Abstract
The dusty regolith covering the surfaces of asteroids and planetary satellites differs in size, shape, and composition from terrestrial soil particles and is subject to environmental conditions very different from those found on Earth. This regolith evolves in a low ambient pressure and low-gravity environment. Its response to low-velocity impacts, such as those that may accompany human and robotic exploration activities, may be completely different than what is encountered on Earth. Experimental studies of the response of planetary regolith in the relevant environmental conditions are thus necessary to facilitate future Solar System exploration activities.We combined the results and provided new data analysis elements for a series of impact experiments into simulated planetary regolith in low-gravity conditions using two experimental setups and a range of microgravity platforms. The Physics of Regolith Impacts in Microgravity Experiment (PRIME) flew on several parabolic aircraft flights, enabling the recording of impacts into granular materials at speeds of ∼ 4–230 cm/s. The COLLisions Into Dust Experiment (COLLIDE) is conceptually close to the PRIME setup. It flew on the Space Shuttle in 1998 and 2001 and more recently on the Blue Origin New Shepard rocket, recording impacts into simulated regolith at speeds between 1 and 120 cm/s.Results of these experimental campaigns found that there is a significant change in the regolith behavior with the gravity environment. In a 10 −2g environment (with g being the gravity acceleration at the surface of the Earth), only embedding of the impactor was observed and ejecta production was produced for most impacts at > 20 cm/s. Once at microgravity levels (<10−4g), the lowest impact energies also produced impactor rebound. In these microgravity conditions, ejecta started to be produced for impacts at > 10 cm/s. The measured ejecta speeds were somewhat lower than the ones measured at reduced-gravity levels, but the ejected masses were higher. In general, the mean ejecta velocity shows a power-law dependence on the impact energy with an index of ∼ 0.5. When projectile rebound occurred, we observed that its coefficients of restitution on the bed of regolith simulant decrease by a factor of 10 with increasing impact speeds from ∼ 5 up to 100 cm/s. We could also observe an increased cohesion between the JSC-1 grains compared to the quartz sand targets.
Highlights
Small airless bodies of the Solar System are known to be covered in a layer of regolith, composed of grains of varying sizes
The free-floating experiment boxes of Physics of Regolith Impacts in Microgravity Experiment (PRIME)-3 offered the best microgravity quality: with only air drag acting on the box moving at very low speeds (< 1 mm/s), no residual acceleration could be detected from our video data
While this is due to the fact that only very few impacts were performed in Johnson Space Center (JSC)-1 in microgravity at speeds > 50 cm/s, the nature of the target plays a role in the ejecta mass produced: compared to quartz sand particles, which are rounded and considered cohesionless in vacuum, JSC-1 particles are more angular and behave like a cohesive powder
Summary
Small airless bodies of the Solar System are known to be covered in a layer of regolith, composed of grains of varying sizes. More recent data collection was obtained with these two experiments during the PRIME-3 (Colwell et al 2016) and COLLIDE-3 campaigns, on the NASA C-9 aircraft in 2014 and Blue Origin’s New Shepard suborbital rocket in 2016, respectively Both these experiment setups generate impacts of approximately centimeter-sized spherical projectiles onto beds of granular material at speeds of 1–230 cm/s and gravity levels ranging from reduced gravity (∼ 10−2g) to microgravity (< 10−4g). They showed that the container side wall location had a vanishing influence on the impactor penetration for a ratio between the tray and projectile of > 5 Given their impact speeds and the size of their projectiles and target grains, their results are applicable to the COLLIDE and PRIME experiments. The sample is jammed and its porosity remains unchanged (0.4 to 0.5 as described in “Projectiles and simulants” section) during loading of the experiment boxes onto the aircraft
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