Chang'e-6, collecting the first lunar farside samples

Highlights

• China's Chang'e-6 spacecraft launched on May 3, 2024 on a mission to collect samples from the far side of the Moon.
• The mission aims to grab samples containing material ejected from the lunar mantle and thus provide insight into the history of the Moon, Earth, and Solar System.
• The mission is a more ambitious followup to China's Chang'e-5 2020 sample return mission

Why sample the far side of the Moon?

The lunar south pole is attracting a lot of attention these days, mainly because of the possibility of the presence of game-changing amounts of water ice trapped in shadowed craters. Water reserves could make lunar habitats much more viable and potentially be used for propellant for sending missions farther out into the Solar System. However, other areas hold clues to intriguing and important science questions.

One of these is the South Pole-Aitken (SPA) basin, a massive ancient impact basin which covers a vast portion of the far side of the Moon. The roughly 2,500-kilometer (1,600-mile) diameter SPA basin was created over 4 billion years ago, contains compositional intrigues, and could offer glimpses into the interior of the Moon.

Accessing the far side is challenging, however. All but one lunar soft landing by any country has set down on the near side which always faces the Earth (this is due to tidal locking, whereby Earth's gravity over time has slowed the Moon's rotation to a speed that keeps it permanently facing us).

The only lunar far side landing came in 2019 with China's Chang'e-4, a backup to the 2013 nearside Chang'e-3 lander and rover mission (Chang'e is a Moon goddess in Chinese legend). Chang'e-4 required a relay satellite, named Queqiao (“Magpie Bridge”), to be launched ahead of the lander and rover into a halo orbit beyond the Moon to bounce signals between ground stations on Earth and the otherwise uncontactable lunar far side.

That mission, together with the 2020 Chang'e-5 nearside sample mission, which successfully collected 1,731 grams (3.81 pounds) of material from Oceanus Procellarum, has set the stage for an even more audacious followup.

Chang'e-6 (嫦娥六号), originally a backup to Chang'e-5 in case of failure, launched on May 3, 2024 to Apollo crater within the SPA basin.

The mission is designed to provide new material from a new area of the Moon to further our understanding of our celestial neighbor, potentially including basalts of diverse ages. Even more exciting to lunar and planetary scientists will be the possibility of collecting exotic lunar material which would have been excavated from depths of the Moon by the SPA basin impact, providing unprecedented insights into the early history and evolution of the Moon and, by extension, the Earth and wider Solar System.

Where exactly will Chang'e-6 land?

Chang'e-6 will target a southern portion of Apollo crater, according to a paper published in Nature Astronomy in July 2023. The site was rumored to be a candidate for the Chang'e-4 mission, which landed in Von Kármán crater. Located at 150–158 degrees west, 41–45 degrees south, Apollo is similar in latitude to Von Kármán and the nearside Chang'e mission landings, meeting engineering criteria. The area could offer a diverse set of samples, provide insight into if and why volcanic activity appears to have ceased on the far side of the Moon much sooner than the nearside, and other asymmetries between the two hemispheres.

“My guess is that the landing will be on the flat basaltic plains in the southern part of the [SPA] basin, so they will collect the first farside basalts,” says Bradley Jolliff, Scott Rudolph Professor of Earth and Planetary Sciences at Washington University in St. Louis.

“As samples of the lunar interior — the mantle where partial melting generated the basalts — these samples will address the question of how different the lunar mantle is on the farside versus the nearside and, potentially, reveal the cause of the great nearside-farside dichotomy.”

How will Chang'e-6 work?

The mission launched from the coastal Wenchang spaceport on a Long March 5 rocket on May 3, 2024. Before this, a dedicated relay satellite named Queqiao-2 was sent into a lunar orbit designed to support the mission.

The four-part Chang'e-6 spacecraft will enter lunar orbit before landing and both scooping and drilling to collect samples. An ascent vehicle will then blast off from atop of the lander, carrying samples into lunar orbit to rendezvous and dock with the waiting service module, while both are traveling at around 1 kilometer per second. That module will then return to Earth and jettison a reentry module to safely deliver the samples through the atmosphere and to the ground.

This was a 22-day mission for Chang'e-5, but indications from the China Aerospace Science and Technology Corporation, which built and will launch the mission, are that the more challenging farside Chang'e-6 profile will be a longer, 53-day odyssey. The mission will, like Chang'e-5, aim to grab 2,000 grams (4.4 pounds) of material.

Chang'e-6 features a few bonus payloads not carried by Chang'e-5. These will be a payload from France attuned to detect radon outgassing from the Moon's crust. The Detection of Outgassing RadoN (DORN) instrument will estimate outgassing from the lunar crust and its contribution to the Moon's exosphere. The Negative Ions at the Lunar Surface (NILS), a payload developed in Sweden with European Space Agency support, will seek to detect negative ions emitted from the lunar surface as a result of interaction with solar wind.

The laner will also carry the INstrument for landing-Roving laser Retroreflector Investigations (INRRI), a passive laser retro-reflector. A similar instrument flew on ESA's Schiaparelli ExoMars lander, which was lost on the surface of Mars. Finally, the Chang'e-6 mission will include the ICUBE-Q cubesat for Pakistan, with involvement from Shanghai Jiaotong University.

Surprise rover

After launch, China shared a picture of the Chang'e-6 spacecraft stack that showed what appeared to be a small rover attached to the lander. The rover was previously undisclosed as a payload. It may carry an infrared imaging spectrometer, which could be used to determine the composition of the lunar rocks and soil, including the presence of water.

After the sample return

Samples will initially be made available to Chinese institutions for science research, as well as through international cooperation. The invaluable material will then be made available to international applications for sample research. The Chang'e-5 samples were opened to international proposals in August 2023, some 2.5 years after landing on Earth.

A final piece of intrigue will be if there will be an extended mission in store for the Chang'e-6 service module. The Chang'e-5 mission orbiter was sent on two groundbreaking side quests after separation of the reentry capsule that delivered samples to Earth.

The service module made China's first foray to an area closer to the Sun than the orbit of the Earth, heading to Sun-Earth Lagrange point 1, also used by NASA's DSCOVR , to test requirements for solar observations.

By early 2022 the spacecraft had returned to the Earth-Moon system and, for the first time, entered a distant retrograde orbit around the Moon. That may have been a test of the orbit to be used for the Queqiao-2 mission to support Chang'e-6. Similarly, a 2014 Chang'e-5 test spacecraft was used in an extended mission to test out a halo orbit at Earth-Moon Lagrange point 2, providing a first hint of China's lunar farside ambitions.