Chang'e 4 (2019): First Landing on the Moon's Far Side

Chang’e 4, a Chinese robotic spacecraft, made the first soft landing on the far side of the Moon on 3 January 2019. It set down in Von Kármán crater, inside the vast South Pole-Aitken basin, and released the small Yutu-2 rover. Because the far side never faces Earth, the mission depended on a relay satellite placed beyond the Moon, an approach no earlier lander had needed.

Background: the hidden hemisphere

The Moon turns once on its axis for every orbit of Earth, so the same face always points towards us. The far side was unseen until October 1959, when the Soviet probe Luna 3 photographed it. In December 1968 the crew of Apollo 8 became the first humans to see it directly.

Orbiters revealed that the two hemispheres are strikingly different. The near side has large dark plains of solidified lava, the maria, while the far side is mostly rugged highlands covered with craters and has a thicker crust. Explaining this difference is one of the main puzzles of lunar science.

The far side is also the site of the South Pole-Aitken basin, about 2,500 km across and one of the largest known impact structures in the Solar System. The impact that made it may have exposed material from deep inside the Moon. Every landing before 2019, from the Soviet Luna 9 in 1966 to the Apollo missions, took place on the near side, where direct radio contact with Earth is possible.

China’s lunar programme

China’s Lunar Exploration Programme, named after the Moon goddess Chang’e, began with the orbiters Chang’e 1 (2007) and Chang’e 2 (2010). In December 2013 Chang’e 3 landed on the near side with the first Yutu (“Jade Rabbit”) rover, the first soft landing on the Moon since 1976. The programme ran in parallel with China’s crewed spaceflight effort, which had put its first astronaut in orbit in 2003, as told in the article on Yang Liwei and Shenzhou 5.

Chang’e 4 was built as the backup to Chang’e 3. After the earlier mission succeeded, Chinese planners chose to send the spare spacecraft to the far side instead. New instruments were added for the mission, some of them supplied by partners in Germany, Sweden and the Netherlands.

Queqiao and the landing

The first step was communications. On 21 May 2018 China launched the Queqiao (“Magpie Bridge”) relay satellite. It was sent to a halo orbit around the Earth-Moon L2 Lagrange point, about 65,000 km beyond the Moon, from which it could see both the far side and Earth.

Chang’e 4 itself was launched on a Long March 3B rocket from the Xichang Satellite Launch Centre in the early hours of 8 December 2018, Beijing time. It entered lunar orbit on 12 December and spent about three weeks lowering its orbit.

On 3 January 2019 at 10:26 Beijing time the lander descended automatically, hovering briefly to pick a safe spot before touching down in Von Kármán crater, about 180 km across. Later that day the Yutu-2 rover rolled down a ramp onto the surface. The landing site was later named Statio Tianhe by the International Astronomical Union.

Science on the far side

The lander carried cameras, a low-frequency radio instrument and a German-built radiation detector, the first to measure radiation doses on the lunar surface for future human visits. Yutu-2 carried a panoramic camera, ground-penetrating radar, a spectrometer and a Swedish instrument that studied how solar wind particles interact with the surface.

In 2019 a team led by Chinese scientists reported in Nature that Yutu-2’s spectrometer had found minerals possibly derived from the Moon’s mantle, perhaps brought up by the basin-forming impact. The interpretation remains debated. The rover’s radar mapped layers of rubble and fine dust down to about 40 metres, recording the crater’s history of impacts and lava flows.

A sealed biological experiment carried seeds, yeast and fruit-fly eggs. A cotton seed sprouted within days, the first plant to germinate on the Moon, but the experiment ended when the first lunar night froze it.

A long-lived rover

Both lander and rover powered down during each lunar night, about 14 Earth days long, and woke again at sunrise. Yutu-2 was designed to last three months, yet it continued to operate for years, becoming the longest-operating rover on the Moon and surpassing the Soviet Lunokhod 1. It travelled well over a kilometre across the crater floor.

Timeline

• 1959, October: Luna 3 returns the first photographs of the Moon’s far side.
• 2013, 14 December: Chang’e 3 lands on the near side with the first Yutu rover.
• 2018, 21 May: The Queqiao relay satellite is launched.
• 2018, 8 December (Beijing time): Chang’e 4 is launched from Xichang.
• 2018, 12 December: Chang’e 4 enters lunar orbit.
• 2019, 3 January: Chang’e 4 lands in Von Kármán crater; Yutu-2 is deployed.
• 2019, May: First results on possible mantle material are published in Nature.
• 2024, June: Chang’e 6 returns the first samples from the far side.

Why it matters

Chang’e 4 achieved a first that had eluded the United States and the Soviet Union: a landing on the far side of the Moon. Its relay-satellite approach became a model for later far-side missions, and the long survival of Yutu-2 gave scientists their first sustained look at the ground of this hidden hemisphere. The mission also marked China’s arrival as a leading power in lunar exploration.