Interview:
Interview Li Qing, deputy director of design for the Chang'e 5 explorer explains the difficulties and goals of retrieving and returning surface samples from the moon to Earth
Lunar Ambitions
At 4:30am on November 24, a Long March-5 rocket blasted off from the Wenchang Spacecraft Launch Site in Southern China's Hainan Province, successfully lifting the four-stage Chang'e-5 lunar probe into its planned Earth-moon transfer orbit. Chang'e-5, named after the goddess of the moon in Chinese folklore, is the country's first attempt to collect soil and rock samples from an extraterrestrial body.
Starting in 2007, China has conducted five lunar missions from Chang'e-1 to Chang'e-5 in staged missions. The Chang'e-5 spacecraft is the final step in China's three-stage lunar program to orbit, land and return from the moon, 380,000 kilometers away.
Landing on the lunar surface on December 1, the lander-ascender completed its mission to collect around two kilograms of rock and soil samples, then lifted-off to rendezvous and dock with the orbiter-returner. After transferring the samples to the return module on December 5, the ascent stage was sent back in a controlled collision with the moon. The orbiter and the reentry capsule will remain in orbit until the optimal time for a return trajectory, and are set to reenter Earth's atmosphere around December 16 for a mission totaling 23 days.
In late November, China News Service's China in Focus program interviewed Li Qing, deputy director of design for the Chang'e 5 explorer at the China Aerospace Science and Technology Corporation (CASC). Li discussed the mission difficulties and what the Chang'e 5 program hopes to accomplish. Newschina has been authorized to reprint the interview.
China News Service: Why was the launch scheduled so early in the morning, and what do you have to take into account when you schedule a rocket launch?
Li Qing: Optimal launch time normally depends on the design of a mission. The main task of the Chang'e-5 probe is lunar surface sampling. To ensure the best measurements and control in the sampling process, we hoped to complete this mission when it was late at night on Earth and daytime on the moon, so the full moon provides the best observation conditions and the best measurement and control conditions. Due to these conditions, we calculated the optimal launch window would be about 4.30am.
CNS: Can you give an overview of China's lunar exploration initiative from Chang'e-1 to Chang'e-5?
LQ: In line with our three-stage lunar program to orbit, land and return, China's lunar exploration project has so far successfully carried out five missions.
Chang'e-1 was launched in October 2007 from the Xichang Satellite Launch Center [Sichuan Province], and hit the moon in a controlled crash in March 2009 after completing its scheduled orbit mission.
China's second moon orbiter, Chang'e-2, as a pilot for Phase II of the program, was successfully launched in October 2010. It started its extended mission in April 2011 after completing its assigned tasks to capture high-resolution images of the Sinus Iridum, or “Bay of Rainbows” on the moon, an area where the Chang'e-3 orbiter was scheduled to land afterwards. After leaving lunar orbit, on its extended flight, Chang'e-2 arrived in outer space and conducted test orbiting around the second Sun-earth Lagrangian Point (L2), where gravity from the sun and Earth balances the orbital motion of a satellite. It then continued its journey to fly by the near-earth asteroid 4179 Toutatis [on December 13, 2012] and sent back images. At present, it is China's furthest probe from Earth, and it opened the first chapter of our multi-target deep space exploration program.
The Chang'e-3 probe, composed of a lander and a rover, was launched in December 2013 and soft-landed safely in the Bay of Rainbows area on the near side of the moon. After landing, the lunar rover separated from the lander and the two completed their scheduled lunar surface probe missions.
The Chang'e-5 test flight capsule was launched in October 2014, mainly to test the return technology. After the Earth-moon transfer,
near-moon navigation and the moon-earth transfer, the return module separated from the service module and reentered Earth's atmosphere, landing successfully in the planned landing area of Siziwang Banner in Inner Mongolia Autonomous Region.
Chang'e-4, [as a backup for Chang'e-3] made adaptive improvements on the basis of Chang'e-3. With the help of a relay satellite named Queqiao, or “magpie bridge,” it landed with a rover on the far side of the moon, the side not visible from Earth. The probe made a successful landing in January 2019 in the Aitken Basin of the south pole region of the moon, the world's first spacecraft to achieve a soft landing on the far side, and it's still working.
This time, Chang'e-5 comprises a lunar orbiter, a lander, an ascender and a return capsule.
CNS: Can you talk more about Chang'e-5's mission?
LQ: The Chang'e-5 probe's task is to collect about two kilos of lunar samples through robotic sampling, carry out ground analysis and research, and then return with the samples.
The mission is divided into 11 stages with a total flight time of about 23 days.
The first stage is to launch the probe into orbit. The probe was launched on a Long March-5 carrier rocket which it separated from and then went into the Earth-moon transfer orbit. This phase was completed on November 24. The second stage is the Earth-moon transfer which takes about five days, reaching a near-moon point with an orbital altitude of about 200 kilometers. We need to make up to three orbital corrections during the flight.
The third is the near-moon braking stage. When it approaches the near-moon point, the probe burns its engines to maneuver into an elliptical orbit. After orbiting three times, the burners fire again to go into a circular orbit at an average 200-kilometer altitude. It takes about a day to complete this process.
The fourth stage is lunar orbit. After four orbits, the lander-ascender and the orbiter-returner are separated with the orbiter-returner remaining in orbit. The lander-ascender transitions into an elliptical orbit, which is about 15 kilometers from the moon at its closest point and 200 kilometers at its furthest.
The fifth stage is the descent. After the lander-ascender continues for about 10 orbits on the elliptical track, it commences the soft landing process, which will take about 15 minutes.
The sixth stage is taking soil and rock samples from the surface and from drilling into the ground, and storing them in the return capsule. This takes about two days.
The seventh stage is the ascent from the moon. The ascent stage lifts off and reenters the elliptical orbit, which takes up to six minutes.
The eighth stage is rendezvous and docking and transferring the samples to the return module. The ascender is guided back to a circular orbit at a 210-kilometer altitude. It takes about two days to remotely guide the ascender. When the ascender is about 50 kilometers ahead of the orbiter-returner and about 10 kilometers above, the docking maneuvers begin. When it has docked and transferred the samples, the ascender detaches from the return stage.
The ninth stage is waiting in orbit for the right window to start the return trajectory to Earth, which will take five to seven days.
The tenth stage is the moon-earth transfer. This takes four to five days, and course corrections are made en route to ensure the reentry coordinates are correct.
The eleventh and last stage is reentry and recovery. When the orbiter-returner is about 5,000 kilometers from Earth, the two stages separate. The orbiter continues with evasion maneuvers, and the reentry module returns to Earth, bouncing once off the atmosphere [to slow it down]. The parachutes open when it reaches an altitude of 10 kilometers, and it is set to land at Siziwang Banner in Inner Mongolia in northern China.
CNS: The mission aims to return samples from the lunar surface. What are the samples like?
LQ: The Chang'e 5 probe has two sampling methods: drill
sampling and surface sampling. Drill sampling involves drilling about two meters into the lunar surface to collect about 0.5 kilos of samples. Surface sampling is multi-point sampling done by a robotic arm. The sampling area includes flat ground, impact pits and small stones. These samples have different distributions, different geological characteristics and different evolutional histories. The sample container is sealed to ensure samples are not contaminated during the return process.
CNS: Could you explain the difficulties of this complex mission? LQ: The technical difficulties of the Chang'e 5 probe are mainly in orbit design, lunar sampling and collection, lunar take-off and ascent, lunar orbit rendezvous and docking and sample transfer.
The first is the complex design of the lunar orbit process. Because of the many flight stages of the Chang'e 5 probe, the orbit design for each stage is closely connected, which involves many restrictions and constraints of measurement and control. It is also necessary to make plans around limited fuel supplies for the orbit design scheme, which is a new challenge to scientists.
The second is lunar surface sampling and packaging technology, which is a new technology and one of the core steps of the Chang'e 5 mission. We have to consider all kinds of constraints, such as measurement and control, illumination, power supply and thermal control. There are a lot of uncertainties.
The third difficulty lies in the moon takeoff ascent verification. The state of the lunar takeoff system derives from the landing conditions, landing posture and position. Lunar takeoff conditions can't be measured and confirmed by ground control on Earth, but depend on the lander-ascender's self-positioning program. Unlike the plume environment [taking off in a vacuum] of conventional spacecraft, the ascent engine ignition in a confined space has a very complex plume thermal effect, which needs a lot of ground simulations and tests.
Fourth, the moon orbit rendezvous, docking and sample transfer must be done automatically due to lack of satellite navigation signals. At this time, ground control support is limited, the size of the docking mechanism is limited, and the precision required for docking is high. So, research into the technology of the integrated design for the docking mechanism and sample transfer mechanism has been challenging.
CNS: The lunar exploration project involves the Jade Rabbit (Yutu) No.2 lunar rover. What's the difference between Jade Rabbit 2 and Chang'e 5?
LQ: Jade Rabbit is the name for the lunar rovers carried by the Chang'e landers. Jade Rabbit No.1 was carried by the Chang'e 3 probe and Jade Rabbit No.2 by Chang'e 4. The Chang'e probes are able to conduct surveys near the landing area. The Jade Rabbit lunar rovers have a mobile device that can expand the range of lunar surface exploration. These two methods complement each other and when we combine them, they improve the efficiency of the mission.
CNS: Based on the lunar research data we will have obtained, do you think we can build a lunar base like in science fiction films?
LQ: The fairy tale of Chang'e, or moon goddess, is a beautiful fantasy the ancient Chinese had about the moon. Based on the achievements of current and future lunar explorations, establishing a lunar base, or a comprehensive lunar research station to support long-term research on the moon is actually on the agenda. Technically speaking, although there are still many breakthroughs needed in key technologies, this ambition is no longer limited to science fiction. This is also the general trend in human space exploration.
But I want to emphasize that consistent long-term efforts matter a lot. With the continuous support of favorable policies, and in an enduring peaceful domestic and international environment, this day is expected to come soon with continued international cooperation and the unremitting efforts of everyone working in the space exploration industry.