As China’s Chang’e-6 became the first spacecraft to bring back samples from the far side of the Moon, it became a significant milestone in space history because that part that the moon the earth never gets to see and there is little study on the far side of the earth’s natural satellite.
The Chinese lander descended on the Moon’s surface on June 1, and spent two days collecting rocks and soil from one of the oldest and largest of lunar craters — the 2,500 km-wide South Pole-Aitken (SPA) basin — using a robotic arm and drill.
The lander then launched an ascent module that transferred the samples to the Chang’e-6 orbiter that was orbiting the Moon. On June 21, the orbiter released a service module that brought back the samples to Earth.
Return missions are not new though. Back in July 1969, the US Apollo 11 mission brought 22 kg of lunar surface material, including 50 rocks, to Earth. In September 1970, the Soviet Luna 16 mission — the first robotic sample return mission — too, brought pieces of the Moon to Earth. In recent years, Chang’e-5, the predecessor of Chang’e-6, brought back 2 kg of lunar soil in December 2020.
All these samples, however, came from the near side of the Moon. Difficult terrain, giant craters, and the difficulty in communicating with ground control made it technically challenging to land a spacecraft on the side that never faces the Earth. The moon is tidally locked with Earth; thus we see only one side of our nearest space neighbour.
Chang’e-4 overcame these difficulties in 2019, putting the Yutu-2 rover on the far lunar surface. Now, Chang’e-6 has not only landed on the far side, but also returned with samples from there. “This is a great achievement by China… Recovering any samples from the moon is difficult, but doing so from the far side, where communications are particularly difficult, is a step taken by no other agency. A real technological feat,” Martin Barstow, a professor of astrophysics and space science at the University of Leicester, told The Guardian.
A sample return mission such as Chang’e-6 aims to collect and return samples from an extraterrestrial location like the Moon or Mars to Earth for analysis. The sample can be rocks or soil — or even some molecules.
In situ robotic explorations — in which landers, orbiters, and rovers carry out experiments in space or on heavenly bodies — can carry only miniature instruments that are not very sophisticated or accurate, and answer only certain types of questions. For instance, they can’t determine the origin or age of a rock.
If the samples can be brought to Earth, on the other hand, scientists can examine them using extremely sensitive laboratory instruments. They can study the “chemical, isotopic, mineralogical, structural, and physical properties of extraterrestrial samples from the macroscopic level down to the atomic scale, frequently all on the very same sample,” Lori S Glaze, director of NASA’s planetary science division, wrote in January 2020.
Also, returned samples can be preserved for decades, and can be examined by future generations using ever more advanced technology. The samples brought back by the Apollo missions in the 1960s and 1970s are still being studied by scientists, who are extracting information on the history of the Moon, Earth, and the inner solar system.
India’s Chandrayaan-4 mission, which is currently under development by the Indian Space Research Organisation (ISRO), will also be a sample return mission. Chandrayaan-3 landed about 600 km from the South Pole of the moon last year.
The lunar far side is geologically different from the near side. It has a thicker crust, more craters, and fewer plains where lava once flowed. But scientists do not know why the two sides are so different — and an examination of the Chang’e-6 samples could throw up some answers.
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