Chang'e-6 returns with the far side of the Moon

On 25 June, at a little after two in the afternoon Beijing time, the return capsule of China’s Chang’e-6 mission touched down on the grasslands of Siziwang Banner, in Inner Mongolia, carrying the first samples ever collected from the far side of the Moon. The probe, launched on a Long March 5 rocket from Wenchang on Hainan island on 3 May, had taken fifty-three days to go to the Moon, land on its hidden face, pick up rocks and soil, and come home.

The far side is the side that always faces away from Earth, and because direct radio contact is blocked by the body of the Moon, no spacecraft had ever brought samples back from it. Chang’e-6 landed on 2 June inside the Apollo crater, in the South Pole-Aitken basin, a vast and ancient depression on the southern edge of the far side that scientists believe holds rocks from deeper in the Moon’s crust than anything returned before. To stay in touch through the landing, the mission relied on Queqiao-2, a relay satellite launched in March and parked in an orbit above the far side, which forwarded commands from Earth and sent the data back.

The sampling itself was brief. The lander used a drill to go into the ground and a robotic arm to scoop material from the surface, working quickly because the far side experiences two-week-long days and nights and the spacecraft had only a short window in which the Sun was at the right angle. When the canister was full, the ascent stage lifted off from the surface, rendezvoused with the waiting orbiter, transferred the sample, and the orbiter headed for Earth. By the time the capsule came down in Inner Mongolia, the sample weighed about 1.9 kilograms — a little less than the roughly two kilograms that the earlier Chang’e-5 mission returned from the near side in 2020, but priceless because of where it came from.

The South Pole-Aitken basin is one of the oldest and largest impact structures in the solar system, and the rocks inside it are expected to tell geologists something about the deep interior of the Moon and about the era of heavy bombardment in the early inner solar system. Until now, every lunar sample ever analysed had come from the near side, and the far side had been sampled only by orbiters and the short visits of unmanned landers. The Chinese space agency said the samples would be shared with international researchers, as Chang’e-5’s had been.

For China’s lunar programme the landing was the middle of a longer arc: a robotic research base near the lunar south pole is planned for the next decade, and a crewed landing by the end of the 2020s has been announced. The immediate prize on 25 June was simpler. A capsule fell through the atmosphere over the same grasslands that have welcomed every Chinese sample-return mission, a recovery crew in camouflage found it within the hour, and the first rocks from the hidden face of the Moon were, finally, on their way to a laboratory on Earth.