Astrobotic’s Peregrine lunar lander failed to reach the Moon due to an issue with a single valve within the propulsion system, according to a mission report released on Tuesday. Company executives said at a news conference that engineers had redesigned the valve and added redundancy to the propulsion system of the subsequent lander, Griffin, to make sure the problem wouldn’t repeat itself.

The report comes from a review board that met shortly after the tip of the Peregrine mission in January. That mission bumped into problems just hours after its Jan. 8 launch, when engineers fired up the spacecraft’s propulsion system for the primary time in orbit.

At that time, the fuel and oxidizer tanks must have been full of helium after opening two pressure control valves, or PCVs. But helium began to flow “uncontrollably” through the second valve into the oxidizer tank, Astrobotic CEO John Thornton explained during a news conference.

“This caused a significant and rapid overpressure of the tank,” he said. “Unfortunately, the tank ruptured and as a result, oxidizer leaked, which continued to leak for the rest of the mission.”

The PCV was unable to reseal, likely due to a mechanical failure attributable to “vibration-induced loosening” between some of the threaded components contained in the valve, said review board chairman John Horack. Telemetry data was able to pinpoint the placement and time of the anomaly, and the information was consistent with the PCV’s autonomous opening and shutting sequence and the valve’s position within the propulsion system. Engineers were also able to recreate the failure during ground testing.

While the oxidizer leak continued, the Astrobotic team managed to stabilize the spacecraft, charge the batteries, and power the payloads. However, the issue ultimately proved fatal to the mission, and after 10.5 days, the spacecraft returned to Earth and burned up within the atmosphere.

The 34-person review board included 26 company insiders and eight outsiders. The board analyzed not only the information collected throughout the mission, but additionally all the information from the flight qualification campaign and component tests. Ultimately, it was determined that the probable cause of the failure was the failure of a single helium PCV within the propulsion system.

Management also laid out a timeline of events leading up to the failure, starting in 2019, when Astrobotic signed a contract with an unnamed supplier to develop a drive power system. When that supplier began experiencing technical and provide chain issues due to the COVID-19 pandemic, Astrobotic made the choice in early 2022 to terminate the contract and complete the partially assembled power system itself.

“At this point, we had already made the decision to do the Griffin propulsion system in-house to do more vertical integration,” said Astrobotic mission director Sharad Bhaskaran. “We had already developed a lot of capabilities to do that propulsion integration. … That also reduced some of the risk in the Griffin program, which is much more complex than Peregrine.”

However, Astrobotic engineers began to encounter problems with the unique supplier’s propulsion components—specifically, the PCVs. In August 2022, they switched to one other, unnamed PCV supplier, and people valves were installed on the lander.

The final set of propulsion system tests showed leaks in a single of the 2 PCVs—but not the one which ultimately leaked in orbit. That one performed well; the one which leaked was repaired. While Bhaskaran acknowledged that the second PCV had been identified “as a risk in our risk register” because the primary one leaked during testing, engineers ultimately deemed the failure minor, because the lander passed final acceptance testing.

The rationale for not replacing the second PCV was that it was situated much further back within the spacecraft and would require “extensive surgery” on the lander, invalidate final testing, and involve the extra risk of disassembly and reassembly.

Horack reiterated that the team’s decision-making was sound throughout: “I really found that when I looked at the team and what happened… I don’t see any decisions made in the lead-up to launch where I could say, ‘Hey, I think you should have done this differently.'”

These findings have already begun to inform development of the much larger Griffin lander, which is currently scheduled to launch to the moon before the tip of 2025. In addition to redesigning the valve, engineers have added a regulator to the propulsion system to control the flow of helium to the fuel and oxidizer tanks, in addition to backup latching valves as additional redundancy in case the PCV problem occurs again.