Just before the Chang’e-4 probe was set to land, Ye Peijian, a consultant to chief commander and chief designer of the Chang’e project, approached Zhang He, executive director of the project, to ask whether she was nervous. “A little,” admitted Zhang. “Don’t worry,” Ye said. “Relax.”
After the “long” 10 minutes during which the probe slowed and landed on the moon, Zhang let out a sigh of relief and turned back to hold hands with Ye and cried with happiness.
Chang’e-4 was originally a backup for Chang’e-3. Back then, a big debate took place to determine whether Chang’e-4 would land on the near side or the far side of the moon. Some questioned why other countries had not gone to the far side. Others asked why China should go and what it would mean.
Zhang He, executive director of the Chang’e-4 probe project at the China Academy of Space Technology, granted an exclusive interview to China-India Dialogue (CID). According to her, exploring the far side of the moon has always been a dream for scientists around the world. The images sent by the probe presented a very different picture of the moon’s far side from what humans had imagined. Furthermore, a comparison of the exploration results from the near side and the far side could kindle many scientific discoveries which will be of great significance for scientists and engineers worldwide.
CID: From launch to landing, when were you most nervous? What do you think was the most crucial factor for the success of Chang’e-4?
Zhang He: Three moments tugged at my heartstrings. The first was the launch of the rocket. It mattered whether the probe could enter into the correct orbit and whether the solar panels could unfold to collect power. The second was space braking. When approaching the moon, if Chang’e-4 didn’t slow down, it would have missed the moon and drifted into space. So, both its speed and position had to be controlled accurately. Otherwise, it wouldn’t orbit the moon. The last but the most nerve-racking moment was the slow lowering and landing process. Contrasting Chang’e-3, Chang’e-4 relies heavily on the Queqiao relay satellite for communications, resulting in a two to three-minute delay. This eliminated any chance to change directions. So the whole process could not be amended or reversed.
Each leg of Chang’e-4 has a moon-touching switch. When we saw the four switches were off, along with the image sent back by the camera, we knew the probe finally landed on the moon. The subsequent challenge was the automatic spread of the solar wings. After this step, we believed we had avoided the most dangerous problems and felt relieved.
Rather bumpy compared to the near side, the moon’s far side is full of craters. So unlike Chang’e-3, we adjusted the powered lowering strategy and made many modifications to control software. Chang’e-3 descended obliquely when landing on the moon. But Chang’e-4 first moved horizontally while slowing down and then arrived six kilometers above its designed landing point. Finally, it descended to the landing point almost vertically in order to avoid possible obstruction.
The workstation on the ground could see the landing of Chang’e-3 with a very short-time delay. The communications for Chang’e-4 had a longer delay and we couldn’t respond to potential malfunctions in real time. So we granted Chang’e-4 some automatic functions to allow it to judge whether the engine was on and whether the sensors worked.
CID: What are the possible problems and challenges during the scientific investigation of the Chang’e-4 lander and the Yutu-2 rover on the far side of the moon? What are the biggest breakthroughs that will be made?
Zhang: Because the evolution history of the far side and near side of the moon is quite different, we hope the Chang’e-4 probe can make new discoveries in the analysis of surface material on the far side of the moon, which mainly relies on the infrared spectrometer loaded on the rover of Chang’e-4 to analyze the composition of the lunar soil. Chang’e-3 also carried such a device.
On the last lunar day, the infrared spectrometer was turned on and will continue to work this month. The Yutu-2 lunar rover is equipped with an electromagnetic radar like that on Chang’e-3. It works by sending out electromagnetic pulses that travel through lunar soil and bounce off objects below to collect information about the layered structure of the lunar crust. We hope that Yutu-2 can go further.
The most difficult part is the low-frequency radio astronomical observation, which is a new payload of Chang’e-4 aiming to observe the low-frequency radio signals from the universe, the sun and other celestial bodies. The difficult factor is that our probe itself also emits a lot of low-frequency electromagnetic signals.
According to data we have already collected, a lot of work has to be done to further eliminate noise and truly separate the low-frequency radio signals from the universe, especially from the sun.
CID: What scientific payloads is the Chang’e-4 probe equipped with? What were the selection criteria for those? What is the significant international cooperation between China and other countries and international organizations?
Zhang: The lander and rover are equipped with four scientific payloads each. For instance, the rover carries a panoramic camera to take pictures for scientific research, and a neutral atom detector developed by Sweden to analyze what neutral atoms exist on the moon. The lander has a neutron radiation detector developed by Germany to test the neutron environment of the moon and discover whether there is water or ice on the moon and an infrared imaging spectrometer to obtain images of the moon’s surface.
China’s State Administration of Science, Technology and Industry for National Defence has collected payloads globally. But because of the limits on weight, the Chang’e-4 probe is equipped with only four international payloads. The Queqiao (Magpie Bridge) relay satellite carries a low-frequency radio astronomical instrument developed by the Netherlands.
CID: What is the biggest difference between Chang’e-3 and Chang’e-4?
Zhang: The two probes look similar, but they are quite different.
Firstly, because the navigation control strategy of the Chang’e-4 landing is different from that of Chang’e-3, navigation sensors, laser range sensors and microwave velocity sensors are specially modified for the far side of the moon.
The second difference is the collection of data about the temperature of the moon night, which Chang’e-3 did not achieve. What’s the temperature of the moon at night? The U.S. Apollo mission on the near side of the moon obtained related data and the country has reported in other materials that the temperature of lunar soil is as low as minus 180 degrees Celsius, but China does not have its own data.
Chang’e-4 uses technological breakthroughs to solve the problem of continuous power supply to the moon night temperature acquisition device. The lander is able to reach the lunar soil by laying out many measuring points to obtain the temperature of the lunar soil and other data.
CID: What are the challenges ahead for China’s lunar probe project and deep space exploration?
Zhang: The work on the Chang’e-5 lunar probe is ongoing—it is expected to bring lunar samples back to Earth. China’s lunar exploration team has basically amassed the knowledge about regular procedures to explore an extraterrestrial body, including “orbiting, landing, roving, and returning.”
Despite having probed the moon over 100 times, human beings possess very limited knowledge about it. Scientific questions waiting to be clearly answered include: How did the moon come into being? Is there water on the moon? What is under the lunar crust? We plan to land a spacecraft at the moon’s poles in the fourth phase of China’s lunar exploration program. The primary challenge is presented by the more rugged landform at the poles and permanently shadowed areas where landers can’t escape once they fall in. Chang’e-3 landed in an optional area of a few hundred square kilometers, and Chang’e-4 landed in an optional area of a few square kilometers. But landing at the moon’s poles requires the lander to find a proper spot within an area of one square kilometer or even smaller, which increases the difficulty.
The lunar poles are also extremely cold because of low sun angles and there is no energy and light in the permanently shadowed areas, which creates new barriers for heat supply and energy sustainment of the probe.
We are also looking to explore more materials deep in the lunar soil, which requires lunar probe robots to be more capable of working independently at deep digging, flying and task planning. And China hopes to complete building a scientific station for long-term operation on the moon by 2030.
As for deep space exploration, alongside a manned moon landing, China also plans to explore Mars, Jupiter, asteroids and even the edges of the solar system. To achieve these goals, we need to tackle enormous technological challenges concerning orbit design, automated navigation, power system, communication for measurement and control, and energy. Each step in deep space exploration requires a great technological breakthrough.
CID: What was the biggest challenge in your personal growth from 2001 when you first joined the lunar probe project to today as you serve as executive director of the Chang’e-4 probe project?
Zhang: Taking part in the lunar probe project and aerospace work has given me more courage to face challenges, and I became more competent in solving problems. In my early days on the Chang’e project, I once took part in an engine test. The squeaking of the engine became extremely loud at certain parts, which made me nervous. My colleagues comforted me by saying, “Take some time and you will get used to it.” Another time in a mechanics experiment, I heard increasing sound as the whole satellite resonated on a different frequency band. When the tremendous noise surrounded me, I could not have felt more nervous and worried that something went wrong with the project. This was a hard exercise on my mental endurance. After enduring more exercises like this, I tend to believe that any problem can be solved no matter how difficult it is.
I have been obsessed with gazing at the stars since I was a kid, and I took many photos of starry nights and celestial bodies. I always dream of going further and seeing more extraterrestrial celestial bodies. So, I feel extremely lucky to work in deep space exploration and be a member of China’s lunar probe team.
CID: Contact with aliens has been frequently discussed. What do you think of the mysterious signals from the universe? Do you believe in extraterrestrial life?
Zhang: I believe that humans are not the only intelligent beings in the universe and that there may be creatures more intelligent than us. We can’t see them now but we may run into them some day.
Human beings should be cautious about aliens. In the beginning of the encounter, I suppose aliens will also “wait and see.” I hope human beings and aliens meet without hostility and live in harmony together in the universe.