By He Jing, Wang Fengjuan
On January 29 and 30, 2019, the Chang’e-4 lander and the Yutu-2 rover reawakened after withstanding a test of temperatures as low as negative 190 degrees Celsius during the first lunar night they spent on the far side of the moon, which lasted two weeks (a singular lunar day lasts an equivalent of 28 Earth days). This was the first time that humans obtained first-hand data on the temperatures of lunar night.
On the afternoon of January 11, the lander and the rover maintained stable operation and took pictures of each other with the assistance of the Queqiao (Magpie Bridge) relay satellite. Clear images taken by the lander and the rover were transmitted back to Earth, scientific payloads operated well, detection data was downloaded effectively, and relevant scientific experiments proceeded well. All of these indicate that the Chang’e-4 lunar mission achieved its preset goals.
Sticking the Landing
Is it hard for a probe to land on the far side of the moon? It’s every bit as difficult as one might imagine.
Blocked communications and huge temperature differences between lunar days and lunar nights were major obstacles impeding a soft landing on the far side of the moon. China’s Chang’e-4 probe chose to land on the moon’s far side, not only because it was untouched land for mankind but also due to its importance for scientific research. “We can compile a complete history of the moon’s evolution if we can determine what changes the celestial body went through four billion years ago based on analysis of the evolution of both its near side and far side,” explained Ouyang Ziyuan, first chief scientist of China’s lunar exploration program and an academician at the Chinese Academy of Sciences (CAS). “This can only be achieved through scientific experiments on the far side of the moon.”
The soft landing of the Chang’e-4 probe on the far side of the moon marked a first in human history of lunar exploration. NASA Administrator Jim Bridenstine expressed congratulations to China’s Chang’e-4 team and called the unprecedented landing an “impressive accomplishment.”
So, just how difficult was the landing? “The far side of the moon is scattered with high mountains and impact craters, making wide flat zones difficult to find,” explained Su Zezhou, chief designer of the Chang’e-4 at the China Academy of Space Technology. “This presented very high requirements for the accuracy of the soft landing on the moon’s far side.” To achieve a highaccuracy soft landing, the probe needed to find a comparatively safe landing point on the rugged lunar surface while maintaining precise control throughout the 450-kilometer descent with its advanced navigation and remote systems. In fact, this was the key to the soft landing of the Chang’e-4.
The Chang’e-4 probe landed in the South Pole-Aitken Basin on the far side of the moon. With a diameter of 2,500 kilometers and a depth of 12 kilometers, the basin is the largest and deepest of its kind on a solid celestial body in the solar system. With 90 percent of its area located on the far side of the moon, the basin boasts high value for scientific research. “Never before has anyone conducted close-up examination of the Aitken Basin,” remarked Zou Yongliao, director of the General Office of China Lunar and Deep Space Exploration of CAS. “We expect many new findings. Moreover, rocks on the lunar surface are older. It will be very helpful for us to understand the evolution of chemical elements on the moon if we can obtain data on the material components of older rocks.”
Scientists are particularly interested in the unique electromagnetic environment and geological features of the moon’s far side, which make it an ideal place to conduct low-frequency radio astronomical observations and scientific research on material components of the moon. “Due to the shielding effect, low-frequency radio observation is impossible on Earth,” declared Zou. “The magnetic environment on the far side of the moon is very clean. Carrying out low-frequency radio astronomical observations on the moon’s far side has been a dream for astronomers for a long time because such low-frequency radio astronomical observation is impossible on Earth.”
First Leaves on the Moon
To expand international exchange and cooperation and enhance openness and sharing, Chang’e-4 carried four international payloads and undertook three scientific and technological tests devised by domestic universities. Six living things—cotton, rapeseed, potato, thale cress, yeast and fruit flies— became the “passengers” on the lunar probe.
On January 15, Chongqing University announced that of the six biological samples aboard Chang’e-4, the green shoots of cotton had sprouted in a sealed can there.
“This was the first biological experiment conducted by humans on the surface of the moon,” said Professor Xie Gengxin, dean of the Institute of Advanced Technology at Chongqing University and chief designer of the biological experiment. “We successfully made the cotton seeds sprout tender shoots on barren land on the moon, which eventually grew the first green leaves on the moon. This provides foundation and experience for one day building a moon base.”
Alongside a minimum goal of sprouting seeds and maturing larva on the surface of the moon, the biological experiment also aimed to lay groundwork for future long-term space biological research. Considering the extremely harsh conditions on the moon including low gravity, strong radiation and drastic temperature changes, animals and plants chosen for the lunar biological experiment had to be tolerant of high temperatures, cold, radiation and disturbances.
“The six living things eventually traveling to the moon were chosen after hundreds of careful selections and numerous biological tests,” illustrated Professor Liu Hanlong, chief director of the biological experiment and vice president of Chongqing University. “From the moment we were committed to the task of selecting species for the experiment and sealing the biological payloads, our research team spent two years trekking areas including deep mountains in Yunnan Province and vast deserts of Xinjiang Uygur Autonomous Region in search of species that could survive in extreme conditions.”
To further enhance the biological payloads’ value for scientific research and science popularization, the research team carried out an Earthmoon comparison experiment at Chongqing University. “Both cotton and rapeseed sprout tender green shoots in sealed cans on Earth, and the cotton, rapeseed and potato also sprout in an open environment.”
Cooperation for Endless Exploration
Wang Wei, an academician with CAS and director of the Research and Development Department of China Aerospace Science and Technology Corporation, announced at the first China Aerospace Conference that the Chang’e-5 lunar probe will be launched in 2019 and complete a lunar sample returning mission, the final step of China’s three-step lunar exploration program.
However, the Chang’e-5 mission will not mark the end of China’s lunar exploration. According to the China National Space Administration, three subsequent missions, Chang’e-6, Chang’e-7 and Chang’e-8, will be launched. Those missions are expected to carry out sample returning, comprehensive detection and other experiments at the moon’s South Pole. Moreover, Chinese scientists will also research the feasibility of building a research base on the moon.
So far, no country has ever built a moon base. As early as 2004, then-U.S. President George W. Bush announced a new plan for space exploration including landing on Mars, returning to the moon and building a permanent moon base. The United States has still yet to achieve its goal of building a moon base. Over the decades, the EU, Japan and Russia also unveiled plans to construct moon bases, all of which haven’t seen any substantial progress.
Chang’e-4 carried payloads developed by Germany and Sweden, making it a successful case for international cooperation in lunar exploration. According to Zhang He, executive director of the Chang’e-4 probe project, China has always advocated international collaboration in space exploration. Led by China’s State Administration of Science, Technology and Industry for National Defence, scientific payloads on Chang’e-4 were collected globally. “This international cooperation enabled Chinese scientists to deepen exchange and promote technological advancement with their German and Swedish counterparts,” said Zhang. “They are also willing to carry out more cooperation in deep space exploration, lunar exploration and other realms.”
The fourth phase of China’s lunar exploration program aims to build a scientific research station that could operate for a long time on the moon— probably near its polar regions. “We hope that other countries will also participate in the construction of the research station and relevant research missions,” added Zhang. “A cooperative project would give it maximal use and produce more research results. More international cooperation is expected in the future.”
After the Chang’e-4 mission, Chang’e-5 will be launched, which is the last step of the three-step lunar exploration project designed to “orbit, land and return samples from the moon.” The lander and rover of Chang’e-4 will stay on the moon forever, but Chang’e-5 will return to Earth and perform a lunar orbital docking after landing on the moon and collecting samples on its surface. Many new technologies are expected to be applied in the Chang’e-5 lunar probe.