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On December 1, 2020, China's Chang'e-5 spacecraft successfully landed on the near side of the moon and sent back images. At 11:11 p.m., the spacecraft landed at the preselected landing area near 51.8 degrees west longitude and 43.1 degrees north latitude. In the final stage of Chang'e-5's landing , the "brake commander", who has been on duty during the landing on the moon, accurately detected the distance between the Chang'e probe and the lunar surface, and issued a brake command at a distance of less than 5m from the lunar surface to make Chang'e "trippingly" land on the moon, realizing the "soft landing" of the probe.
The "brake commander" is a gamma shutdown sensor developed by CASIC. Prior to the lunar exploration mission, the gamma altitude control device developed by the team has been successfully applied in the Shenzhou spacecraft mission to ensure the personal safety of the astronauts through "soft landing" of the return capsule.
γ-rays have the characteristics of little in external interference. It is precisely because of the "strongest" accuracy of γ-rays that this mechanism of using γ-rays for altitude measurement has been applied to manned space flight and lunar exploration missions that require the highest accuracy and reliability.
The lunar surface is complex and special. From the "geodesy" to the "selenodesy", it is necessary to further break through the control technology of "soft landing" on the lunar surface to ensure accurate ranging and precise control. When Chang'e-5 turned from the circumlunar flight to the lunar landing, it controlled itself to slowly descend to the moon by turning on the engine. As the landing process began, the gamma shutdown sensor at the bottom of Chang'e-5 measured the distance between Chang'e-5 and the lunar surface in real time. When the sensor detected that the distance from the lunar surface was less than 5m, it issued a shutdown command to shut down the orbit and attitude control engines. The issuance of this shutdown command is the key to the "lunar landing of Chang'e", determining the success or failure of the "lunar landing" mission. With the withdrawal of reaction thrust of the engines, the probe decelerated and made a "soft landing", ensuring that Chang'e gracefully and "trippingly" landed on the moon.
