New details emerge about failed lunar landings
HOUSTON — The Indian government has offered new details about what happened during its first attempt to land on the moon in September.
In a written response to questions Nov. 20 to the Lok Sabha, the lower house of India’s Parliament, Jitendra Singh, minister of state for the Department of Space, said that the Vikram lander “hard landed” on the moon Sept. 6 because of a problem with the lander’s braking thrusters.
“The first phase of descent was performed nominally from an altitude of 30 km to 7.4 km above the moon surface,” he wrote. The lander slowed from 1,683 meters per second to 146 meters per second during that time.
“During the second phase of descent, the reduction in velocity was more than the designed value,” he continued. “Due to this deviation, the initial conditions at the start of the fine braking phase were beyond the designed parameters.”
“As a result, Vikram hard landed within 500 m of the designated landing site,” he concluded. Singh’s statement did not elaborate on that caused that “deviation” in the performance.
That statement is the first formal acknowledgment by the Indian government that the lander crashed during its landing attempt. The Indian Space Research Organisation (ISRO), part of the Department of Space, made few statements about the fate of the lander since the Sept. 6 landing, and in them referred to only a loss of communications with the lander, not a failed landing.
“Vikram lander has been located by the orbiter of Chandrayaan-2, but no communication with it yet,” ISRO said in a Sept. 10 statement. “All possible efforts are being made to establish communication with lander.” Contact with the lander was never restored after being lost during the landing attempt.
In an Oct. 21 interview at the International Astronautical Congress (IAC) in Washington, S. Somanath, director of ISRO’s Vikram Sarabhai Space Centre, said engineers were using simulations to reconstruct what happened to the lander after contact was lost 2.1 kilometers above the lunar surface.
ISRO, he said then, was holding off on an announcement about the lander until that work is finished, but he acknowledged the lander likely hit the lunar surface at a high velocity, “beyond its survivability.”
The loss of the Vikram lander took place less than five months after Israel’s first lunar lander, Beresheet, also crashed during landing. Officials with Israel Aerospace Industries (IAI), which built the lander, and SpaceIL, the nonprofit organization that conducted the mission, later said a computer glitch during the lander’s descent led it to crash.
While SpaceIL has abandoned plans for a second Beresheet mission, IAI signed an agreement with Firefly Aerospace to use the Beresheet lander design as the basis for a lander called Genesis that Firefly is offering as part of its NASA Commercial Lunar Payload Services (CLPS) contract. Firefly was one of nine companies selected by NASA nearly one year ago for CLPS contracts, although the company has yet to win a task order for a mission.
During a Nov. 20 panel at the SpaceCom Expo here, Shea Ferring, vice president of mission assurance at Firefly, said correcting the problem that doomed Beresheet won’t be difficult. “The Israelis got a lot of data from that mission. They learned and know exactly what went wrong,” he said. “The fix is relatively easy, so we can build off all the successes they did have all the way up until those final moments.”
Ferring declined to go into details on exactly what happened with Beresheet, but IAI discussed the landing failure in a paper presented at the IAC in October. In that paper, the company said one of two inertial measurement units (IMUs) on the spacecraft malfunctioned during descent and was shut down by the onboard computer. Controllers uploaded commands to turn the unit back on.
“This led to a cascade of resets in the spacecraft avionics, which shut off the main engine and prevented proper engine activation,” the paper stated.
A review of the lander telemetry found that the decision to turn the IMU back on “triggered a communication block” between the IMU and the central processing unit, according to the paper, keeping data from the other, working IMU from reaching the system and thus causing the thrusters to turn off and the computer to reboot.
“As result, all accumulated [software] updates that were stored on a volatile memory (SRAM) were eliminated during reboot,” the paper stated. “Therefore, the computer did not contain all the essential changes implemented during the flight, which made autonomous recovery impossible.”
Among the changes recommended in the paper were to allow full updates of the lander’s software during flight, and storing that software in non-volatile memory that is not wiped during a computer reboot.
“The big thing with us is teaming with the Israelis, with all of the successes they did have on Beresheet and fixing the problems that had,” Ferring said. “Taking the history, taking the lessons learned, building from that and doing better the next time.”