Today marks the fifth anniversary of the soft landing of the Chang'e-4 probe on the back of the moon. On January 3, 2019, the Chang'e-4 probe successfully landed in a pre-selected area on the far side of the Moon, and transmitted the world's first close-up image of the moon's back through the Queqiao relay satellite, unveiling the mystery of the ancient moon's back. The Chang'e-4 mission has achieved the first soft landing of a human probe on the far side of the moon and the first relay communication between the moon and the earth, and China has carried out international cooperation of great significance with many countries and international organizations, opening a new chapter in human lunar exploration. In 2019, "Chang'e-4 has received many scientific and technological achievements that have attracted worldwide attention, and has become a star in the international aerospace community."
The panoramic camera of the Yutu-2 rover images the Chang'e-4 lander
The Chang'e-4 lander's topographic camera images the Yutu-2 rover
Why explore the far side of the Moon?
If the ancients and the present people are flowing water, they will see the bright moon together. Interestingly, not only did the ancients and modern people look at the same moon, but they also see the same side of the moon, because the rotation period of the moon is consistent with the rotation period around the earth, so the side of the moon facing the earth is always the same, and human beings cannot directly observe the scene on the far side of the moon.
In the past 60 years, countries around the world have carried out more than 110 lunar exploration activities, successfully carried out dozens of unmanned lunar exploration and manned lunar landings, and obtained a large amount of scientific information on the lunar topography, landform, element distribution, gravity field, magnetic field and other scientific information. However, the results of exploration of the far side of the moon are very limited, the former Soviet Union and the United States have used probes around the moon to observe the terrain and map the far side of the moon, but no probe has been able to land on the far side of the moon, and the exploration of the far side of the moon is like a veil and has never been able to penetrate.
The exploration of the far side of the Moon has important scientific value and has been diligently pursued by the scientific community. On the one hand, the geological conditions of the lunar back are different from those of the lunar surface, and the study of the geological evolution history of the lunar back is of great significance for understanding the formation of the moon, the early history of the Earth-Moon system, and the deep structure of the moon. The far side of the Moon is dotted with a large amount of highland terrain, full of impact craters and craters. The famous South Pole-Aitken Basin is located in the south of the back of the moon, is the largest and oldest impact basin in the solar system, it was formed 3.92 billion years ago, the deepest depth is 13 kilometers, representing the impact event it experienced may dig out the deep lunar crust or even the slow lunar material, is the key and window to study the evolution history of the moon and the composition of deep materials.
Image map of the naming of geographic entities in the Chang'e-4 landing area
On the other hand, astronomical observations of electromagnetic waves from the far side of the moon can obtain information that has never been exposed to humans, and will reveal a wealth of scientific content. For a long time, scientists have mainly used electromagnetic waves to carry out astronomical observations to understand the universe. Observations in most spectral bands such as ultraviolet, radio, X-ray, infrared and millimeter waves have been carried out, but low-frequency radio astronomy observations have not been effectively carried out due to ionospheric blocking and terrestrial radio interference. Low-frequency observation has important research value for all-sky imaging, cosmic dark period characteristics, solar physics and space weather. The far side of the moon avoids the electromagnetic interference on the earth's surface, making it an ideal place for cosmic low-frequency radio observation, which provides the possibility for the study of the sun, planets and extrasolar objects, and will also provide important information for the study of the origin of stars and the evolution of nebulae.
Why is there no country landing on the far side of the moon? Because it's very difficult. Why did Chang'e-4 have to challenge the landing on the back of the moon? Because no one has been there yet, in order to explore the unknown, "no one has ever been" is the reason for the challenge.
After 10 years of sharpening the sword of the lunar exploration project, Chang'e-1 has drawn up a topographic image map and a three-dimensional map of the whole moon, and clearly detected the outer space environment of the moon and the composition, type and distribution of rocks on the lunar surface. Chang'e-2 "completed a 7-meter resolution digital image and a three-dimensional map of the whole moon, and visited the Sun-Earth Lagrange point;" Chang'e-3 successfully landed on the moon and opened up the history of "surveying the sky, observing the earth, and measuring the moon" on the mainland. In the accumulation and progress of lunar exploration missions, after rounds of long and arduous repeated demonstrations and rigorous experiments, "Chang'e-4" aimed at landing on the far side of the moon and took a new step for mankind.
How to explore the far side of the Moon?
Every step of lunar exploration requires overcoming countless scientific and technological problems. Communications, landing zone selection, lunar timing, orbit control, and landing technology are all major challenges for Chang'e-4.
Communication is the primary concern. The back of the moon is always facing away from the earth, and the probe is blocked by the lunar sphere and cannot communicate directly with the earth, so the far side of the moon has not left the "footprints" of human probes. In this regard, the mainland launched the Queqiao relay satellite, and overcame a series of challenges such as orbit, communication link design, and transmission delay to build an Earth-Moon communication bridge. The "Queqiao" runs in the Halo orbit around the L2 point of the Earth and the Moon, and is located on the Earth-Moon extension line at a distance of more than 400,000 kilometers from the Earth and 65,000 kilometers from the Moon. This orbital concept was proposed by the United States in 1967, but it was never realized due to the high risk and high technical complexity. "Queqiao" bravely chose and successfully entered the mission orbit, becoming mankind's first lunar communication relay satellite.
On the far side of the Moon, with its mountainous peaks and rugged terrain, where is it easier to carry out a soft landing with good scientific benefits?" The Chang'e-4 team comprehensively investigated the scientific exploration value, the visibility of relay communications, the thermal and illumination environments, the undulation of the track elevation during the landing process, the dispersion range of the landing site, the monitoring and control of the probe, the orbital accessibility, the solar altitude angle during landing, and the adaptability of delaying the landing.
Chang'e-4 will have to cross long distances between the Earth and the Moon, switch multiple orbits, and make a soft landing during the day on the far side of the Moon, which requires calculating and maintaining a relative relationship with the Sun, Earth, and Moon throughout the flight. Among them, the lunar orbit should adjust the perilunar position and flight time, so that the orbit around the moon of "Chang'e-4" passes over the lunar point and arrives above the lunar point during the day on the far side of the moon; During the flight to the moon, the Chang'e-4 solar sail maintains a certain range of angles with the direction of sunlight incidence to ensure energy supply. In the process of going to the moon and landing on the moon, it is necessary to maintain the communication connection between the ground and the "Chang'e-4", and carry out accurate measurement and control in advance under the interference of communication delays. In order to meet a variety of constraints, researchers have calculated four launch time windows totaling 6 minutes distributed in 2 days, which poses a severe challenge to the adaptability of the launch vehicle, to be able to quickly switch between different launch time windows in two days, to adapt to the needs of the mission, to improve the reliability of the main subsystems and important single aircraft of the rocket, to ensure that the launch is on time within the window of as short as 1~2 minutes.
Landing on the back of the moon is the ultimate challenge. The topography and elevation map of the Antarctica-Aitken Basin are extremely complex, with the terrain undulating dramatically, the altitude changing faster than any landing in history, and the landing on the back of the moon is like surfing in a stormy ocean. Chang'e-4 needs to climb over lofty mountains to reach the landing area and overcome navigation errors caused by undulating terrain. In this regard, the Chang'e-4 team optimized the orbit design and control to enable the probe to enter the landing area; The in-orbit calibration of engine thrust was refined, the strategy of the whole probe power descent and landing process was optimized, and relay obstacle avoidance methods such as optical obstacle avoidance, laser 3D imaging obstacle avoidance, and two-wheel obstacle avoidance were designed to ensure the final accurate landing. In the final landing stage, Chang'e-4 first descended and then ascended, and then entered a long-distance vertical descent process, during which it completed the whole process of obstacle avoidance, hovering, precise obstacle avoidance, and slow landing, and went through a series of complex "positioning" to achieve a "fixed-point, timed, and accurate" landing.
Bringing ideas to life – a list of Chang'e-4 missions
Overcoming many difficulties and breaking through various problems, "Chang'e-4" has moved from concept to reality step by step.
On May 21, 2018, the Queqiao relay satellite was launched, first entering the predetermined Earth-Moon transfer orbit, after four days of flight and near-moon braking, it successfully entered the transfer orbit from the Moon to the Earth-Moon L2 point, and flew to the Earth-Moon L2 point with the help of the force, and then entered the Halo mission orbit after three orbit capture and correction controls, becoming the first spacecraft in human history to visit this space, and building a "lifeline" of Earth-Moon communication for Chang'e-4.
On December 8, Chang'e-4 was launched into the Earth-Moon transfer orbit, and after a mid-orbit correction and a five-day journey, it successfully entered the circular lunar orbit through near-moon braking, and then changed its orbit again to enter the lunar elliptical orbit after completing two communication tests with Queqiao. On January 3, 2019, "Chang'e-4 successively completed the main deceleration stage, attitude adjustment and descent stage, hovering stage, slow descent stage, buffer landing stage and other processes, and landed steadily in the predetermined area, completing the feat of the first soft landing on the far side of the moon."
After landing on the lunar surface, with the support of Queqiao, the Yutu-2 rover left the lander and stepped on the lunar surface, and the surveillance camera on the lander took the first trace image of the "Yutu-2" on the back of the moon and transmitted the world's first close-up map of the back of the moon back to the ground. From January 4th to 11th, during the first month-to-day working period, the payload of the Chang'e-4 lander started to start the exploration work, and completed the round-up of the terrain around the landing site: "Yutu-2" successfully completed the relay satellite link connection, the payload was started, the scientific experiment project was successfully carried out, the detection data was transmitted, and the lander completed the task of mutual shooting. So far, the Chang'e-4 mission has been a complete success, and China's lunar exploration program has achieved "five victories in five battles".
On January 29 and 30, the Yutu-2 and Chang'e-4 landers completed their autonomous awakening and safely spent the first moonlit night. The lander's moonlit night temperature collector successfully monitored the temperature change of the first moon's night, and in the long-term extremely low temperature environment of the lowest -190 °C, the lander and the rover withstood the test, the key equipment was powered on according to the predetermined procedure, and the communication and data transmission status to the ground through the Queqiao relay satellite was stable, and the scientific exploration of the second moonday continued as planned.
Since then, Chang'e-4 has continuously undergone the test of day and night, carried out a series of scientific experiments, and continuously achieved new results. On December 20, Chang'e-4 successfully woke up on its own and entered the thirteenth day, quietly breaking the world record held by the Soviet Union's "Lunar Rover No. 1" for 49 years, becoming the lunar rover with the longest working time on the lunar surface. The lunar neutron and radiation dose detector and low-frequency radio spectrometer on the Chang'e-4 lander were started up as planned to carry out the scheduled scientific exploration. During the inspection, the Yutu-2 lunar rover carried out scientific exploration work on a number of detection points, carried infrared spectrometer to image and infrared detection of a lunar surface stone, panoramic camera, neutral atom detector, and lunar radar obtained a large amount of scientific detection data.
The Chang'e-4 lander and the Yutu-2 rover completed their work during the thirteenth day and went into a moonlit night
The Chang'e lunar exploration project is one of the most iconic large-scale scientific research projects in mainland China, from "Chang'e-1" to "Chang'e-5", China's lunar exploration has never stopped, and the Chang'e-4 mission has taken a breakthrough step for China's space flight. In 2020, Chang'e-5 achieved China's first lunar sample return mission. Around 2024, Chang'e-6 is scheduled to be launched. It will go to the far side of the moon again to carry out a sample return mission on the back of the moon to improve human understanding of the moon.
During the September day, Yutu-2 discovered unidentified gelatinous material in an impact crater on the far side of the Moon
This article is an excerpt from "Exploring the Moon's "New World": A Review of the First Anniversary of Chang'e-4's Lunar Landing", published in SpaceX, Issue 2, 2020.
Text: Yang Shirui
Editor/Yang Sishuang
Audit/Mutan
Executive Producer/Jiang Jun