China's Chang'e-6 lunar mission has carried out a successful far side touchdown and is already busily gathering specimens for send-off to Earth.
The first batch of data has been relayed from the Chang'e 6 lander-ascender combination following its arrival on Sunday morning (Beijing Time). The landing was aided by China's newly positioned Queqiao-2 relay satellite to provide a stable communications link to mission controllers on Earth.
Chang'e 6's lander-ascender combination soft landed in the South Pole-Aitken (SPA) Basin, at an impact crater known as the Apollo Basin, located within the SPA Basin.
Related: Watch China's Chang'e 6 probe land on far side of the moon in dramatic video
Landing process
A six-stage landing process enabled the craft's soft landing: deceleration, quick adjustment, approaching, hovering, obstacle evasion and slow descending. Collectively, those stages spanned about a 900 second time period.
Deceleration happened at roughly 10 miles (16 kilometers) from the lunar surface, with the probe adjusting its angle, making it nearly perpendicular to the surface.
During the descent, an autonomous visual obstacle avoidance system was used to automatically detect obstacles, with a visible light camera selecting a comparatively safe landing area based on the brightness and darkness of the lunar surface.
Obstacle avoidance
When reaching within two to three kilometers from the lunar surface, the probe entered the approaching stage and its optical sensors began scanning the landing area for obstacles.
The probe then entered a hovering stage when descending ever-closer to the moon, with sensors working to scan the surface and quickly select a suitable space where all four of its legs would land on an even surface while evading obstacles.
At under a 100 feet (30 meters) altitude above the lunar surface, the probe entered its final landing stage, with its descending speed reduced to 4.5 miles per hour (two meters per second) until making the soft landing.
"So far, the entire moon landing process is very smooth and successful. The power descent control is relatively accurate, while the relay link is uninterrupted. And the landing is smooth and steady. The ideal landing location creates a favorable environment for subsequent lunar sample collection," said Li Xiaoning, an engineer from the Beijing Aerospace Control Center during a China Central Television (CCTV) interview.
China space watcher, Seger YU, suggests on X (formerly Twitter) that the Chang'e-6 landing point is located northeast of a pre-selected landing point, about 10 miles (16.7 kilometers) away.
Rapid sampling
China space officials have noted that the Chang'e-6 mission aims to realize key technologies of intelligent and rapid sampling, as well as takeoff and ascent from the far side of the moon.
"The mission is quite tough, primarily due to the need to establish relay communications between the Earth and the far side of the moon," Li said. "We launched the Queqiao-2 relay satellite in the early stages, providing the necessary communications support for soft landing and sampling on the far side."
Ge Ping, deputy director of the Lunar Exploration and Space Engineering Center under the China National Space Administration (CNSA) added: "We adopted rapid intelligent sampling and take-off and ascent from the moon's surface, which have laid a solid foundation for technologies, such as soft landing and sampling on extraterrestrial bodies."
It is anticipated that by honing those skills they will be mirrored by China's Mars sample return initiative in the comings years.
Substantial challenge
Now busily at work, the Chang'e-6 lander is slated to finish lunar sampling tasks within two days. To do so, the spacecraft has two methods of lunar sampling: a drill to snag subsurface samples and grabbing samples on the surface with a robotic arm.
The landing site within the SPA Basin is expected to yield unique scientific data; the far side has been subjected to a longer history of impacts, indicating an older geological age for in-depth exploration. Also, the landing area – and flatness of the landscape — offers communications and telemetry conditions to execute the mission.
"It's the first time in human history to retrieve lunar samples from the far side of the moon," Ye Peijian, an academician of the Chinese Academy of Sciences told CCTV.
"If Chinese scientists succeed in obtaining lunar regolith samples from the far side, there will be numerous technological achievements. This represents a significant advancement for us, but also poses a substantial challenge," Ye added.
Landing/sampling area
Chang'e-6 carries instruments to facilitate scientific research.
Zuo Wei, chief engineer of the Chang’e-6 mission, told CCTV one of that trio of instruments is the foreground camera, which captures high-definition images of the landing and sampling areas.
Another is the mineral spectral analysis instrument, Zuo said, which obtains spectral data at the sampling area to aid in analyzing the mineral composition there.
Another instrument is the structural detector, installed at the bottom of the lander, which detects the structure of the soil beneath the moon's surface and provides information for further drilling and sampling procedures, Zuo explained.
International payloads
Also onboard the lander are several international payloads including a detector from France, a lunar surface negative ion analyzer from the European Space Agency (ESA) and a laser angle reflector from Italy.
As the first French-made instrument to land on the moon, the Detection of Outgassing RadoN (DORN) is dedicated to measuring the radioactive gas of radon on the moon’s surface.
The lunar surface negative ion analyzer developed by the ESA/Sweden will be used to detect negative ions and study the interaction between plasma and the lunar surface.
The laser retro-reflector developed by Italian scientists will be used for positioning and distance measurement in future lunar missions.
In addition to scientific instruments from European countries, the probe also carried a cube satellite from Pakistan known as ICUBE-Q, which separated from the Chang'e-6 orbiter on May 8 to carry out exploration activities, such as capturing images of the moon.
"Samples into our pocket"
The multi-component Chang'e-6 mission consists of an orbiter, a returner, a lander and an ascender.
Since the mission's departure from China on May 3 it has carried out Earth-moon transfer, near-moon braking, lunar orbiting and landing descent. The lander-ascender combination separated from the orbiter-returner combination on May 30, said the China National Space Agency (CNSA).
"The drilling and collecting work are reaching an end," advised Ren Depeng, a space expert from the China Aerospace Science and Technology Corporation (CASC).
"Now we have samples and the last action is to put the samples into our pocket. We can say that the drilling and collecting are progressing smoothly," Ren told CCTV.
Moon-to-Earth journey
Following Chang'e-6's drilling and sample collection duties, the probe will seal all the samples off before the ascender performs take-off preparations.
Ren added that the ascender will then blast off from the moon's surface, connect with the returner combination that has been waiting in orbit and transfer those collected samples into the returner combination.
That combination is set to conduct a Moon-to-Earth journey and return the specimens to the Earth under parachute, now eyeing a June 25 (Beijing Time) targeted landing.
With samples on Earth, they will be transferred to a lunar receiving lab in Beijing for examination. The far side specimens are to be described and documented, prior to publication of a Chang'e-6 sample catalog.
The Chang'e 6 mission profile is essentially identical to China's Chang'e 5 robotic moon mission in late 2020. That earlier effort returned 1,731 grams of lunar materials from Northern Oceanus Procellarum near a huge volcanic complex, Mons Rümker, located in the northwest lunar near side.
Spectacular accomplishment
According to Jim Head, a lunar exploration authority at Brown University, the Chang'e 6 landing and sampling operations represent a spectacular accomplishment.
Head pointed out that China, with its earlier Chang'e 4 mission, demonstrated the first successful landing on the far side of the moon, followed immediately by surface operations with the deployed Yutu-2 rover.
The Chang'e 4 mission accomplished two very difficult tasks five years ago in 2019, Head said, with the help of the Queqiao-1 far side relay satellite.
"Now, with the help of the new Queqiao-2 far side communication satellite, China has successfully landed a sample return spacecraft, Chang'e 6, in the Apollo basin within the huge South Pole-Aitken basin, an area of intense scientific interest and importance," Head told Inside Outer Space.
Mission critical steps
Successful return and analysis of these samples, currently undergoing collection and stowage, Head said, "will help to address age-old fundamental questions about the many differences between the lunar near side and far side, and what they mean about the origin and evolution of the moon."
The return of lunar samples from the nearside of the moon by China's Chang'e-5 mission in 2020, now followed by the successful landing on the lunar far side by the Chang'e-6 sample return mission, helps to orchestrate future projects, Head advised.
"It's clearly very important practice for Chinese engineers and operations personnel, as each step of these missions duplicates important and mission-critical steps in the Chinese plans to land taikonauts on the moon and robotically return samples from Mars," Head said.
