONE’s goal, to determine how stable and energy-saving this orbit actually is.

Last week Friday, on October 7, recovery commands were executed and the initial telemetry from CAPSTONE points to a successful maneuver, indicating the spacecraft has stopped its spin and regained full 3-axis attitude control. This means CAPSTONE’s position is controlled and its solar arrays are pointing towards the Sun and the communications antenna is pointing towards Earth.  

The team is now monitoring the spacecraft status and making any needed adjustments to procedures in order to account for and mitigate the effects of the partially open thruster valve. The mission team also will work to design possible fixes for this valve-related issue in order to reduce risk for future maneuvers. CAPSTONE remains on track to insert into its targeted near rectilinear halo orbit at the Moon on Nov. 13.

Further reading: NASA CAPSTONE blog post