Light on the “Dark Side” of the Moon

On January 11, 2019, the Chang’e-4 lander and Yutu-2 rover took pictures of each other with the support of the Queqiao (Magpie Bridge) relay satellite, marking overwhelming success of China’s Chang’e-4 mission. China’s lunar program has already reaped remarkable achievements. Chang’e-4 is the first probe from Earth to soft-land on, rove and explore the far side of the moon. In the coming years, China’s lunar program will continue to develop quickly. By around 2020, the Chang’e-5, China’s first sample return mission, will bring lunar samples back to Earth. After that, subsequent missions of China’s lunar program such as Chang’e-6 and Chang’e-7 will be launched in succession.

China’s lunar exploration program was formally launched in early 2004. Compared to the United States and some European countries, China’s lunar program started late. Why do humans spend so much on lunar exploration? In general, moon exploration is tremendously significant for scientific, technological, and economic fields as it promotes social development and influences future life.

For example, with almost zero atmospheric and geological activities on the moon, rocks are kept in pristine condition. Studying moon rocks is like studying samples from Earth some four billion years ago. Research of the moon helps provide more early information about Earth and enables people to better understand the origin and evolution of life, Earth, the solar system and even the whole universe. Since the moon has no global magnetic field, its geological structure is extremely stable, and because the moon’s far side is free from radio signal interference from Earth, it is an ideal place to set up large observatories and other scientific research bases.

Exploring the moon is a complicated systematic project. Lunar missions can promote and facilitate the development of basic scientific research and high-tech realms including cosmology, space astronomy and space physics. Furthermore, since the moon is home to abundant natural resources, lunar missions can set a foundation for future usage of moon resources.

It is easy to grasp the great significance of lunar missions. According to Agreement Governing the Activities of States on the Moon and Other Celestial Bodies by the United Nations Committee on the Peaceful Uses of Outer Space, the moon and its natural resources are the common heritage of mankind. However, the countries that develop the celestial body first will certainly benefit first. Consequently, increasing numbers of countries are actively working on lunar programs. India, Israel, and Japan all have plans to launch lunar probes in 2019.

China now leads the world’s second-tier lunar exploration group composed of Europe, Japan and India. China has reached a lunar orbit, landed and roved on the moon and conducted both general surveys from afar and detailed regional investigations at a close range. It is worth mentioning that Chang’e-4’s soft landing on the far side of the moon has left a major impact on the world.

The far side of the moon, which always faces away from Earth, starkly contrasts the near side of the moon that we see almost every night. In general, the moon surface is composed of two geological forms: lunar maria (Latin for “seas”) and terrae (Latin for “earth”). Compared to maria, terrae are older and preserve more of the original appearance of the moon. The far side of the moon has a battered, densely cratered appearance with few maria. Conducting research and investigations of the morphology, substance composition, soil and shallow structure of the moon’s far side will help people learn more about the moon’s early evolution and holds important value for the research on the early history of Earth.

Since the moon rotates on its axis at the same rate that it orbits Earth, only the near side of the moon is visible from Earth. Thus, for astronomical researchers, the far side of the moon is ideally quiet. The far side of the moon is shielded from electromagnetic interference from Earth where low-frequency radio waves less than 10MHz can be monitored, so it’s an ideal place to research the origin of stars and nebula evolution. Major astronomical breakthroughs are expected to emerge from there.

However, because people can never “see” the far side of the moon from Earth, probes have a difficult time establishing radio communication with Earth from there. This factor has presented a persistent difficulty hindering a landing on this part of the moon. Although more than 100 probes and orbiters have been launched to the moon by various countries since the 1950s, never before had a probe landed on its far side. Furthermore, the exceptionally complicated landforms on the far side of the moon pose great challenges for soft landing, roving and researching in this area.

China dived headfirst into solving this worldwide problem. On June 14, 2018, China launched the Queqiao relay satellite into a halo orbit around the Earth-Moon L2 point. In this orbit, the relay satellite serves as a mirror facing both Earth and the moon’s far side at the same time, which enables a communication link to Chang’e-4 after it landed. Since Chang’e-4 was planned to land in a rugged environment, the probe was designed to achieve a high-degree landing precision. To avoid “cliffs” on the moon surface, Chang’e-4 is equipped with strong abilities in automatic navigation and obstacles avoidance. It was able to find a comparatively flat place independently before performing a nearly vertical landing.

The Chang’e-4 mission is a first for both a human probe visiting the moon’s far side and a communication satellite operating at the L2 point of the Earth-Moon system. It has provided scientists with a new work platform on the moon’s far side and produced abundant original scientific research data. The mission serves as a critical step for humans to uncover the mysteries of the moon’s far side. Scientists expect that after the Chang’e-4 probe conducts detailed exploration on the far side of the moon, the long-standing scientific mystery of the spike in impact rate 3.9 billion years ago on both the moon and the Earth can be solved. 

The author is chief science popularization expert on space exploration technology at the China Association for Science and Technology and a former researcher with the China Academy of Space Technology.