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Forget everything you learned playing Lunar Lander: Chinese boffins reveal secrets of Chang'e 5 probe's touchdown

Landing with liquid fuel aboard made slosh-avoidance essential. Plus: China names first woman to visit Tiangong space station

Chinese space boffins have revealed details of how the guidance navigation and control (GNC) system in the Chang'e 5 got the probe onto the surface of the Moon despite its propellant sloshing about inside.

The goal of the Chang’e-5 mission was to collect approximately 2kg of stones and soil, inclusive of samples sourced from two metres below the Moon's surface, and take it back to Earth for analysis.

Chang'e 5's 15-minute powered descent and soft landing on the near side of the Moon went off smoothly back in December 2020, as did its return with the samples back to Earth. Its return made it the third nation to retrieve Moon rocks, and the first in over 40 years.

Chang'e 5 differed from its predecessors Chang'e 4 and Chang'e 3 by using a lander and ascender module (LAM) that actually landed on the Moon before returning to the orbiter and return module (ORM) in lunar orbit – a necessity for bringing back lunar samples. This meant the LAM needed an extra tank to carry a larger quantity of propellant.

And as explained in the journal Space: Science & Technology, during the controlled deorbit to the lunar surface, the LAM needed to perform an attitude reorientation – a turn from horizontal to vertical – and do that without sloshing its go-go juice because doing could potentially throw the craft off balance.

It's not as if the Moon needs more craters.

The GNC therefore incorporated a three-part reconfigurable attitude control consisting of a quaternion partition control that managed the LAM's angle and angular velocity while avoiding gimbal lock, plus a phase and gain stabilization filter to keep the sloshing at bay by controlling pitch, yaw and roll. Other systems kept an eye on things to predict any centre of mass changes or other disturbances.

And just to make sure everything was working as intended, the LAM was kitted out with extra sensors. The system compared all the sensor data, and any outliers were deemed faulty and disregarded.

Luckily for China's National Space Administration (CNSA) the GNC worked. The landing of the lander and ascender module was mission-critical – without safe retrieval, no samples would have arrived on Earth, depriving scientists and propagandists alike of their payoff.

"We hope that the GNC methods presented in this paper will find application in future lunar exploration, crewed space exploration, and other missions," said Dr Honghua Zhang of the Beijing Institute of Control Engineering and first author of the paper. He believes the data fusion method used for the sensors monitoring the velocity and inertial measurements might have uses on future aerial vehicles.

In other cool news for Chinese space endeavours, the crew for its upcoming Shenzhou-13 mission made a public debut. Among the trio is the first Chinese female taikonaut to visit China's space station and participate in extravehicular activities.

Wang Yaping is joined by her male colleagues Zhai Zhiguang and Ye Guanfu. All will live and work for six months in the country's under-construction Tiangong space station. ®

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