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Asteroid sampling plays an important role in deep space exploration. For example, two sample-return missions are undergoing, NASA's OSIRIS-REx to asteroid (101955) Bennu (Beshore, et al., 2015) and JAXA's Hayabusa2 to asteroid (162173) Ryugu (Watanabe, et al., 2017), which use robotic spacecrafts to collect samples from the surface of asteroid and take them back to earth. Based on OSIRIS-REx touch-and-go (TAG) mission design, OSIRIS-REx would fly a TAG trajectory to the asteroid’s surface to obtain a regolith sample (Berry, et al., 2013). And according to detailed description of the OSIRIS-REx asteroid sample return mission, when sampler touched the surface, materials on it (may contain hydrated minerals, organics, etc.) would be collected by Touch-And-Go-Sample-Acquisition Mechanism (TAGSAM), which had been tested many times during microgravity flights (Beshore, et al., 2015). The samples are stored in a separate sealed container inside the sample-return capsule and then taken back to earth.
According to available findings, 3D shapes of asteroids are of great differences. Although a few of asteroids are nearly spherical, most asteroids are with irregular shapes, which is mainly due to the weak gravitational force, e.g., Itokawa is like a peanut (Saito, et al., 2006) and Bennu is more like a walnut (Daly, et al., 2020). In addition, almost all asteroids’ surfaces are pitted and cratered, which means many kinds of stone formations existed in surface terrains (Fujiwara, et al., 2006). The prior concern in missions of asteroid exploration is how to land safely. However, the places of great scientific interest are usually surrounded by rocks and craters (Cui, Ge, and Gao, 2017). Therefore, precise and safe landing must be applied by robot automatically. Obviously, to deal with this complex situation, the spacecraft should be capable of detecting hazards, estimating its touchdown performance, and choosing a safe spot with high accuracy to land based on kinds of sensors (Lauretta, et al., 2017). Four candidate sampling sites on asteroid Bennu were selected by NASA for OSIRIS-REx mission, based on their scores for safety, sampleability, deliverability, and science value. In December 2019, the OSIRIS-REx team chose sites Nightingale and Osprey as the mission’s primary and backup sample collection sites.
Specifically, the surface of asteroid Itokawa was investigated (Zeng, Li, Martin, Tang, and Wang, 2019), which aimed to provide the reference for asteroid landing and sample-return missions. The robust planning of the Hayabusa2-Ryugu proximity operation and landing site selection process was presented (Yamaguchi, Saiki, Tanaka, Takei, and Tsuda, 2018), considering unknown environment on asteroid and the vehicle constraints. The proximity operation was decomposed a landing site selection and in-situ observation. An optimal landing site selection method based on safety index during planetary descent was proposed (Cui, et al., 2017). It developed a criterion for evaluating the safety index of landing area, including terrain safety, fuel consumption and touchdown performance during descent. The motion of surface particles on the asteroid Bennu was investigated (Zhang, Zeng, Circi, and Vulpetti, 2019). A probability-based hazard avoidance guidance algorithm for planetary landing method was developed, which exploited information on the uncertainty of lander position and calculated the probability of collision with each hazard (Yuan, Yu, Cui, Xu, and Luan, 2018). Besides, it indicated that site selection based on images of asteroid is useful and important for validating the method of fuel-optimal control for soft landing on an irregular asteroid, especially for a highly irregular gravitational field (Yang and Baoyin, 2015). In Yang and Baoyin’s research, with the help of asteroid’s images, a convincing fuel-optimal descent trajectory can be determined.