Chang'e 4

• 1. Introduction

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  Chang'e 4 touched down on the far side of the Moon at 02:26 UTC on the 3rd of January 2019, becoming the first spacecraft in history to land there. That hemisphere of the Moon is permanently turned away from Earth. Radio signals cannot pass through the lunar body, so the entire operation had to be conducted blind from the ground. What made it possible, and what did the mission find once it arrived? Those are the questions this documentary sets out to answer.

  The mission carries the name of Chang'e, the Chinese Moon goddess, a tradition that runs through each spacecraft in the Chinese Lunar Exploration Program. Chang'e 4 was not, strictly speaking, designed from scratch. It was originally built as a backup spacecraft for its predecessor, Chang'e 3, and only became available for reassignment when Chang'e 3 landed successfully in 2013. Chinese engineers then adapted it for a far more ambitious target: the unexplored far side, inside a crater named after a Hungarian-American aerospace pioneer whose own student would go on to found China's space program.

• 2. The Far Side Problem

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  Direct radio communication between Earth and the lunar far side is physically impossible. The Moon itself blocks every transmission. To solve this, the China National Space Administration launched a dedicated relay satellite called Queqiao, which translates as "Magpie Bridge," on the 20th of May 2018. The name was drawn from the Chinese folk tale of The Cowherd and the Weaver Girl.

  Queqiao weighs 425 kg and carries a 4.2 m antenna to pick up X-band signals from the lander and rover, then relay them to Earth on the S-band. Rather than flying directly to its destination, the satellite used a lunar swing-by maneuver to conserve fuel. It took 24 days to reach the Earth-Moon L2 point, completing its final orbit-insertion burn on the 14th of June 2018. The halo orbit it settled into sits roughly 65,000 km from the Moon, giving the satellite a line of sight to both the landing zone and Earth simultaneously. No relay satellite had ever been positioned at this location before.

  Riding alongside Queqiao were two microsatellites, each weighing 45 kg, named Longjiang-1 and Longjiang-2, developed by Harbin Institute of Technology. Their job was to observe the sky at very low radio frequencies, between 1 and 30 megahertz, wavelengths that Earth's ionosphere prevents any ground-based or Earth-orbiting telescope from studying. Longjiang-1 failed to reach lunar orbit, but Longjiang-2 operated successfully until the 31st of July 2019, when it was deliberately directed to crash into the Van Gent crater, leaving a 4 by 5 metre impact scar on the surface.

• 3. Von Karman and the Aitken Basin

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  The landing site chosen for Chang'e 4 sits inside a crater 180 km across called Von Karman, itself nested within the South Pole-Aitken Basin on the lunar far side. The basin formed from an ancient collision so violent that it excavated material thought to be roughly 13 km deep, likely exposing the deep lunar crust and possibly the mantle beneath. Studying that material could offer an unprecedented window into how the Moon was built.

  The crater carries a particular resonance for China's space program. Theodore von Karman was the doctoral advisor of Qian Xuesen, the scientist regarded as the founder of Chinese spaceflight. The selenographic coordinates of the touchdown point are 177.5991 degrees east, 45.4446 degrees south, at an elevation of minus 5,935 m. Landing happened shortly after lunar sunrise broke over the crater, at 02:26 UTC on the 3rd of January 2019.

  In February 2019, the landing site was formally named Statio Tianhe. Four surrounding features received names during the same process: the mountain Mons Tai and three craters designated Zhinyu, Hegu, and Tianjin. In May 2019, the mission achieved its primary scientific aim when Chang'e 4 identified what appear to be mantle rocks on the surface, material no spacecraft had ever sampled in place from the far side.

• 4. Lander, Rover, and Science Payloads

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  The lander and Yutu-2 rover together had a total landing mass of 1,200 kg. Yutu-2, whose name translates literally as "Jade Rabbit No. 2," measures 1.5 by 1.0 by 1.0 metres and weighs 140 kg. Six wheels drive it across the surface; solar panels generate its electrical power, while a radioisotope heater unit keeps it warm through the long lunar night. The rover was manufactured by the China Academy of Space Technology.

  Yutu-2's nominal operating period was three months, but Chinese engineers, drawing on their experience with the original Yutu rover in 2013, improved the design and expressed hope it would last several years. That confidence proved well-placed. On the 21st of November 2019, Yutu-2 broke the lunar longevity record of 322 Earth days previously held by the Soviet Lunokhod 1, which operated from the 17th of November 1970 to the 4th of October 1971.

  The rover carried four scientific instruments. Its Panoramic Camera captures stereoscopic 3D imagery across wavelengths from 420 to 700 nm. The Lunar Penetrating Radar can probe the subsurface to a depth of about 30 m at 30 cm vertical resolution, or to more than 100 m at 10 m resolution. A Visible and Near-Infrared Imaging Spectrometer covers 450 to 950 nm and can identify surface materials. The Advanced Small Analyzer for Neutrals, provided by the Swedish Institute of Space Physics, measures energetic neutral atoms to reveal how the solar wind interacts with the lunar surface, which in turn may help explain how lunar water forms.

  The lander's payloads included a neutron dosimeter developed by Kiel University in Germany, designated the Lunar Lander Neutrons and Dosimetry instrument. Its measurements showed that the radiation dose on the lunar surface is 2 to 3 times higher than what astronauts experience aboard the International Space Station, a data point directly relevant to planning future human expeditions. Science payloads for the overall mission were supplied by international partners from Sweden, Germany, the Netherlands, and Saudi Arabia.

• 5. Life in a Cylinder

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  Among the lander's payloads was a sealed biosphere container weighing 3 kg, 18 cm long and 16 cm in diameter, designed by 28 Chinese universities. The Lunar Micro Ecosystem held seeds and insect eggs representing six types of organisms: cottonseed, potato, rapeseed, Arabidopsis thaliana, yeast, and fruit fly eggs. The container maintained an Earth-like atmosphere, temperature, and humidity, while the organisms experienced low lunar gravity and elevated radiation.

  The plan was elegant in its simplicity. If fly eggs hatched, the larvae would exhale carbon dioxide; the germinating plants would convert it back to oxygen through photosynthesis. Yeast would regulate that gas exchange and break down waste from the flies and dead plant matter, creating an additional food source for the insects. The three elements together were intended to demonstrate a minimal closed ecological loop, directly relevant to developing life-support systems for long-duration crewed missions.

  Within hours of landing on the 3rd of January 2019, the container's temperature was adjusted to 24 degrees Celsius and the seeds were watered. On the 15th of January 2019, it was reported that cottonseed, rapeseed, and potato seeds had all sprouted, though only images of the cottonseed were publicly released. The first plants had ever grown on another world. One day later, on the 16th of January, the experiment was terminated. The lunar night had dropped the external temperature to minus 52 degrees Celsius, and the container's heating system could not maintain the seeds' environment. The experiment ended after nine days rather than the planned 100, but the data gathered was recorded as scientifically valuable.

• 6. International Dimensions

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  Chang'e 4 arrived at a delicate moment in the relationship between China and the United States in space. A Congressional ban enacted in 2011 had largely frozen cooperation between NASA and Chinese space agencies. Chang'e 4 became the first major exception to that ban, making it the first significant US-China collaboration in space exploration since the restriction was imposed.

  Scientists from both nations maintained regular contact in the period before the landing. One proposed collaboration involved using NASA's Lunar Reconnaissance Orbiter to image any plumes of surface particles kicked up by Chang'e 4's descent engines, a measurement that could be compared against theoretical models of rocket exhaust interacting with the lunar regolith. The LRO was not in the correct orbital position during the actual landing, so that observation did not occur. The Americans provided Chinese scientists with data on the positions of US satellites orbiting the Moon, and the Chinese shared the precise longitude, latitude, and timing of the landing. China subsequently agreed to NASA's request to make use of the Chang'e 4 probe and the Queqiao relay satellite for future American lunar missions.

  NASA Administrator Jim Bridenstine called the mission's success an "impressive accomplishment." Martin Wieser of the Swedish Institute of Space Physics, the principal investigator for the ASAN instrument aboard the rover, framed the scientific stakes plainly: "We know the far side from orbital images and satellites, but we don't know it from the surface. It's uncharted territory and that makes it very exciting." In November 2019, the Chang'e 4 mission team received the Gold Medal from the Royal Aeronautical Society. In October 2020, the International Astronautical Federation awarded the team its World Space Award. Both were the first times a Chinese mission had won those respective honours.