Producer: Popular Science China
Crafted: Space Craft Brew
Producer: Computer Network Information Center, Chinese Academy of Sciences
At 2:23 on December 8, 2018, China's Chang'e-4 probe successfully took off from the Xichang Satellite Launch Center and began its journey to the moon!
According to Xinhua News Agency, at 10:26 on January 3, 2019, the Chang'e-4 probe autonomously landed in the von Kármán impact crater in the South Pole-Aitken Basin on the far side of the moon, achieving the first soft landing of a human probe on the far side of the moon.
Successful launch site
How significant is it to china's lunar exploration, and even to the entire human lunar exploration? The top ten highlights of this article are all-round interpretations for you.
Watch point 1: Why did you choose to explore the back of the moon?
Every day, countless people on the earth look up at the bright moon and give it all kinds of emotions. But very few people realize that from the first time they saw the moon to the present, except for the change in the "shape" (phase) of the moon, the "pattern" of the lunar surface has never changed, and even the moon seen by the earliest human distant ancestors and ancient apes is no different from that of modern people.
NASA's deep-space weather exploration satellite observed the Moon at a "fixed point" at the Lagrange point 1.5 million kilometers away, skimming the Earth, and it is obvious that the Moon is only facing the Earth on one side
The reason is simple:
Through hundreds of millions of years of efforts, the giant Earth has firmly achieved tidal locking of the moon, so that its orbital period around the earth is completely equal to its own rotation. From the earth, only the side of the earth that was fixed towards the earth can be seen. Although a small part of the back side can be glimpsed due to the moon's balance movement and parallax, most of it is invisible. We call it the "far side of the moon", and in some literary works, it is also called the "dark side of the moon" in art. It should be noted that the far side of the moon can also be illuminated by the sun, and it is not "dark", but it is not visible to humans.
Watch point two: "I chose to land behind the moon, not because it was simple, but because it was more difficult"
The pinnacle of human lunar exploration, and even human spaceflight, is the famous Apollo lunar project, which left three sentences that have gone down in history:
President Kennedy said twice: "We will send astronauts to the moon before the end of this decade, and then safely send them back," and "We chose to go to the moon not because it was simple, but because it was difficult."
After the efforts of countless astronauts, finally on July 20, 1969, Armstrong said the most famous sentence: "This is a small step for me, but a big step for all mankind."
By the time of China's Chang'e-3, humans had already had 20 landers on the front of the moon, but zero on the back
But all of the Apollo manned missions, and even other uninhabited lunar missions of the Soviet Union and the United States, were concentrated on the front of the moon, where humans already knew a great deal. Now, not only can top scientists accurately monitor the distance and surface of the Earth and moon with lasers every day, but ordinary astronomical enthusiasts can also use telescopes to understand every moon sea and every crater on the front of the moon. The Apollo program even landed 12 astronauts on the front of the moon and brought back 382 kilograms of monomer samples.
An important reason why I have been detecting the front and landing on the front, but never going to the back, is that the difficulty of landing on the back is much more difficult than the front.
Because the direction of the Soviet-American space race gradually became "expensive but limited", the two gave up lunar exploration. It also leaves a big challenge for human lunar exploration: who can successfully land on the far side of the moon? Are we going to stop going because it's hard?
Chang'e-4 replied: "I chose to land behind the moon not because it was simple, but because it was more difficult."
Watch point three: What will the exploration behind the moon bring to the scientific research community?
Direct emphasis:
The far side of the Moon is almost entirely craters/craters (97.5 per cent), much more than the front (69 per cent), with only theoretical explanations for the causes and current situation, and lack of field validation;
b. The South Pole-Aitken Basin on the far side of the Moon, the second largest supercrater in the Solar System, was discovered by NASA's Lunar Explorer as early as 1998 to have water ice here and is closer to the most primitive situation of the Moon;
c. The actual landing site of Chang'e-4 is located in the von Kármán Crater in the middle of the region, which is named after Qian Xuesen's mentor. After a strong collision, it is very likely that it has been submerged by lava, and it is rich in various substances, perhaps leaving the most primitive components of the mantle and preserving the deepest secrets of the moon;
d. The statement that "the development of helium tri-energy on the moon can provide human energy demand xx million/billion years", I believe you have seen a lot, and theoretically the environment on the far side of the moon is more likely to produce helium trinium;
e. There is a natural "shield" on the far side of the Moon, without any radiation interference from the Earth, which means that it has an incomparably perfect quiet environment for astronomical observation;
f. The lunar soil on the far side of the Moon is also very likely to be very different from the front.
The front of the Moon (left) and the far side of the Moon (right) look very different
The above basically means that the first probe to achieve landing on the far side of the moon will harvest many unique scientific research results, which will be a huge contribution to the world's aerospace and astronomical communities.
Watch point four: the moon is blocked, what about the signal?
Since the moon is blocking the far side, a signal relay satellite must be arranged to prepare for the lander and support the whole signal. Since the 1960s, NASA has been proposing ideas and arguments, but has never practiced them. So far, China has completed this task: as early as May this year, it successfully deployed the "Queqiao" relay satellite, operating in a halo orbit cluster near the second point of lagrange in the Earth-Moon 65,000 kilometers behind the moon.
The orbital relationship between the Queqiao relay star and the Earth and the Moon
This is the first lunar signal relay satellite in human history, which has erected the "Queqiao" behind the earth and the moon, which will witness the detection process of Chang'e-4 and will also be responsible for the full communication of Chang'e-4.
Watch point five: How complex is the landing area?
The dense craters on the far side of the Moon, especially the Topography and Elevation Map of the South Pole-Aitken Basin, are extremely complex and require a variety of systems with landing sensors. In particular, the speed of change in altitude exceeded that of any previous landing, with a maximum drop of up to 16.1 km. Von Carmen Crater belongs to one of the core areas, and for Chang'e-4, it is equivalent to crossing the lofty mountains to find it.
If the previous landing of the probe on the front of the moon is described as a small boat gently paddling across the lake, the landing of Chang'e-4 on the far side of the moon is like surfing in a stormy ocean, and the difficulty is not the same.
The topographic map of the Antarctic-Aitken Basin is far more complex than elsewhere, and is marked with the von Carmen Crater
In order to successfully land and release the lunar rover exploration, the difficulty and significance of overcoming can be imagined.
Watch point six: Why is it that it takes four days to reach the moon, and it is still delayed?
Chang'e-4 set off as early as December 8, using the strongest rocket currently detected in deep space in China, the Changsan-3B iii. Weighing 3.8 tons, it took only 4 days and 14:16 to complete the launch, orbit correction, peril moon braking, and entry into the working orbit, which is the shortest of the four major missions. However, the landing mission did not take place until January 3, 2019.
Why does it take so long?
It's to bide your time.
The Moon is tidally locked, rotating in the same way as its rotation, causing it to be one day a year, half night and half day. In other words, Earth time is the alternating day and night of the last 14 days. After entering lunar orbit, Chang'e-4 tested the instrument while waiting for the moon to enter daylight, after all, its main energy source is solar energy.
The shadow formed by the light illumination is also an important image reference for judging the terrain when landing
On the other hand, since the landing site is near the South Pole, it needs to gradually adjust the orbital inclination to pass through here, which is more time-consuming. At the same time, it is also necessary to wait for the sun's illumination to reach the ideal state at the angle of the moon, so that all the landforms will have clear shadows, and the optical equipment will be assisted to select and locate the landing site to the greatest extent.
There is a saying on the Internet that the timing of the Apollo moon landing refers to the traditional Chinese calendar - the lunar calendar, to some extent, this statement is quite reasonable, because the lunar calendar is consistent with the moon, and the moon phase explains the sun's illumination on the moon. This is an important reference for lunar landing devices.
Watch point seven: How to "move" on the landing of Chang'e-4?
Due to the greatly increased difficulty of landing, the landing method of Chang'e-4 will also be changed. Due to the unique geomorphological characteristics of the landing site, its obstacle avoidance needs are much higher than other landing tasks.
Comparison of the power descent process of Chang'e 3 and 4
It can be seen that after entering the preparatory stage, the trajectory of Chang'e-4 does not descend but rises, and then enters a process that almost achieves a long-distance vertical descent, during which the whole process of obstacle avoidance, hovering, fine obstacle avoidance, and slow landing is completed, which is extremely difficult. Such a complex "walking position" has extremely high requirements for autonomous navigation guidance and control.
Watch point 8: How does the energy of Chang'e 4 come from?
The combination of solar panels and rechargeable batteries is a basic feature of many lunar probes. During the day, solar energy on the Moon is extremely abundant, solar panels are charged, surplus electrical energy is saved, and all systems are working properly. Lack of energy at night, can only hibernate or reduce the intensity of work. China's Chang'e-3 and Yutu missions, as well as many unmanned probes in the Soviet Union and the United States, mostly use this scheme.
The Chang'e-3 and Yutu lunar rovers, launched on December 2, 2013, rely on solar panels for energy
In order to cope with the low temperature at night, the probe also needs to carry the radioactive isotope element plutonium-238, which has a half-life of up to 88 years and will continuously release heat for thermal insulation, such as Chang'e-3. At the same time, its heat can also be collected to generate electricity, which is the famous "nuclear battery", the five messengers of the solar system: Pioneer X, Pioneer XI, Voyager I, Voyager II, New Horizons, all rely on it to achieve deep space journey.
On Chang'e-4, China will experiment with its own "nuclear battery" technology for the first time. Although it is not the primary source of energy, this breakthrough is already significant.
Watch point nine: backup of counterattack!
Chang'e-4 was originally a backup version of Chang'e-3, but there is no doubt that its current scientific and engineering goals have far exceeded the former. During the power decline of Chang'e-4, it needs Queqiao to cooperate to complete all the actions, which requires a new upgrade of the on-satellite sensors and core landing aids.
Lunar micro-biosphere with potato, silkworm, Arabidopsis thaliana and life support systems
Based on past experience, Chang'e-4 has made certain improvements to the lunar rover in order to extend its service life and expand scientific research results. The solar panel system has also been upgraded to a high-efficiency three-junction gallium arsenide material for a more powerful energy source. Upgrading Chang'e-3's existing lunar radar and other equipment.
In addition, on scientific instruments, in order to make full use of the "clean" astronomical observation conditions behind the moon, it has added a series of devices for detecting neutrons and radiation doses, low-frequency radio and neutral atoms. In addition to scientific research, it also carries a micro-biosphere of the lunar surface with the main goal of popular science, which contains several organisms, and is also the first time that humans have conducted lunar biological experiments.
Highlight 10: A great breakthrough in international cooperation
As one of China's most iconic large-scale scientific research projects, the Significance of chang'e lunar exploration project in expanding China's international influence in space and even directly promoting international cooperation is irreplaceable for China, and the Chang'e-4 mission (in essence, the Queqiao relay satellite is also part of the Chang'e-4 mission) is a breakthrough step for China's spaceflight.
The Chang'e-4 mission has a number of international agencies on the rocket fairing, which can be seen as a highly international mission
In science, the low-radio detector is in cooperation with the Netherlands, the lunar neutron and radiation dose detector is in cooperation with Germany, the neutral atom detector is in cooperation with Sweden, and the lunar small optical imaging detector is in cooperation with Saudi Arabia.
After the success of the mission, the first-hand scientific research data collected by Chang'e-4 will be fully disclosed to the public, and scientists around the world are welcome to work together. In a word: Chang'e Project is the largest business card of China's aerospace industry in the world in recent years, causing a wide impact, but also receiving a lot of attention inside and outside the industry, and is the best platform for China's aerospace opening up to attract cooperation.
Therefore, the Chang'e-4 in front of us is not only China's, but also the world's. Its scientific and engineering significance is a major breakthrough for humanity as a whole.