Fifty years ago this month, NASA’s Apollo 8 mission allowed human eyes to view the far side of the Moon directly for the first time. The crater-pocked land that the crew saw as they looped around the Moon was completely hidden until 1959, when the Soviet Luna 3 mission transmitted black and white images of the gray landscape, idle and virtually unchanged for billions of years. But the next chapter of exploration on this untrodden ground is expected to kick up a little of the long-standing lunar dust.
Today, a Chinese Long March 3B rocket lofted a spacecraft that will attempt the first ever soft landing on the lunar far side: Chang’e-4. The craft is expected to make its landing sometime between January 1 and 3 after a five-day cruise to the Moon, lunar orbit insertion, and a few course corrections in preparation for sunrise over the presumed landing site at Von Kármán crater in late December.
Since initiating its lunar project in the early 2000s, China has advanced rapidly to become the only nation to have soft landed (as opposed to intentionally impacted) anything on the Moon since the Soviet Luna 24 sample-return mission in 1976. After successfully setting down the robotic Chang’e-3 lander and rover on Mare Imbrium in December 2013, China became only the third country to achieve a lunar landing. The success allowed the country to repurpose a backup spacecraft manufactured at the same time for an even more ambitious mission.
In 2015, China announced that the spare Chang’e-4 spacecraft, a lander and small rover, would be used for a pioneering mission to land on the far side of the Moon—a feat that has never been attempted due to its complexity.
The spacecraft, with a combined mass of nearly four metric tons, will be carrying eight instruments and science payloads. These tools will be employed in low-frequency radio experiments, taking advantage of the unique radio-quiet far side, shielded by the Moon from the cacophony of terrestrial radio signals. Science experiments on Chang’e-4 will also investigate interactions of solar wind particles with the lunar surface, conduct geological analysis from the ground and carry out mineralogical studies of the regolith and rock.
Joining the ride will be the first lunar biosphere experiment, designed by students in universities across China, which consists of a canister containing silk worm eggs, potato seeds and Arabidopsis—a flowering plant used widely in space research. With water, air and nutrients for the organisms, scientists will study the respiration and photosynthesis of the first life to voyage to the surface of the far side.
Landing and operating a spacecraft on the far side of the Moon is particularly challenging thanks to the phenomenon of tidal locking. The Earth’s gravitational influence has, over time, slowed the Moon’s rotational period to equal the time that it takes to complete an orbit around Earth. As a result, the same side of the Moon is always facing those of us on the ground. To send commands to the other side, a mission like Chang’e-4 requires some innovative communications solutions.
To contact the spacecraft during and after the landing, China launched a relay satellite named Queqiao in May with a large parabolic antenna. The satellite is positioned in a halo orbit at a stable point some 65,000 to 85,000 kilometers beyond the Moon, known as the second Earth-Moon Lagrange Point. From this perch, Queqiao enjoys constant line-of-sight with both terrestrial ground stations and Chang’e-4’s landing site.
But the extra effort to get there should be worth it. Not only will the mission allow space observations without electromagnetic interference from Earth, but the lunar far side also promises insights into the history and evolution of the Moon and the Earth. For decades, scientists have been eager to investigate the vast area known as the South Pole-Aitken Basin (SPA), stretching from the South Pole up to Aitken crater.
The SPA basin is around 2,500 kilometers in diameter and 12 kilometers deep, making it one of the solar system’s largest and oldest impact craters. The ground in this area could contain exposed material from the deeper lunar mantle, thrust to the surface during the large impact billions of years ago. And considering many astronomers believe the Moon is formed of the same material as the Earth, the SPA basin could potentially provide information about what the proto-Earth looked like, just after the theorized collision that formed the Moon. Sometime after that collision, life took root, and the far side of the Moon can add a few pieces to that puzzle.
The landing site, though not officially confirmed, is understood to be the southern floor of the Von Kármán crater. Bradley Jolliff, Professor of Earth and planetary sciences at Washington University in St. Louis, notes that the selected site within Von Kármán is relatively smooth due to ancient lava flow, offering a safer landing.
Meng Zhiguo, a professor at the College of Geoexploration Science and Technology, Jilin University, and co-author of papers on the Chang’e-4 candidate landing sites, is particularly interested in the possible insights the mission could offer into the composition of the lunar far side made possible by two spectrometers flying on the spacecraft, the Visible and Near-Infrared Imaging Spectrometer (VNIS) and the Low-Frequency Spectrometer (LFS).
“The compositions of the crater floor with mare basalts will provide direct proof about the mare volcanism of the lunar far side, which is useful to further understand the difference of the mare volcanism between the near side and the far side,” Meng says.
The near side of the lunar surface has many vast basaltic plains known as maria. Each mare was created by volcanic eruptions in the distant past, and they can be seen as dark flat areas with the naked eye. The far side, however, has relatively few maria. The unique landscape Chang'e-4 will encounter meant extra challenges in both selecting sites and executing the landing.
The mission could also further our understanding of the formation of rocky bodies and the development of layers such as crust, mantle and core. “The compositions may [have] originated from the different depth of the shallow layer of the Moon,” Meng says, “which is important to understand the procedure of the history of the lunar magma ocean and the global differentiation hypothesis,” (which is the hypothesis that proto-Earth collided with a Mars-sized body known as Theia, leaving the Earth-Moon system to form from the debris).
Unfortunately, Jolliff says, one of the main reasons for visiting this area of the Moon will not be addressed. “A key long-standing science goal of a mission to the SPA basin is to determine its age and [the] composition of the original impacted materials, which potentially contain rocks excavated very deeply from within the Moon, possibly its mantle,” he says. “Unfortunately, the CE-4 mission is not equipped to address these issues, nor is it going to a particularly good location within SPA to address these issues.”
The type of mission that could really provide a picture of what the far side is made of would require a sample return. Jolliff has proposed such a SPA basin sample-return mission through the NASA New Frontiers program, but as of now, no such mission has been approved for funding.
China is preparing a lunar sample return on the near side with Chang’e-5, which was planned to launch before Chang’e-4 in November 2017. However, a failed launch in July of that year of the Long March 5—a heavy-lift rocket required for the bigger payload—has delayed the mission to late 2019 at the earliest.
If all goes well with Chang’e-5's flight to the Mons Rümker region in Oceanus Procellarum, its backup, Chang’e-6, could attempt a sample return on the lunar far side, or possibly at the Moon’s south pole, in the early 2020s. The expansion of Chinese robotic lunar exploration plans is leading toward the objectives Jolliff and others espouse, but for today's mission, it’s all about sticking the landing.
“I would emphasize the significance of the far side landing, something none of the other space-faring countries has done. Exercising command and control of the mission from Earth, remotely via a communications satellite, is definitely a stepping stone for the Chinese space program,” Jolliff says, noting that Chang’e-5 will follow swiftly after. “These missions are taking place in rapid succession, and that also demonstrates the resolve of this program to move forward toward the eventual goal of putting Chinese astronauts on the lunar surface.”
As China prepares for three missions to the surface of the Moon in the next half decade or so, the nation is likely to become a significant global leader in lunar exploration. Meng says that the data from the Chang’e-4 mission will be made available to scientists outside of China, and the spacecraft is carrying instruments from Germany and Sweden as part of an international partnership that includes the Netherlands and Saudi Arabia.
“Successful completion of this mission will establish China as a leader in the international exploration of the Moon,” says James Head, a professor of Geological Sciences at Brown University. “[They] have already offered the use of their relay satellite infrastructure to the international space community”.
China’s lunar missions are at the front of a wave of renewed interest in our celestial neighbor, with NASA, the European Space Agency, Russia and India all working toward various robotic and crewed missions in the coming years. China’s plans are taking definitive shape, however, with a launch earlier today, and plans already in the works for Chang’e-7, a technology demonstrator to test the feasibility of long-term goals, like in-situ resource utilization of water ice or metals.
“Every new mission to the Moon provides surprises, Head says. “Chang'e 4 will not only be a monumental technical accomplishment, but will no doubt bring new discoveries and surprises.”
Andrew Jones is a journalist who reports on China's space program, writing for SpaceNews, Gbtimes and contributing guest blogs at the Planetary Society. He is based in Finland.