Image source @ Visual China
Text | Academic headlines, author | Cooper, Editor, | Kou Jianchao
To date, many manned and unmanned space missions have surveyed the near hemisphere of the moon, but little is known about the far side of the moon, and the far side of the moon, which is popularly understood as the far side of the moon, is always full of mystery.
Probing the far side of the Moon is even more challenging, as harsh extraterrestrial environments and longer communication distances require effective local movements of lunar rovers to explore scientifically significant lunar geological features.
On January 3, 2019, the Chang'e-4 lander and the "Yutu-2" lunar rover of the Chinese lunar exploration project successfully landed softly at the von Carmen impact crater in the South Pole-Aitken Basin on the far side of the moon, setting a number of "firsts" and "firsts" in one fell swoop. For example, this is the world's first soft landing and patrol survey on the far side of the moon, and it is also the first landing in the high latitude polar region of the moon, and it is also the first relay communication between the back of the moon and the earth, and in December 2019, Yutu-2 also became the longest working lunar rover on the lunar surface.
Figure | topographic map of the back of the moon, red represents the high area, blue is the low area, and the purple circle and gray circle are the inner and outer wall ranges of the basin respectively (source: Wikipedia)
The area explored by the "Yutu-2" lunar rover is one of the largest known impact craters in the solar system, and is recognized as the largest, oldest and deepest impact basin on the moon, formed about 4.55-3.92 billion years ago, with great scientific research value, and the super survivability of "Yutu-2" also fulfilled its mission, sending back a wealth of information about the far side of the moon' regolith, craters and rocks.
On January 20, the latest cover article published in the authoritative scientific journal Science Robotics analyzed the relevant information detected by the Yutu-2 lunar rover, hinting at significant differences in the surface geology of the far and near sides of the moon, which may greatly improve the understanding of the far side of the moon and also make reference suggestions for improvements to the lunar rover.
The main researchers of the paper are from the State Key Laboratory of Robotics and Systems of Harbin Institute of Technology, Beijing Aerospace Control Center, The Key Laboratory of Aerospace Flight Dynamics Science and Technology, and the State Key Laboratory of Remote Sensing Science, Institute of Aerospace Information Research, Chinese Academy of Sciences.
A new chapter of lunar exploration
Planetary exploration using orbiters or rover probes has been pushing beyond the boundaries of science and technology over the past few decades, and the Moon, as the closest object to Earth, has potentially exploitable chemical and mineralogical resources to become one of the most important destinations for human space exploration missions.
The U.S. Apollo lunar landing program pushed the lunar exploration work to the first climax, and since then, 20 successful lunar landers and probes have landed at the proximal end of the moon. The remote lunar exploration of Yutu-2 has undoubtedly opened a new prelude to lunar exploration.
Photo | lander and the Yutu-2 lunar rover (Source: Science Robotics)
The Yutu-2 is the lightest lunar rover to date (135 kg) and is itself a six-wheel high-performance off-road robot with four steering motors on the turning wheel, a maximum speed capability of 200 m/h, it can climb a slope of 20° and span obstacles up to 20 cm, carrying 4 scientific payloads to obtain high-resolution images and high-precision data, including panoramic cameras (Pancam), visible and near-infrared image spectrometers (VNIS), Lunar Exploration Radar (LPR) and advanced Small Neutral Particle Analyzer (ASAN).
The high-reliability mobile system enabled Yutu-2 to survive beyond the designed 3-month lifespan, and at the end of the first 2 years, its detection range expanded to 600.55 meters, gaining richer scientific insights, and as of now, the Yutu-2 lunar rover has traveled more than 1000 meters.
During the first two years of exploration, approximately 16.46 gigabytes of scientific data were returned for transmission and analysis, filling in the gaps in geological knowledge of the far side of the moon and deepening understanding of the formation and evolution of the moon.
The researchers presented locomotive data and images collected by the Yutu-2 lunar rover, which highlighted the unique features of the far side of the moon, and also introduced the results of wheel sliding and sinking, as well as the analysis of the lunar topsoil properties using the wheel-terrain interaction model.
Through a comprehensive review of Yutu-2's scientific research in craters, rocks and strata, the theoretical puzzles of more powerful rovers, scientific payloads and complex wheel-to-ground interaction mechanisms have been revealed, which may be necessary for the development of lunar exploration robots in the future.
The situation in the Moon's largest impact crater
Yutu-2's actions are mainly controlled by upload commands generated by ground teleoperation, but of course, it can also work in autonomous mode on relatively flat terrain, using its laser detection or risk-avoidance camera (Hazcam) to plan traversable routes.
The physical properties of the lunar topsoil strongly influence the lateral trajectory of Yutu-2, the sliding and sinking of wheels, which can be used to infer tangential and normal topographic properties. The data shows that Yutu-2 experienced relatively small wheel slippage and slippage, with slip rates ranging from 0.15 to 0.15, and struggled to explore some local craters without accurate visual location of waypoints.
Figure | the path of Yutu-2 and the traveling slide on the far side of the moon (Source: Science Robotics)
The wheel traces left by Yutu-2 provide clues to the wheel slip rate experienced during complex movement, and the fine-grain slip rate based on clear and complete traces can be estimated by extracting the trace unit and using the slip rate estimation model.
Based on this, comparing the relevant data obtained during the Chang'e-3 mission, more lunar soil physical properties can be inferred.
Figure | Compared with the first generation of Yutu (B), the wheel soil cohesiveness of Yutu II (A) is different (Source: Science Robotics)
The researchers compared and analyzed that during the Chang'e-3 mission, the sticky fine particles on the Yutu wheel were most likely caused by electrostatic adhesion, but during the Chang'e-4 mission, the large lunar soil on the Yutu-2 ship was unusual, and more than 46% of the wheel surface was covered, which was much larger than the coverage rate on the Yutu wheel at that time (about 2%).
Therefore, it is reasonable to assume that at the Chang'e-4 landing site, the greater viscosity of the soil is regional rather than local, and that the increase in the cohesion of the lunar regolith in this area may be due to the higher percentage of agglutinites in the lunar regolith.
Although Yutu-2 is not equipped with a specific soil parameter identification instrument, some parameters of the soil can be identified according to the wheel-terrain interaction, and the normal bearing characteristics of the topsoil around the landing site can be inferred from the amount of wheel sinking.
Based on the topographic mechanical model of the towing wheel and the wheel-terrain interaction parameters, the researchers predicted curves reflecting the parameters of the topsoil carrying characteristics under different sinking conditions, based on this analysis, the topsoil of the Yutu-2 landing site was harder than the typical lunar soil in the Apollo mission, and the topsoil carrying characteristics of the landing site on the far side of the moon were similar to the dry sand and sand loam soil on Earth.
Figure | Soil Parameter Identification (Source: Science Robotics)
Unexpected discoveries of substances
Yutu-2 conducted in situ studies of lunar rocks, soil, impact craters and high-energy neutral atoms launched from the lunar surface on the far side of the moon, and most of the mission's main scientific objectives have been completed, including:
(1) Low-frequency astronomical research on the lunar surface;
(2) Shallow structural survey of the flowing area on the far side of the Moon;
(3) Topographical and mineral composition survey of the flowing area on the far side of the Moon.
These results provide the basis for subsequent in-depth research.
During the 2-year adventure, Jade Rabbit II also discovered an unexpectedly highly reflective gelatinous substance at the edge of a 2-meter-deep crater. After two strenuous approaches, a dark green, sparkling substance was observed, showing a different shape, color, and texture from the surrounding topsoil blocks. The material's appearance resembles lunar samples 15466 and 70019 obtained on the Apollo 15 and 17 missions, suggesting that it may be a bonded weathered breccia that impacts molten or glass-coated, or perhaps a complex structure formed by welding, cementing, and bonding lunar weathered rocks and breccia produced by impacts.
Picture | crater encountered by Yutu-2 on its route (Source: Science Robotics)
However, due to the maneuvering failures that may occur nearby, Yutu-2 abandoned further exploration, because driving down the steep crater wall may lose control due to wheel slippage, even if it can successfully enter the crater, leaving the crater is very difficult, climbing in the case of serious slippage of the wheel, it is easy to cause the wheel to sink severely, resulting in complete inability to walk, especially when traveling on the soft and deformable lunar surface.
In response to this question, the researchers say that one of the core jobs of the future is to model the fundamental principles of motion of the substrate with complex properties such as solidification or fluidization during the wheel-ground interaction, which relies on further research and system testing of "robotic physics". New theories based on dynamic particle intrusion models and lunar rover and unconventional gait strategies developed based on robotic physical analysis are expected to solve these challenges.
From the surrounding environment, the Luna-2 landing site has no surface boulders, but there are many craters, and 88 impact craters can be quantitatively observed within a 50-meter travel range, ranging from 4.68 meters to 61.83 meters in diameter, of which about 60% of craters are less than 10 meters in diameter, and craters with a diameter greater than 20 meters are very rare. From more detailed panoramic stereoscopic images, 20 craters within 10 meters of the rover can be measured, all at depths of less than 0.6 meters.
These craters have a wide range of morphologies and can be divided into three main categories: the first is severely degraded craters, and the other two are evenly distributed or obliquely distributed on one side. High-resolution observations suggest that some of the craters that were relatively close at some point are almost all oriented northwest, so it is inferred that they come from the same set of affecting events. In the Chang'e-4 mission, few relatively large rocks buried by the regolith were observed, but some small rocks were seen everywhere.
Pictured| Yutu-2 encounters rocks on the crossing route (Source: Science Robotics)
A unique rock measuring more than 20 cm in size measured by Yutu II is called olivine sutzel, and the analysis of its spectral pattern (mineral composition similar to that of the regolith layer) and its fine to medium-grained texture may be the result of relatively rapid crystallization, or originate from impact molten pools.
The underground structure of the Yutu-2 landing site is mainly composed of low-loss, high-porosity, granular materials, and with various sizes of rocks, according to the double-channel LPR data in the shallow topsoil and deep geological layers, the researchers analyzed that at least 5 formations can be detected at a depth of 328 meters on the moon, of which there may be a uniform basal layer between 38 meters and 52 meters, followed by more basal layers from different lunar geological periods.
In future lunar missions, on-site sampling through large-depth drilling and contact measurements is expected to accelerate the progress of stratigraphic research in the lunar external environment.
Pictured| The Yutu-2 lunar rover leaves the Chang'e-4 lander
The next generation of Jade Rabbit lunar rover is worth looking forward to
The researchers said that the data returned from the Yutu-2 clearly revealed the differences in soil properties on the near and far sides of the moon, which complemented the human understanding of the moon.
Although the exploration of Yutu-2 can make a regional analysis of interesting soil cohesion phenomena, further research into the chemical composition and physical properties of the lunar topsoil is needed in order to conduct further analysis and make full use of the rover's capabilities, and more important discoveries are expected.
A deep understanding of the lunar distal surface characteristics is important, and future lunar probes will require greater kinematic capabilities, greater intelligence and advanced scientific payloads. For example, leg robots, hybrid wheeled-legged robots, or bolt-train rovers could be considered for future lunar crater or cave exploration.
In addition, in order to cope with the unknown nature of the environment, it is necessary to develop more advanced physical intelligence (understanding the scene, beyond pure geometric cognition), allowing the lunar rover to effectively select and target targets of interest, conduct some exploration in the autonomous paradigm, and enhance the ability of autonomous scenario experiments to conduct assurance or stability exploration assessments through more aggressive detection strategies.
In terms of scientific payloads, a variety of payloads for field sampling and chemical, physical, and biological analysis can be used with powerful actuators and advanced data processing subsystems to push content, depth, and efficiency limits to meet growing comprehensive survey requirements.
The researchers look forward to the potential for multidisciplinary knowledge to combine to make sweeping improvements to the lunar rover in order to uncover entirely new insights into the moon in future autonomous exploration.
bibliography:
https://www.science.org/doi/10.1126/scirobotics.abj6660