Scientists have long pondered the mystery of the moon’s far hemisphere, the so-called “dark side,” which is never directly visible from Earth. The Soviet Union’s Luna spacecraft took the first grainy photographs of the far side of the moon in 1959, but it has never been explored. Late last week, however, the Chinese news agency Xinhua announced that the space probe Chang’e 4—named after China’s mythological moon goddess—will launch a mission to the moon in 2018 and make a soft landing on its far side.
As Earth’s only natural satellite, the moon has been the object of human fascination—and scientific observation—for centuries. Although from our perspective the moon does not appear to spin, in reality it rotates about every 27 days, approximately the same amount of time the moon takes to orbit the Earth once. Over the course of this “synchronous rotation,” as the phenomenon is known, we see only about 59 percent of the moon’s surface; the other 41 percent, sometimes called the “dark side” of the moon, is never visible from Earth.
Soon after the Soviet satellite Sputnik became the first spacecraft to orbit Earth in 1957, both the Soviet and U.S. space programs began focusing on the next great objective: the moon. The Soviet Union initially had more success, as its first two Luna probes made the first escape from Earth’s gravity and the first lunar impact in 1959. That same year, Luna 3 achieved another first, taking a photographic survey of the moon’s far side. Despite their grainy quality, these early images revealed that the previously unseen hemisphere had few of the smooth, dark spots that we observe on the moon’s surface. Scientists initially mistook these volcanic plains for lunar seas, and called them maria (from the Latin word for sea).
As reported in the Washington Post last year, the National Aeronautics and Space Administration (NASA) now has tens of thousands of images of the far side of the moon, which has allowed them to make better predictions about what that distant surface might look like. Now, China’s growing space program has announced its plans to make a historic landing on the far side of the moon in 2018—two years earlier than originally planned.
Since 2003, when the country launched its first astronaut, the multibillion-dollar space program run by the Chinese military has been right on schedule with achieving the landmarks it set for itself. In late 2013, the unmanned spacecraft Chang’e 3 made a soft landing on the lunar surface, making China the third nation (after the United States and the former USSR) to reach the moon. The rover Yutu or “Jade Rabbit,” which deployed from Chang’e 3 after the landing, recently discovered a new type of basaltic rock during its exploration of a volcanic crater in the Mare Imbrium (what we see as the right “eye” of the “Man in the Moon”). While earlier moon rock samples had either high or low concentrations of titanium, the new rock’s titanium level falls somewhere in between, suggesting that the upper mantle of the moon is more diverse in its composition than that of Earth.
Despite such advances in lunar knowledge, the Chinese space program has so far mostly repeated feats that its U.S. and Soviet counterparts achieved decades ago. The Chang’e 4 mission to make a soft landing on the far side of the moon, however, will represent a first in the history of space exploration. As Liu Jizhong, dean of China’s Lunar Exploration & Aerospace Engineering Center, told Agence France-Presse: “The implementation of the Chang’e 4 mission has helped our country make the leap from following to leading.”
The mission will be the first one in the history of China’s space program to be partially funded by private investors. Chang’e 4’s goal is to make a soft landing with a lander and rover on the far hemisphere of the moon, so that China can study its geology and possibly place a radio telescope there. As the moon itself would block any interfering waves from Earth, its far side would be an ideal location to study radio waves. But this same aspect makes it more difficult to land a spacecraft there, as the probe would be unable to receive radio signals to communicate with Earth. In order to confront this challenge, Liu said China plans to add a relay satellite to the probe in order to enable communication between it and Earth.
Clive Neal, chair of the NASA-affiliated Lunar Exploration Analysis Group, spoke to Agence France-Presse of the unprecedented nature of the Chinese mission: “I am sure the international lunar science community will be very excited about this mission. I know I am.” The moon’s far hemisphere is extremely different from the near side, according to Neal, largely because of the “biggest hole in the solar system–the South Pole-Aitken basin, which may have exposed mantle materials–and the thicker lunar crust.” At nearly 2,500 km wide, the basin is the largest known impact crater on the moon, and one of the largest in the entire solar system. The distance from its depths to the tops of the highest surrounding peaks measures some 15 km (or eight miles), almost twice the height of Mount Everest.