China leaves for the other side of the moon | Science

SHANGAI, CHINA-China's ambitious lunar exploration program is about to attempt a first space mission: on December 8, it will launch a probe intended to land on the other side of the moon. In addition to owning rights, the LG and Chang rover's e-4 should provide a wealth of new information on the composition and history of the moon. "Chang-e-4 is a historic mission," said Bernard Foing, director of the International Working Group on Lunar Exploration of the European Space Agency (ESA) in Noordwijk, the Netherlands .

Remote observations have shown that the invisible face of the moon, invisible from the Earth, has a much thicker and older crust and is characterized by craters more numerous and deeper than the near face, where large dark plains called Maria, formed from ancient lava flows cleared many craters. The big difference "is still a mystery," says Foing, and Chang's journey "can give clues".

China began its lunar program three decades after the end of theirs by the United States and the Soviet Union. Chinese geologists eager to study the moon have convinced the government to set up the lunar exploration program under the authority of the Chinese Space Administration (CNSA) in 2004. agency launched Chang-e-1 and Chang-e-2, named after a Chinese moon goddess, in 2007 and 2010, respectively; The two scientists produced "a lot of good scientific data," including high-resolution lunar imagery and new altimeter measurements, says planetary scientist James Head of Brown University.

In 2013, Chang & # 39; e-3 became the first boat to land on the moon since the return mission of Luna 24 samples from the Soviet Union in 1976. The LG and its little rover carried data on the topography, mineralogy and abundance of the elements of the moon. In one, the rover was equipped with a ground-penetrating radar that profiled the buried lava flows and the regolith, the broken rocks and the dust constituting the lunar soil.

Chang & # 39; e-4 was designed as a backup copy identical to Chang & # 39; e-3, but when this mission was successful, Chinese planners became more ambitious. Going on the other side has promised a "unique and original science" as well as a chance to "develop China's deep space observation capabilities," said Li Chunlai. , deputy director general of the National Astronomical Observatories of China (NAOC) of the Chinese Academy of Sciences Beijing, who advises the CNSA on the scientific objectives of the program.

The moon blocking any direct radio contact with the LG and the mobile, Chang's e-4s will support a communications relay satellite launched in May. Called Queqiao, it travels 65,000 kilometers beyond the moon at point 2 of Lagrange (Earth-Moon), a point of gravitational equilibrium. Chang-e-4 itself will land in the crater of Von Kármán in the South Pole-Aitken basin. Probably formed by a giant asteroid impact, the basin has a width of about 2,500 kilometers and a depth of 12 kilometers. "It's the largest, deepest, and oldest impact structure of the moon," said Xiao Long, a global geoscientist at China's University of Geosciences in Wuhan.

The impact may have brought materials from the upper mantle of the moon 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 the soil on the other side, which is likely to differ on the other side because of the same processes that produced the difference in crust thickness.

The radar penetrating the ground of the rover – similar to that of Chang – e-3 – will provide another look at about 100 meters below the surface, probing the depth of the regolith and searching for underground structures. Combining radar data with surface images from lander and mobile cameras could provide scientists with a better understanding of the cratering process.

Going on the other side also opens "a whole new window for radio astronomy," says NAOC radio astronomer Ping Jinsong. On Earth, and even in the near-Earth space, natural and anthropogenic interferences hinder low-frequency radio observations. The moon blocks this noise. The mission therefore comprises a trio of low frequency receivers: one on the landing gear, a – a collaboration with the Netherlands – on Queqiao and a third on a microsatellite released from Queqiao in a lunar orbit. (The contact with a second microsatellite carrying a fourth receiver has been lost.) The receivers are listening to solar radio bursts, auroral signals on other planets and weak signals from the primordial clouds of gaseous hydrogen that have occurred. gathered in the first stars of the universe.

China's ambitious lunar program will continue with Chang-e-5, a return-of-samples mission, which should be launched next year. It will recover up to 2 kilograms of soil and rocks in the Oceanus Procellarum, a large lunar mare located nearby, spared from previous landings, and one of the youngest volcanic rivers of the moon . "It's a great goal and potentially a fantastic science," says Bradley Jolliff, a scientist in planetary science at Washington University in St. Louis, Missouri, who urged the US to launch its own mission back home. lunar samples.

If China continues its tradition of developing lunar missions in pairs, a second mission back samples, Chang-e-6, could follow. Head notes that NASA, ESA, Japan, Russia and India have all manifested a renewed interest in the companion of our planet, which holds clues about the history of the Earth. "The Chang & # 39; e-4 & 5 are an important part of this rebirth," says Head, "and in many ways are the current vanguard."