SHANGHAI, CHINA—China's ambitious program of lunar exploration is about to attempt a spacefaring first: On 8 December it will launch a probe intended to land on the far side of the moon. Besides boasting rights, the Chang'e-4 lander and rover are expected to produce a host of new insights into the moon's composition and history. "Chang'e-4 is an historical mission," says Bernard Foing, director of the European Space Agency's (ESA's) International Lunar Exploration Working Group in Noordwijk, the Netherlands.
Remote observations have shown that the far side of the moon, invisible from Earth, has a much thicker, older crust and is pockmarked by more and deeper craters than the near side, where large dark plains called maria, formed by ancient lava flows, have erased much of the cratering. The big difference "is still a mystery," Foing says, and Chang'e's trip "can give clues."
China started its lunar program 3 decades after the United States and the Soviet Union ended theirs. Chinese geologists eager to study the moon convinced the government to establish the Lunar Exploration Program under the China National Space Administration (CNSA) in 2004. The agency launched Chang'e-1 and Chang'e-2, named after a Chinese moon goddess, in 2007 and 2010, respectively; both produced "a lot of good science," including high-resolution lunar images and new altimetry measurements, says planetary scientist James Head of Brown University.
In 2013, Chang'e-3 became the first craft to land on the moon since the Soviet Union's Luna 24 sample return mission in 1976. The lander and the small rover it carried gathered data on the moon's topography, mineralogy, and elemental abundance. In a first, the rover was equipped with a ground-penetrating radar that profiled buried lava flows and regolith, the broken up rock and dust that makes up the lunar soil.
Chang'e-4 was designed as an identical backup to Chang'e-3, but when that mission proved successful, China's planners became more ambitious. Going to the far side promised "unique and original science" as well as a chance to "develop China's deep space observational capabilities," says Li Chunlai, deputy director-general of the Chinese Academy of Sciences's National Astronomical Observatories of China (NAOC) in Beijing, which advises CNSA on the program's science objectives.
Because the moon will block direct radio contact with the lander and rover, Chang'e-4 will rely on a communications relay satellite, launched in May. Called Queqiao, it's traveling in a loop 65,000 kilometers beyond the moon at Earth-moon Lagrange Point 2, a gravitational balance point. Chang'e-4 itself will land in the Von Kármán crater within the South Pole–Aitken basin. Likely formed by a giant asteroid impact, the basin is roughly 2500 kilometers across and 12 kilometers deep. "It's the moon's largest, deepest, and oldest impact structure," says planetary geoscientist Xiao Long of the China University of Geosciences in Wuhan.
The impact may have brought material from the moon's upper mantle to the surface, a scenario that data from a visible and near-infrared imaging spectrometer might be able to verify. The imaging spectrometer will also explore the geochemical composition of far-side soil, which is likely to differ from the near side because of the same processes that produced the difference in crust thickness.
The rover's ground-penetrating radar—similar to that on Chang'e-3—will provide another look down to about 100 meters beneath the surface, probing the depth of the regolith and looking for subsurface structures. Combining the radar data with surface images from cameras on the lander and rover might advance scientists' understanding of the cratering process.
Going to the far side also opens "a totally new window for radio astronomy," says Ping Jinsong, a NAOC radio astronomer. On Earth, and even in near-Earth space, natural and humanmade interference hampers low-frequency radio observations. The moon blocks this noise. So the mission carries a trio of low-frequency receivers: one on the lander, one—a collaboration with the Netherlands—on Queqiao, and a third on a microsatellite released from Queqiao into a lunar orbit. (Contact with a second microsatellite carrying a fourth receiver has been lost.) The receivers will listen for solar radio bursts, signals from aurorae on other planets, and the faint signals from the primordial clouds of hydrogen gas that coalesced into the universe's first stars.
China's ambitious lunar program will continue with Chang'e-5, a sample return mission, due for launch next year. It will retrieve up to 2 kilograms of soil and rock from the Oceanus Procellarum, a vast lunar mare on the near side untouched by previous landings, and one of the moon's youngest volcanic flows. "It's a great objective and will potentially yield some fantastic science," says Bradley Jolliff, a planetary scientist at Washington University in St. Louis, Missouri, who has urged the United States to launch its own lunar sample return mission.
If China continues its tradition of developing moon missions in pairs, a second sample return mission, Chang'e-6, might follow. Head notes that NASA, ESA, Japan, Russia, and India have all taken a renewed interest in our planet's companion, which holds clues to Earth's own history. "Chang'e-4 and 5 are a major part of this renaissance," Head says, "and in many ways are the current vanguard."