For the first time in the world, how difficult is it to "dig soil" on the back of the moon?

On May 3, 2024, the Chang'e-6 probe boarded the Long March 5 carrier rocket and began an unprecedented journey to the moon. Chang'e-6 has many similarities with Chang'e-5, but the difference is that this time the "stage" was moved from the front of the moon to the back of the moon.

The Long March 5 carrier rocket takes off in the wind and rain (Photo by Su Dong)

After more than 30 days of flight, Chang'e-6 landed steadily in the pre-selected landing area, and the work of sampling the far side of the moon is about to begin. The work of collecting lunar soil is simply "digging soil", but once Chang'e-6 lands, it can no longer move. No one knows whether enough lunar soil samples can be collected around and under the probe. Although there are many unknowns, the exploration of the far side of the moon is of great significance to human understanding of the lunar structure, geological characteristics and the origin of the universe.

Chang'e 6 landed in the South Pole-Aitken Basin, one of the largest impact basins on the moon. First, no one has ever been here. Second, it was formed a long time ago, and there will be some different understandings in scientific analysis. However, this huge impact crater has a rugged terrain. If the probe accidentally lands on a large rock, it will affect the subsequent sampling work. How can the probe "land accurately"?

Chang'e-6 cannot follow the path opened by Chang'e-5 to reach the far side of the moon. Designers redesigned a route for it, namely the "retrograde circumlunar orbit" plan. In simple terms, the flight direction of the probe in the circumlunar orbit is opposite to the direction of the moon's rotation. The designers' ingenuity lies in the orbit design. In order to adjust the orbit without increasing the consumption of propellant, Chang'e-6 "brakes" three times. This is one more time than Chang'e-5, which can make full use of the more than 20 days of flight time from the capture of the moon to the descent, constantly adjust the orbit parameters, aim at the landing point with high precision, and wait for the best time to land.

Secondly, judging from the results of human lunar exploration, the material composition of the South Pole-Aitken Basin is somewhat different from that of other regions. The bottom of the basin contains more elements such as iron, titanium, and thorium; from a mineral perspective, there are more orthopyroxene and orthorhombic ore. These characteristics are different from Apollo samples, Soviet lunar samples, and lunar meteorite samples. At present, humans still lack in-situ samples brought back from the back of the moon. How to carry out sampling work when the characteristics of the lunar soil on the back of the moon are unknown?

Composition of the Antarctic-Aitken Basin

The Chang'e-6 probe has two sampling modes: drilling and surface sampling. Surface sampling uses a robotic arm carrying a sampling device to collect soil from the lunar surface, while drilling uses a drill rod to drill into the lunar surface to a predetermined depth to collect soil from the deeper layer. There is a big difference between drilling and surface sampling. The lunar soil collected by surface sampling can be seen with the help of a camera, but drilling is to collect lunar soil below the lunar surface, which is equivalent to working with eyes closed. To make a simple analogy, when we mechanically turn something, we encounter different materials such as iron, steel, and copper, and the process dimensions used are all different. The same is true for lunar soil. Drilling different lunar soils, such as stones, small particles, fine powder, etc., requires different processes. Therefore, it can be seen that the difficulty of drilling sampling by Chang'e-6 is very great.

In addition, since the front of the moon is directly visible to the earth, Chang'e 5 can use all 48 hours on the moon for work. On the back of the moon, although there is a relay satellite "paving the way", the relay satellite has a "visual blind spot" when orbiting the moon, and the "digging" time left for Chang'e 6 is shortened to 36 to 40 hours, which puts higher requirements on the work efficiency of ground personnel and probes. Therefore, Chang'e 6 adopts fast intelligent sampling technology, which reduces the number of instructions sent from the earth to the moon by hundreds, reduces the time of interaction between the earth and the sky, reduces the occupancy of communication links, and greatly improves the efficiency of sampling.

The lunar surface shows a Chinese character "中"

On the distant back of the moon, Chang'e-6 carried out exploration and sampling missions, just like a careful embroiderer who used her "needle and thread" to "embroider" a big "Zhong" character on the lunar soil, leaving a unique mark of China on the desolate and mysterious lunar land.

(Authors: Deng Yunan, Lin Jiaxin, Image source: National Space Administration, Review Expert: Jia Yang, Researcher of the General Department of China Academy of Space Technology)