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China Opens a New Page in Lunar Exploration

On January 3 at 10:26 AM Beijing time, China’s Chang’e-4 (嫦娥四号) spacecraft, housing the 140 kilogram Yutu-2 (玉兔-2) rover, set down in the polar region on the far side of the Moon. This high-risk, challenging mission has never before been attempted by any space agency.


The non-Earth facing side of the Moon has previously only been observed from orbit by various unmanned craft and Apollo astronauts. The visibly distinct differences between the near and far sides of the Moon have posed unanswered questions to scientists, who are still not even sure how the Moon formed. This ground-breaking mission is mankind’s first opportunity to take {in situ} measurements and high-resolution photographs of the far side.


The Chang’e-4 mission is also a demonstration of China’s further “opening up” of its civilian space program, by the China National Space Administration. Historically, the civilian side of China’s space program was run largely by the People’s Liberation Army (PLA), which chose and trained the astronauts, and ran the closed launch sites. By contrast, NASA was created at the opening of the space age in 1958 as a civilian agency. However, it appears that the PLA will be playing less of a role. The next group of Chinese astronauts will include civilian scientists and engineers. And China’s new launch site, on Hainan Island, will be open to the public, and include a hotel for tourists, museum, and a space-themed park.


China’s lunar exploration has also been ‘opening up,” encouraging international participation. In 2015, when the Chang’e-4 spacecraft was under development, China invited other nations to participate in the mission. As a result, major scientific instruments on the spacecraft were contributed by Europe, with European scientists as lead investigators.


Some commentators have described the mission as a “Sputnik moment.” For years, American media have derided China’s manned missions as nothing new; just reliving what NASA had accomplished in the 1960s. Now, China has gone out front with this far side mission, doing what Americans, including Apollo 17 astronaut and geologist Harrison Schmitt, had proposed, in 1972, but was never carried out by the U.S.

Recognition of the accomplishment has come in congratulations from the heads of the U.S. and Russian space agencies, and the governments of Argentina and Brazil.

 

The Far Side

Although the far side is often described as the “dark side” both sides of the Moon receive an equal amount of sunlight, as the Moon rotates on its axis. We never see the far side because due to its “tidal locking” with the Earth, the Moon’s period of rotation on its axis (its four week full day) is the same as its revolution around the Earth (also, one month), which results in the same side facing the Earth. The far side is only “dark,” in the sense of our lack of knowledge about it.

        

The far side of the Moon is distinctly different from the hemisphere that is seen from Earth, which is yet to be explained. There are few smooth plains, or mare, on the far side, the “seas” of volcanic lava, which are prominent on the side we see. The far side is characterized instead by craters, with numerous smaller craters within larger, older ones. Chang’e-4 sits inside the 186-kilometer diameter Von Karman crater, within the huge 2,500 km wide South Pole-Aitkin basin, which is the oldest and largest lunar impact crater. Previous orbital observations suggest there are large caches of water ice there.


Samples of soil from the lunar near side, brought back by the Apollo astronauts, are very similar in chemical composition to that of the far side, which has been preliminarily measured from orbit by spacecraft such as NASA’s Clementine and the European Space Agency’s SMARt-1. But the chemical composition of both sides is not identical. Unlike the near side, which is shielded by the Earth from a certain amount of galactic cosmic radiation, the far side is completely exposed. Perhaps this exposure to radiation could account for some of the difference in the geologic history of the two different hemispheres of the Moon, such as volcanic activity. Or could it account for differences in chemistry?

 

Planetary scientist, Mark Robinson at Arizona State University, told Scientific American in December 2018 that the mare regions of the Moon are cooled lava from volcanic eruptions billions of years ago. Brought up from deeper layers of the Moon, the mare basalt provide data on the overall composition of the Moon, but also potentially, the mantle, which is the layer between the core and the crust. Although the far side has few frozen lava “seas,” if Yutu-2 can explore one, it would help explain the differences between the two sides of the Moon. Yutu-2 may also be able to explore more recent impact craters inside the basin, to add to what it discovers at the Von Karman crater.

 


Landing on the far side posed several new challenges for the Chinese scientists and engineers. One is the communications challenge. Because it is out of sight, communications with spacecraft on the far side—both commands sent from Earth to the spacecraft, and data transmitted from the Moon to Earth--cannot be done directly. Therefore, last May, China launched a relay satellite named Queqiao, (鹊桥) or Magpie Bridge, which flew about 60,000 km past the Moon to the gravitationally-stable L-2 region, where it needs little fuel to maintain its position. From that vantage point, it can communicate with both the spacecraft on the Moon’s far side and the scientists and engineers on Earth, simultaneously. China has offered use of the relay satellite to any other nation with a far side mission in the future.

The second challenge was to land Chang’e-4 autonomously. Even though the lag time in communication via the relay satellite is only 60 seconds, choosing a safe landing site had to be done so quickly, the spacecraft was equipped with the navigational capability to hover while it determined by itself where it was safe to land.

 

Science on the Moon

The rover is equipped with an imaging spectrometer, which will enable an analysis of the chemical composition of the lunar surface. The ground-penetrating radar aboard rover Yutu-2, will create images of the various layers of the Moon, to reveal the history and geologic features below the surface. These can be compared to radar data that Yutu has been gathering from the near side of the Moon, on the Chang’e-3 mission.

 

In 2015, when the Chang’e-4 mission was under development, China invited other countries to contribute experiments. The Advanced Small Analyzer for Neutrals, contributed by Sweden, will explore how the solar wind interacts with particles and the soil on the Moon. And the German Lunar Lander Neutrons and Dosimetry instrument will make measurements of the radiation environment in the vicinity of the landing site. As the scientists note, the poles of the Moon are the ideal place for lunar bases. The Chinese plan is to create a robotic science base, which would periodically be visited by astronauts.

 


The lunar poles have concentrations of water ice inside perpetually dark craters, due to the migration of water molecules, delivered by comets, meteorite, or the effects of the solar wind. The far side south pole region has mountains with “peaks of eternal light” that are in sunshine the majority of the time, rather just two weeks a month. For a base visited by crew, access to water and solar power is key.


A third international scientific payload takes advantage of the fact that, never facing the Earth, the far side is not bombarded with radio and other man-made electromagnetic noise. The Moon’s quiet far side is the perfect place for radio astronomy. Aboard the relay satellite is the Netherlands-China Low-Frequency Explorer. Its job is to map the radio sky to study large-scale noise in our galaxy, and to detect and explore solar and planetary radio bursts.


The Chinese scientists have pledged that what is learned about the Moon will be shared with the scientific community internationally. This has not always been the case. The fact that two of the experiments are from foreign institutions will most likely spur timely release of data, since the principal investigators traditionally have first access to the raw data as it is received from the relay satellite by China’s mission control.


And there are also living creatures now on the Moon, for the first time. They hitched a ride to the Moon aboard the Chng’e-4 spacecraft. A small container holds a variety of plant seeds and silkworm eggs. The goal is to create a miniature biosphere, in which as the plants grow, they produce the oxygen needed by the silkworms, and as the silkworms grow, they will produce the carbon dioxide needed by the plants. The experiment was the result of a collaboration of 28 Chinese universities.

Chang’e-4 is just starting its exploration of the lunar far side. The lander and the rover should reveal some of the secrets of the far side.


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