In June 2024, a small Chinese spacecraft called Chang'e-6 became the first machine in human history to return samples from the far side of the Moon — drilling and scooping roughly 2 kilograms of lunar soil from a region no spacecraft had ever sampled, and delivering it back to Earth, giving humanity its first physical material from the half of the Moon that has been permanently turned away from our planet since it became tidally locked billions of years ago

For roughly the last three billion years, half of the Moon has been hiding. Earth’s gravity caught the Moon early in its existence and slowed its spin until the same face was permanently turned toward us — a process called tidal locking, completed somewhere between three and four billion years ago, which left every human civilisation that ever existed staring at the same single hemisphere of the same single satellite for the entire duration of human history. No human being saw the other half until 7 October 1959, when the Soviet Luna 3 probe transmitted the first photographs from a flyby trajectory. No human or human-built object touched the far side until 3 January 2019, when China’s Chang’e-4 mission soft-landed Yutu-2, a small rover, into Von Kármán crater. No spacecraft of any nationality had ever collected and returned physical material from the far side to a laboratory on Earth — until 25 June 2024, when the return capsule of Chang’e-6 parachuted into the grasslands of Inner Mongolia carrying 1,935.3 grams of lunar soil and rock fragments scooped and drilled from the South Pole-Aitken Basin.

According to a comprehensive reference summary of the Chang’e-6 mission architecture and operational sequence, the mission’s four-component spacecraft — a lander, an ascent vehicle, an orbiter, and an Earth-return capsule — launched from Wenchang Space Launch Site on Hainan Island aboard a Long March 5 rocket on 3 May 2024. The spacecraft entered lunar orbit on 8 May, then spent approximately three weeks mapping potential landing sites and refining its trajectory. On 1 June, the lander and ascent vehicle separated from the orbiter and descended autonomously toward the surface, communicating with mission control in Beijing through the Queqiao-2 relay satellite that had been launched into a high lunar halo orbit in March 2024 specifically to bridge communications between far-side surface operations and Earth. Direct radio contact with the far side is impossible — the lunar mass itself blocks the signal — and every far-side mission, beginning with Chang’e-4 in 2019, has required a dedicated communications relay positioned to see both Earth and the lunar surface simultaneously.

Where the lander touched down

Chang’e-6’s landing site was within the Apollo Basin, a 538-kilometre-wide impact crater that sits inside the much larger South Pole-Aitken Basin — the largest, oldest, and deepest confirmed impact structure in the entire solar system. As reported by Scientific American’s coverage of the Chang’e-6 sample return and the scientific significance of the landing site, the South Pole-Aitken Basin stretches approximately 2,500 kilometres across the lunar surface and reaches depths of approximately 8 kilometres below the surrounding terrain. The basin was formed approximately 4 billion years ago, in the period of intense bombardment that followed the formation of the solar system, by an impact substantial enough to potentially have excavated material from the Moon’s mantle and brought it to the surface. If true — and the question of whether SPA Basin ejecta actually includes mantle material has been an open scientific question for decades — then samples from the basin would represent the first physical material from beneath the lunar crust that humans had ever held in a laboratory.

The lander spent approximately 49 hours on the surface, using a robotic scoop to collect material from the immediate surface and a drill to extract samples from up to two metres below ground. Per the Planetary Society’s overview of the Chang’e-6 surface operations and sample-handling sequence, a small 5-kilogram rover named Jinchan (“Golden Cicada”), which had been kept secret during the launch preparations, was deployed from the side of the lander and photographed the spacecraft from a distance of several metres — producing the first close-up image of a lunar far-side lander taken from the lunar surface. The collected samples were transferred into a sealed container inside the ascent vehicle, which lifted off from the top of the lander on 3 June at 23:38 UTC, rendezvoused with the orbiter in lunar orbit on 6 June, transferred the sample container into the Earth-return capsule, and was subsequently crashed back into the lunar surface. The orbiter and return capsule then began the journey back to Earth, with the return capsule separating from the orbiter shortly before atmospheric entry and executing a skip-reentry trajectory — bouncing off the upper atmosphere once to shed velocity before completing the descent — to a soft landing in Inner Mongolia on 25 June. Total mission duration was 53 days.

Why far-side samples matter

The asymmetry between the lunar near side and far side is, by every available scientific measure, one of the larger unsolved problems in planetary science. The near side — the half of the Moon humans see from Earth — is covered for approximately 30 percent of its surface by basaltic maria, the dark plains of solidified ancient lava that the early astronomers mistook for seas. The far side has almost none. As detailed in a planetary scientist’s analysis of what the Chang’e-6 samples could reveal about lunar history, less than 1 percent of the far side is covered by maria. The far side’s crust is substantially thicker than the near side’s. The far side contains substantially lower concentrations of the geochemically distinctive elements collectively known as KREEP — potassium, rare earth elements, and phosphorus — that dominate the near-side surface chemistry. Why the two halves of the same body should differ so dramatically in their geological and chemical composition has been a subject of active research and unresolved theorising since the first far-side photographs were transmitted by Luna 3 in 1959.

Initial analyses of the Chang’e-6 samples have already begun producing scientifically substantial results. As covered in a November 2024 paper in Science by Cui et al. of the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, fragments of volcanic basalt from the samples have been dated to approximately 2.83 billion years old — substantially younger than most far-side material had been expected to be, suggesting that volcanic activity persisted on the far side for far longer than existing models predicted. A parallel analysis led by Zhang et al. at the Institute of Geology and Geophysics, published in Nature the same month, produced essentially the same age estimate using a different sample fraction. Earlier work led by Wang et al. on volcanic glass beads from the Chang’e-5 mission to the lunar near side had identified evidence of unexpectedly recent volcanism dated to approximately 120 million years ago. Whether comparably young volcanism occurred on the far side remains an open question that the continued analysis of Chang’e-6 samples may eventually address. The far-side basalts that have been dated so far contain substantially lower concentrations of radioactive heat-producing elements than near-side basalts of comparable age, which raises a new puzzle: if the far-side magma source was as chemically cool as the analyses suggest, what kept it molten long enough to produce volcanism extending into the relatively recent geological past? The current working hypothesis, which the analysis of the remaining samples over the coming years will substantially refine, is that the thin crust beneath the South Pole-Aitken Basin allowed mantle material to remain partially molten and to erupt through the comparatively thinner overlying rock long after the rest of the far side had cooled.

The 1,935 grams of lunar soil in a laboratory in Beijing now represent the only physical samples humans have ever recovered from a hemisphere of the Moon that, for the entire duration of human civilisation and for billions of years before it, has been permanently hidden from view. The next missions in the series — Chang’e-7 in 2026 and Chang’e-8 in 2028 — will focus on the lunar south pole, searching for water ice and testing in-situ resource utilisation in preparation for the International Lunar Research Station that China plans to begin building by 2035. The same techniques that produced the Chang’e-6 sample return will be used, with modifications, for the Tianwen-3 Mars sample return mission scheduled to launch in 2028 — a mission that, on its current schedule, will deliver the first samples from the surface of another planet to Earth in approximately five years.