The Chang'e-6 probe landed in the Aitken Basin at the south pole of the moon on June 2, 2024, completed the orbital docking around the moon and the transfer of in-orbit samples on June 6, and returned to Earth on June 25.
During the more than 50-day mission period, Chang'e-6 successively carried out difficult operations such as retrograde circumlunar motion, intelligent sampling, and take-off and landing on the back of the moon, bringing lunar soil samples near the moon's south pole back to Earth.
Image source: Xinhua News Agency
The landing site of Chang'e-6 on the moon is located in the Ait Ken Basin, which is characterized by a large-scale impact that has formed a crater with a diameter of about 2,500 kilometers and a depth of 13 kilometers.
Approximate location of the Ait-Ken Basin in Antarctica (Image source: NASA)
It is conceivable that after such a violent impact on the south pole of the moon, a large amount of lunar mantle material may be ejected to the surface. According to the data of the lunar orbit survey, the abundance of iron, titanium and other elements in the Aite Ken basin is relatively high, and other ores are also more abundant than the surrounding areas, so collecting samples and studying in the basin is of great significance to unravel the mystery of the evolution of the moon.
In addition, the Ait-Ken Basin is located at the south pole of the moon, which means that there may be icy material in the surface material. According to data from the American LCROSS probe that hit the Capius crater at the south pole of the moon in 2009, there is evidence of water ice material in the crater. and in the permanently shadowed zone of the moon's south pole, there is also the possibility of water ice material. These are the latest research results on whether there is water ice material at the south pole of the moon.
By bringing back these lunar soil samples from Chang'e-6, we can further confirm the presence of water ice material.
The difficulty of returning from the moon
With the return experience of Chang'e-5, the Chang'e-6 process has also been carried out very smoothly. First, the ascender is activated to take off from the lunar surface, dock with the orbit-return assembly in lunar orbit, transfer the collected sample container to the return capsule, and then the ascender is separated from the orbit-return assembly. At this time, the orbit-return assembly should wait for the right time in the circumlunar orbit and start the engine to enter the moon-to-earth transfer orbit.
The circumlunar orbit is similar to the outer ring highway such as the third and fourth rings of a large city, and the moon-to-earth transfer orbit is like a highway connecting two cities. Due to the distance between the Earth and the Moon, the orbit-return assembly in the Moon-Earth transfer orbit needs to undergo multiple orbit corrections in order to effectively control the direction and angle of entering the Earth's atmosphere.
Schematic diagram of a spacecraft returning to Earth from lunar orbit (Photo source: Xinhua News Agency)
Finally, the orbit-return assembly is expected to unlock and separate at a distance of 5,000 kilometers from the earth, the orbiter will perform an evasive maneuver, and the return capsule will continue to move forward, entering the earth's atmosphere at a distance of about 120 kilometers from the ground, and finally landing in the predetermined area after a series of procedures such as aerodynamic deceleration and taxiing.
There are several difficulties in returning to this process, such as the multiple assemblies on Chang'e-6, which need to be docked and separated at each stage. One of the difficulties is to dock the ascender when returning, to separate the sample after transferring, and to "break it when it is close to the Earth" when the orbital return assembly is close to the Earth.
In addition, the rendezvous and docking of the ascender and the orbiter in the lunar orbit 380,000 kilometers away without the assistance of navigation satellites requires us to master advanced tracking and control technology, spacecraft docking technology, etc., in order to ensure that this process is foolproof.
Schematic diagram of the docking of the Chang'e-6 ascender and the return assembly (Image source: CCTV)
When returning from the moon-to-earth transfer orbit, the speed of the spacecraft is very fast, close to the second cosmic speed, which is much faster than the speed of our previous manned spaceflight to return to the atmosphere, so the thermal control, measurement and control brought by it have a greater impact on the spacecraft. As a result, we got about 2 kilograms of lunar soil samples, and Chang'e-5 brought back 1,731 grams of samples.
The unpacking site of 1,731 grams of lunar soil samples brought back by Chang'e-5 (Photo source: Xinhua News Agency)
Another detail to note here is that when bringing back samples from extraterrestrial objects, it is important to ensure that the container containing the samples is not contaminated by the Earth's environment before launch and after returning to Earth, otherwise if there are some major discoveries, it will not be possible to determine whether they are from an extraterrestrial body. Japan's Hayabusa probe has encountered a similar problem, with the return capsule potentially contaminated by the Earth's environment, making it impossible to conclude the results and determine whether the material in question came from an asteroid.
At present, in the direction of lunar exploration, only China, the United States and the Soviet Union have brought back samples from the lunar surface, the United States has brought back lunar rock samples during the Apollo lunar landing program, and the Soviet Union has obtained samples from the lunar surface through a series of lunar unmanned probes, and then our Chang'e-5 and Chang'e-6.
However, the significance of Chang'e-6 lies in the fact that we have obtained lunar soil samples near the South Pole from the far side of the Moon, and from the perspective of scarcity and science, the significance of Chang'e-6 is very significant.
Highlights and strategic significance of the Chang'e-6 mission
The highlight of the Chang'e-6 mission is to land near the south pole on the far side of the moon and take samples to return, first, human beings have not obtained the lunar soil on the far side of the moon, and the lunar soil samples brought back by the United States and the Soviet Union are all from the side of the moon facing the earth; Second, the lunar soil near the south pole of the moon is a long-term focus of the scientific community, and it is also a pre-selected area for NASA's Artemis program landing, which is related to how human civilization will develop the moon in the future, after all, water is an essential substance to maintain the normal functioning of the human body; Third, we have further verified the operation process of returning from the lunar surface to Earth, laying the foundation for future manned lunar landings.
According to data obtained by NASA in the past study of the moon's south pole, key evidence of water ice material in the lunar soil has been found. In 2009, the LCROSS probe hit the south pole of the moon, and in the huge plume that formed, scientists detected spectral signatures of hydroxyl groups, indicating evidence of the presence of water ice material at the bottom of the Capius impact crater.
India's Chandrayaan-1 probe also detected low concentrations of hydroxyl groups on the lunar surface. NASA's Stratospheric Infrared Observatory has also confirmed that water may be distributed on the surface of the moon and not limited to permanently shadowed areas near the edges of some impact craters.
EVIDENCE OF THE DISTRIBUTION OF WATER ICE MATERIAL INSIDE THE LUNAR IMPACT CRATER DRAWN BY INDIAN CHDRAYAAN-1 (IMAGE SOURCE: WIKI)
Obviously, these scientific achievements have made a manned landing on the moon more strategic, and if we can solve the problem of water on the moon, there will be no need to use launch vehicles to transport water from Earth to the moon to meet the needs of lunar habitat. This is crucial for humans who are expected to be active on the lunar surface for a long time in the future.
In addition to this, scientists also believe that water can also be used to produce hydrogen, which in turn can be used as rocket fuel. On the one hand, it can solve the problem of water resources for human beings to stay on the moon for a long time, and on the other hand, it can also contribute to the production of rocket fuel. From these two aspects, the strategic significance of Chang'e-6 is very significant.
We can directly obtain real lunar soil samples from the area near the south pole of the moon, which is important to grasp the true content of water resources on the lunar surface, and at least we can determine whether there is water ice at the Chang'e-6 landing site. If so, it could be used as a pre-selection area for our future manned lunar landing or lunar habitat.
What else will we do in the future
After Chang'e-6, we have already unveiled the Chang'e-7 and Chang'e-8 probes. According to publicly available data, Chang'e-7 will consist of an orbiter, a lander, a small flying probe and a rover, with a launch mass of about 8.2 tons and a design life of up to 8 years.
From this combination, it can be seen that Chang'e-7 focuses on the integration of open space and ground, surveys the environment and resources of the moon's south pole, further grasps the lunar surface environment of the moon's south pole, the distribution and stock of water ice materials, and establishes a comprehensive resource perception capability of the entire lunar south pole.
The Chang'e-8 probe will establish the basic prototype of China's lunar south pole research station, which is expected to be launched around 2029, marking a milestone step in the direction of China's lunar research station.
The mainland is expected to build a lunar scientific research station in the 2030s (Image source: CCTV)
From the perspective of China's development plan for the direction of space and the moon, the lunar soil samples retrieved by Chang'e-6 are extremely critical, and the subsequent lunar exploration project will be carried out based on the results achieved by Chang'e-6, laying the foundation for the final establishment of the lunar South Pole scientific research station and lunar habitat.
Source: Popular Science China