Why can’t we see the far side of the moon from Earth? How difficult is it for Chang’e-6 to land on the far side of the moon?

Why can't we see the far side of the moon from Earth? The reason why we can't see the far side of the moon from Earth is that the moon's rotation period is the same as its revolution period, a phenomenon called tidal locking. The moon's revolution period around the Earth is about 27.32 days, and its rotation period is also 27.32 days, so the moon always faces the Earth with the same side. This means that the far side of the moon is always out of sight from Earth, and humans cannot directly see the far side of the moon unless through space probes or other means.

So how does the tidal locking of the Moon come about? The tidal locking of the Moon is a long-term process involving the gravitational interaction between the Earth and the Moon. Here are the basic steps in the formation of tidal locking:

1. Gravitational tidal effects: The Earth exerts gravity on the Moon, and the Moon also exerts gravity on the Earth. Since the Earth's gravity is stronger on the Moon, it causes tidal deformations on the Moon's surface. This deformation changes constantly as the Moon orbits the Earth, as different parts of the Moon alternately move closer to and farther away from the Earth.

2. Friction effect: As the Earth's gravitational pull on the Moon changes, the Moon's tidal deformation also changes. This gravitational tidal effect causes friction inside the Moon, converting rotational energy into heat energy.

3. Momentum transfer: Due to the friction effect, the rotation speed of the moon gradually slows down, while the revolution speed remains unchanged. This causes the rotation period of the moon to gradually increase until it is equal to its revolution period. When the rotation period of the moon is equal to its revolution period, the moon reaches a tidal locking state.

4. Achieve tidal locking: In the tidal locking state, one hemisphere of the moon always faces the earth, while the other side always faces away from the earth. This is because the moon's rotation period is the same as its revolution period, so that the moon's long axis is always along the line connecting the earth and the moon, thus avoiding further deformation and friction caused by tidal forces.

Tidal locking is a slow process that takes billions of years to occur. The Moon has reached a tidal locking state, so we can only see one side of the Moon. This also explains why we can only see the same side of the Moon from Earth forever.

Image source: China Lunar Exploration and Deep Space Exploration Network

The difficulty of Chang'e 6 landing on the far side of the moon is very great, mainly facing the following difficulties:

1. Communication difficulties: Since the back of the moon cannot communicate directly with the earth, Chang'e 6 needs to rely on the "Queqiao" relay satellite to transmit information. This increases the complexity of the mission, because the relay satellite cannot be "online" at all times and may not be able to receive instructions and send back data in time.

2. Complex terrain: The terrain on the far side of the moon is complex and changeable, with a large number of obstacles such as mountains and impact craters, which puts extremely high demands on the design of the lander. Historically, the Soviet Union had suffered consecutive failures in unmanned lunar exploration due to the complex terrain on the far side of the moon.

3. Insufficient light: The back of the moon lacks direct light, which makes the navigation and positioning of the lander more difficult, and also affects the energy supply of the lander's solar panels and the temperature control of the lander.

4. Extreme temperature difference: The environment on the far side of the moon is extreme, with a huge temperature difference between day and night, which places extremely high demands on the materials and electronic equipment of the lander. This is like the first time humans climbed Mount Everest, full of unknowns, and a test of human courage and wisdom.

5. Autonomous navigation capability: Since it is impossible to communicate directly with the Earth, it is an important requirement for Chang'e-6 to have autonomous navigation capability. Autonomous navigation capability can ensure that Chang'e-6 can independently complete tasks such as flight trajectory control, safe landing point selection, and precise control.

6. Sampling time limitation: Due to terrain restrictions, Chang'e 6's sampling time on the back of the moon is compressed, which means that it becomes more difficult to complete the sampling mission within a limited time.

The difficulty of Chang'e-6 landing on the far side of the moon lies in the challenges it faces in communication, terrain, lighting, temperature difference, autonomous navigation, sampling time, etc. Despite this, China Aerospace Science and Technology Corporation successfully overcame these difficulties and successfully landed Chang'e-6 on the far side of the moon and began sampling on the far side of the moon.

Chang'e-6 faced a unique challenge when performing the sample return mission on the far side of the moon: due to the rotation of the moon, the far side cannot directly establish line-of-sight communication with the earth. To solve this problem, the scientific research team took the following measures:

1. Deployment of relay satellites: The scientific research team carefully designed and deployed the Queqiao-2 relay satellite, which was precisely positioned at the Earth-Moon Lagrange L2 point, about 65,000 kilometers from the moon. The L2 point is a relatively stable gravitational balance point that can see the Earth and the back of the moon at the same time, making it an ideal location for information transmission.

2. Use of high-gain antennas: In order to ensure stable data transmission and reception, the Queqiao-2 relay satellite is equipped with high-gain antennas. This type of antenna can transmit and receive signals in a concentrated manner, significantly enhancing the strength and quality of communication signals, ensuring high-quality communication links during long-distance transmission.

3. Data protocol design: In order to improve data transmission efficiency, the research team combined the characteristics of deep space communication and designed a data protocol suitable for long-distance transmission. This protocol compresses and optimizes data during transmission, greatly improving the data transmission rate and reducing transmission delay.

4. Construction of a global receiving network: In order to further improve the efficiency and reliability of data transmission, multiple receiving stations on Earth work together to form a global receiving network. In this way, at any time, at least one receiving station is in the best condition to receive and process data from the relay satellite for analysis and use by scientific researchers.