After "digging" on the far side of the moon and displaying the five-star red flag, the Chang'e-6 ascender carried the "native products" on the back of the moon to dock with the orbit and return to the returner, and transferred the lunar soil on the back of the moon to the returner. On June 25, 2024, the Chang'e-6 returner successfully returned to Earth with "native products" on the back of the moon, marking the complete success of the Chang'e-6 mission. As a result, China became the world's first probe to land on the far side of the moon, the first to take samples on the far side of the moon, and the first country to hang a national flag on the far side of the moon.
The character "Zhong" left behind after sampling on the back of the moon and the Chinese national flag made of basalt as the core material
Chang'e-6 was originally a backup to Chang'e-5, and after the success of the Chang'e-5 mission, it was given a new mission - to go to the edge of the Apollo crater in the South Pole-Aitken Basin on the far side of the moon to carry out scientific exploration and sample collection, in addition to drilling a part of the lunar soil with a drill bit, and maintain its original layered structure as much as possible to create a profile of the surface lunar soil. Eventually, Chang'e-6, like Chang'e-5, sent back about 2 kilograms of lunar soil to Earth.
As we all know, the Moon only faces the Earth on one side because of tidal locking. Prior to the Chang'e-6 mission, human samples of the Moon were all on the front side of the Moon. The South Pole-Aitken Basin, located on the far side of the Moon, is the largest and oldest known impact basin on the Moon, forming about 3.9 billion years ago. Scientists believe that material from the deep part of the moon may have been brought to the surface during a violent impact. Chang'e-6 will bring back older lunar samples, which will help mankind further analyze the structure, physical properties, and material composition of lunar soil, and deepen the study of the origin and evolution of the moon.
The Apollo crater, the landing site of this mission, is located on the northeast side of the interior of the Antarctica-Aitken Basin, and is a "basin within a basin" formed by multiple impacts, so it may be one of the thinnest locations of the lunar crust. The name was given in honor of the "Apollo" program of the United States landing on the moon, not the landing target of the American "Apollo" program, after all, the Apollo crater is located on the far side of the moon, and the landing site of the "Apollo" program is on the front side of the moon. As early as 2012, the remnants of the Apollo 11 moon landing could be seen in the frontal images of the moon transmitted by the mainland's Chang'e-2 spacecraft.
Schematic diagram of the Antarctica-Aitken Basin and the Apollo Basin
Chang'e-6 is not only equipped with payloads such as landing cameras, panoramic cameras, mineral spectrum analyzers, lunar soil structure detectors, and national flag display systems, but also carries payloads such as ESA's lunar surface negative ion analyzer, French radon gas detector, Italian laser angle reflector, and Pakistani CubeSat in-orbit imaging, with a mission period of 53 days, with many innovations, high risks and great difficulties, which are highly anticipated by all parties. In these 53 days, Chang'e-6 spent about 10 days to complete the journey from the Earth to the Moon, and the rest of the time was used for orbit adjustment and testing, scientific expeditions, communication tests, navigation constellation technology tests, and scientific research missions for international cooperation payloads. Compared with the 23 days of the Chang'e-5 mission, the mission has more flight stages, more scientific tasks, and more technical difficulty, which makes the Chang'e-6 mission more timeless than Chang'e-5.
Some readers may have questions, isn't the far side of the moon permanently dark? How to solve the problems that require light such as power supply and photography of the detector? In fact, this is a misunderstanding of the far side of the moon, and it is a completely wrong statement. Judging from the exploration of the moon by human beings, human beings are not completely unable to see the far side of the moon. Thus, at least 10 to 20 percent of the marginal strip on the far side of the Moon can be seen from Earth. And only when we see the entire front side of the moon, that is, when the moon is full, the other side of it is completely dark, when there is no moon at the beginning and end of the moon or only the crescent moon, the far side of the moon is full of sun, which is why Chang'e-6 chose to land on the back of the moon on June 2 (the twenty-sixth day of the fourth month of the lunar calendar). At this time, from the earth, the moon is just a small crescent, and even gradually disappears into the night sky, but from the back of the moon, the sunlight is directly on the far side of the moon, which is very conducive to the development of various exploration work.
In fact, if you want to reach the back of the moon and carry out your work, the key constraint is the problem of communication. The huge body of the moon will block all communication signals, and any probe that flies to the back of the moon and falls to the back of the moon cannot directly establish contact with the earth, and must rely on relay communication satellites. It is worth taking pride in the fact that the mainland has the only two relay satellites around the moon, one is Queqiao-1, which operates in the Halo orbit near the L2 point of the Earth and the Moon, tens of thousands of kilometers away from the Moon, and can see both the Earth and the back of the Moon, so it can realize full-time relay communication on the far side of the Moon. However, due to the distance between Queqiao-1 and limited communication capabilities, it could not meet the needs of Chang'e-6 lunar back sampling, so the mainland launched Queqiao-2 on March 20 this year. Queqiao-2 did not orbit near the Earth-Moon L2 point, but chose a lunar large-inclination elliptical orbit and became a lunar satellite. The perilunar point is in the high latitudes of the northern hemisphere of the Moon, and the apogee point is in the high latitudes of the southern hemisphere of the Moon, with a period of about 24 hours, which is much closer to the lunar surface than Queqiao-1.
Schematic diagram of the track of Queqiao No. 1 and Queqiao No. 2
Queqiao-2 is equipped with an antenna with a diameter of up to 4.2 meters, which allows it to communicate at a speed of more than 1 gigabit per second on the far side of the moon. Of the 24 hours of the flight around the moon, 20 hours are near the apogee, which can ensure stable communication between the Earth and Chang'e-6. The remaining 4 hours are near the perilunar point, and there is a communication blind zone, and the relay cannot be provided. In this way, the communication of Chang'e-6 will be subject to some restrictions, and the key operations must be completed during the time period when Queqiao-2 can provide services, which will need to wait for some time windows, which will invisibly extend the time around the moon. Coupled with the fact that there are 14 days of day and night on the moon, Chang'e-6 is designed to complete sampling during the moon's day, and if the landing window meets the moonlit night, it will have to continue to wait. It is precisely because of the common limitations of the relay and the sunshine time window, coupled with the need for scientific observations, that the Chang'e-6 mission can only be carried out according to the 53-day schedule mentioned above, and it cannot be faster.
The Chang'e-6 mission not only left Chinese flags and imprints on the back of the moon, but also accumulated experience for future operations to obtain samples from Mars, and the precious lunar samples collected will provide more valuable data and information about the moon for mankind, and are expected to deepen the study of the origin and evolution of the moon.
China advocates the peaceful nature of the space exploration program, emphasizes international cooperation and sharing scientific research results, and the win-win cooperation model of space has been well known to other countries in the world, not only the Chang'e project, China's space station, China's Mars exploration and other missions, but also invite the world to carry out together, after the Chang'e-6 sample collection back, China will start a new round of world sharing. China is making positive contributions to building a community with a shared future for mankind, the peaceful use of space, and promoting global cooperation and development in space exploration.
Author: Zheng Qingshun (The author is a telemetry technician at Xichang Satellite Launch Center)
Editor in charge: Wang Kunpeng