Look, this is the back of the moon! Five years ago today, Chang'e 4 successfully landed on the moon

At 10:26 on January 3, 2019, my country's Chang'e-4 probe autonomously landed on the far side of the moon, in the Von Karman crater. This marked the first time in human history that a spacecraft softly landed and conducted a patrol survey on the far side of the moon, the first time that measurement and control communication between the Earth and the far side of the moon was achieved, and the first time that a laser ranging technology test beyond the distance between the Earth and the Moon was conducted.

The Chang'e-4 probe set a record for the longest time working on the far side of the moon, and obtained a large amount of scientific research data on the moon, providing more abundant information for human beings to understand the moon. It provided Chinese solutions and Chinese wisdom for the international "lunar exploration journey", and left a strong and colorful mark in the history of lunar exploration for all mankind.

A tortuous innovation journey

Because the moon's rotation period and revolution period are equal, and it is tidally locked to the Earth, the Earth's strong gravity causes the moon to always face one side towards the Earth and face away from the Earth. The back of the moon has become a place that humans have always dreamed of.

In April 2019, Israel's first lunar exploration mission, Genesis, successfully orbited the moon, but unfortunately failed to land during the powered descent phase and crashed on the lunar surface. In September of the same year, India's first lunar lander, Chandrayaan-2, lost contact after powered descent to 2.1 kilometers from the lunar surface, and ultimately failed to softly land and also crashed on the lunar surface. From orbiting to landing, this "a little closer" distance of dozens of kilometers, countless probes have failed here.

Because the landing process of Chang'e-4 cannot be directly seen from the Earth, the entire process must rely on the autonomous control of the probe, and there will be a delay in the image transmission, so for ground personnel, the landing process is almost a "blind landing" process. Although the planned landing area of ​​Chang'e-4 is located inside the relatively flat Von Karman crater, the Von Karman crater is located inside the rugged South Pole-Aitken Basin on the back of the moon, and the terrain under the entire landing track fluctuates up to 6 kilometers. This means that Chang'e-4 has a smaller range of optional landings, and it must land very accurately - once it deviates a little, it may not land in the flat scheduled landing area, but hit the rugged mountains and valleys.

To further reduce the uncertainty of the landing position, the R&D personnel considered adding a correction orbit during the lunar orbit phase when designing Chang'e 4, and made certain adjustments to the power descent control strategy. "When conducting the test, many factors were also considered, such as how much the maximum tilt of the landing place of the lander can be? If it is a terrain like Zhangjiajie, when the vehicle goes down, it follows two ladders. Are these two ladders necessarily parallel? If the ladders form an eccentric angle, what is the maximum angle?" - The team members analyzed thousands of possible situations, exhausted various extreme situations, and conducted experiments with increased difficulty for the calculated possible situations.

More reliable energy supply

The Chang'e-4 probe consists of a lander and a rover, with a total of eight payloads, including two international cooperation payloads. The lander is equipped with four payloads, including a topographic camera, a landing camera, a low-frequency radio spectrometer, and a lunar surface neutron and radiation dose detector developed in cooperation with Germany; the rover is equipped with a panoramic camera, a lunar radar, an infrared imaging spectrometer and a neutral atom detector developed in cooperation with Sweden.

These instruments conduct low-frequency radio astronomy observation and research on the far side of the moon through in-situ and patrol detection, research on the morphology, mineral composition and shallow structure of the patrol area, and experimentally conduct lunar environment research such as neutron radiation dose and neutral atoms on the far side of the moon. In addition, the lander also carries a lunar surface biological science experiment payload.

A moonlit night is equivalent to 14 days on Earth. At the same time, the lowest temperature on a moonlit night can reach minus 180 degrees Celsius. In the long dark night without sunlight, it is a big challenge for the Chang'e-4 lander and rover, which rely on solar energy for power, to safely survive the moonlit night by relying on their own stored energy.

Faced with this problem, researchers have proposed the concept of dormancy awakening. When the sun rises, the lander and rover will begin their busy work - the lander will carry out scientific exploration in situ, and the rover will start "running around". When the moonlit night falls, the rover will find a place to stay, put away the mast, close the solar wing, and start dormancy until the sun shines on the solar cell of the lunar rover, awakening the "sleeping" rover and lander to start another survey.

"Chang'e-4 uses the most advanced high-efficiency triple-junction gallium arsenide solar cells in China, and the photoelectric conversion efficiency has been increased from the original 28.6% to 30.84%." Chen Cheng, the person in charge of the solar cell circuit of the lunar exploration project, said that the new battery is superior to the original battery in terms of technical indicators such as photoelectric conversion efficiency, output voltage, output current, and radiation resistance, and its reliability has also been verified by multiple tests. "In addition, the new battery reduces the thickness of the solar cell sheet, which reduces the weight of the solar cell by 10%, and the power margin is increased from the original 6% to 9%."

"Queqiao" relay satellite

Another key reason why the Chang'e-4 probe was able to achieve a precise landing was the early launch of the relay satellite, Queqiao, which built a communication bridge between the ground and the Chang'e-4 probe.

Sun Ji, chief designer of the relay satellite "Queqiao" of the Fifth Academy of China Aerospace Science and Technology Corporation, said: "Although Queqiao is not the protagonist, it is an important component. Its mission is to provide communication services for the lander and rover on the back of the moon. This is equivalent to moving a ground station to the sky."

Zhang Lihua, chief designer of "Queqiao", said: "This relay satellite is not large, weighing only more than 400 kilograms, but one end of it is connected to a detector as far as 80,000 kilometers away, and the other end is connected to the earth more than 400,000 kilometers away. This distance is very far, so the aperture of the communication antenna must be large enough." However, due to the space limitations of the rocket, the antenna on the satellite faces many difficulties.

Taking many factors into consideration, the R&D team went through many design tests and finally chose a new solution of umbrella-shaped antenna. In this way, the antenna can be folded to the smallest size during launch and unfolded after entering orbit, but the processing and assembly process of such an antenna is very complicated. Because there are thousands of rope control points on the antenna, in order to ensure that it meets the surface accuracy requirements after unfolding, it must be manually adjusted point by point. "The work intensity and pressure of umbrella antenna development and satellite testing are huge, and the time is relatively tight. The scientific researchers in the team often work overtime for several months in a row. When it is most tense, they work around the clock without weekends or holidays." Zhang Lihua said.

The Queqiao satellite was finally launched from the Xichang Satellite Launch Center on May 21, 2018. In addition to providing relay services for the Chang'e-4 mission, the Queqiao satellite can also conduct scientific and technological experiments. It carries a low-frequency radio detector jointly developed by Dutch and Chinese scientists, which can "listen" to the "sounds" from the depths of the universe. In addition, it also carries a laser corner reflector developed by Sun Yat-sen University to carry out long-distance laser ranging experiments, laying a technical foundation for future applications. Ye Peijian, an academician of the Chinese Academy of Sciences and chief scientist of space science, also said that if foreign spacecraft want to explore the back of the moon, they can also get help from this relay satellite, which is China's contribution to the world.