The Moon has two faces, and they are not the same. The near side — the one always facing Earth — is relatively flat, covered with dark volcanic plains called maria that erupted billions of years ago. The far side is heavily cratered, ancient, and almost devoid of maria. The reason for this asymmetry has been debated for decades, and the rocks to test the leading hypotheses were always frustratingly out of reach: every Moon sample returned to Earth, from Apollo through Luna to Chang'e-5, came from the near side. On June 25, 2024, China's Chang'e-6 mission changed that.
The mission landed in the Apollo crater within the South Pole–Aitken Basin — a structure 2,500 kilometers in diameter and up to 8 kilometers deep, making it the largest and oldest confirmed impact crater in the solar system. The basin is so old and deep that material from the Moon's mantle may have been excavated and mixed into its surface. If so, samples from the floor of SPA contain rocks that formed not in the crust, but far below it — giving access to the Moon's interior composition in a way that no near-side sampling can replicate.
The asymmetry problem
The near-far dichotomy is one of the oldest unsolved problems in lunar science. The leading explanation involves a second smaller Moon, sometimes called Theia's companion, that formed at the same time as the Moon and eventually merged with it at low velocity, piling ancient highland material onto the far side. Another hypothesis invokes a difference in heat-producing radioactive elements between the two hemispheres, driven by early convection in the lunar magma ocean, which would explain why the near side melted more completely and generated more volcanic maria. A third possibility is that a large impactor struck the near side specifically and excavated the low-lying basins that later filled with lava.
Testing these hypotheses requires knowing the age and composition of far-side rocks. The South Pole–Aitken Basin is so ancient that it predates almost everything else preserved on the lunar surface — its formation age is estimated at roughly 4.1 to 4.3 billion years. Chang'e-6 collected approximately 1.9 kilograms of material from the basin floor using a drill and surface scoop, then launched that material from the far side (using a relay satellite to maintain communications, since the far side has no line-of-sight to Earth) and returned it to Inner Mongolia via a reentry capsule.
Early results
Analysis of the samples began immediately after recovery and is ongoing. Preliminary reports indicate that the material includes a mixture of basaltic volcanic rock and impact-processed regolith — consistent with the basin's long and violent history. Initial geochronology suggests that volcanic activity in the SPA Basin may have continued to a more recent date than far-side crater counts alone had indicated, pointing to a more geologically active far side than the cratered surface implied. Comparison with near-side samples from Chang'e-5 (returned in December 2020 from a relatively young near-side volcanic region) and Apollo will allow direct compositional comparisons between the two hemispheres for the first time.
The broader significance of Chang'e-6 extends beyond the samples themselves. Landing on the far side requires relay satellite infrastructure that China has now established and demonstrated operationally. The same architecture will be needed for any future crewed or robotic missions to the lunar south pole region — the focus of both NASA's Artemis program and China's own crewed lunar goals. Chang'e-6 proved the communication architecture works under operational conditions. The Moon's two faces are no longer equally inaccessible.
The road forward
Chang'e-6's success alters the trajectory of lunar science in ways that go beyond the samples themselves. It proves that the relay satellite architecture enabling far-side surface operations works — the Queqiao-2 relay satellite that supported Chang'e-6 communications will remain operational for future missions. China has announced Chang'e-7 and Chang'e-8, both targeting the south polar region. The far side, which had been the Moon's scientific blind spot for the entire history of lunar exploration, is now open to systematic investigation. The geochemical asymmetry that has puzzled planetary scientists for six decades will be resolved not with more theory but with more samples, and Chang'e-6 established that collecting them is achievable.
