China leaves for the other side of the moon | Science

SHANGHAI, CHINA- China's ambitious lunar exploration program is about to attempt a first in the world. Space: On December 8, he 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 & # 39; e-4 is a historic mission," says Bernard Foing, director of the lunar exploration international working group of the European Space Agency (ESA) in Noordwijk, the Netherlands.

Remote observations have shown that the hidden face of the moon, invisible from the Earth, has a much thicker and older crust and is surrounded by craters more numerous and deeper than the near, where large Dark plains called maria, formed by ancient lava flows, have erased much of the crater. The big difference "remains a mystery," says Foing, and Chang's journey "can give clues".

China launched its lunar program three decades after the end of the program of 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 within the framework of the China 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; According to James Head, planetary scientist at Brown University, both produced "a lot of good scientific knowledge," including high-resolution lunar images and new altimetry measurements.

In 2013, Chang & # 39; e-3 became the first aircraft to land on the moon. since the return mission of Luna 24 samples from the Soviet Union in 1976. The lander and his little rover were carrying data on the topography, mineralogy and abundance of 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 fragmented rock 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 had borne fruit, 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.

As the Moon will block any direct radio contact with the LG and the mobile, Chang's e-4 will support a communications relay satellite, launched in May. Called Queqiao, it travels a 65,000-kilometer loop beyond the moon to the Lagrange Moon-Earth point 2, a point of gravitational equilibrium. Chang-e-4 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 2500 km and a depth of 12 km. "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 might have brought materials from the Moon's upper mantle to the surface, a scenario according to which visible and near-infrared imaging spectrometer might be able to verify. The imaging spectrometer will also explore the geochemical composition of the soil from the other side, which may differ from the near side due to the same processes producing the difference in crust thickness.

The rover ground penetration radar, similar to that on 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 on-board cameras on landers and rovers could help scientists better understand the cratering process.

Going on the other side also opens a "new window for radio astronomy," says Ping Jinsong, NAOC. radio astronomer. 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 includes three low frequency receivers: one on the landing gear, one – 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. merged into the first stars of the universe.

China's ambitious lunar program will continue with Chang & # 39; e-5, a sample return mission, which is expected to be launched next year. It will recover up to 2 kilograms of soil and rocks in the Oceanus Procellarum, a large nearby lunar mare, spared from previous landings, and one of the newest volcanic rivers in the world. the moon. "This is a great goal that could eventually lead to a fantastic science," says Bradley Jolliff, scientist in planetary science at Washington University in St. Louis, Missouri, who urged the US to launch its own mission to lunar sample return.

If China continues its tradition of developing lunar missions in pairs, a second sample return mission, 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."