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 −2 g 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 −4 g), 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 occured, 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 cm/s up to 100 cm/s. We could also observe an increased cohesion between the JSC-1 grains compared to the quartz sand targets.

Context. In situ observations of small asteroids, such as Itokawa, Ryugu, and Bennu, show that surfaces covered by boulders and coarse terrain are frequent on such bodies. Regolith grain sizes have distributions on approximately mm and cm scales, and the behavior of such large grains in the very low-gravity environments of small body surfaces dictates their morphology and evolution. Aims. In order to support the understanding of natural processes (e.g., the recapturing of impact ejecta) or spacecraft-induced interactions (e.g., the fate of a small lander), we aim to experimentally investigate the response of coarse-grained target surfaces to very-low-speed impacts (below 2 m s−1). Methods. We present the outcome of 86 low-speed impacts of a cm-sized spherical projectile into a bed of simulated regolith, composed of irregular mm- and cm-sized grains. These impacts were performed under vacuum and microgravity conditions. Our results include measurements for the projectile coefficient of restitution and penetration depth, as well as ejecta production, speed, and mass estimation. As part of our data analysis, we compared our data set with impacts performed in similar conditions with fine grain regolith targets to determine the dependence of our measurements on the target grain size. Results. We find that impact outcomes include the frequent occurrence of projectile bouncing and tangential rolling on the target surface upon impact. Ejecta is produced for impact speeds higher than about 12 cm s−1, and ejecta speeds scale with the projectile to target the grain size ratio and the impact speed. Ejected mass estimations indicate that ejecta is increasingly difficult to produce for increasing grain sizes. Coefficients of restitution of rebounding projectiles do not display a dependency on the target grain size, unlike their maximum penetration depth, which can be scaled with the projectile to target grain size ratio. Finally, we compare our experimental measurements to spacecraft data and numerical work on Hayabusa 2’s MASCOT landing on the surface of the asteroid Ryugu.

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&lt;p&gt;Several lines of evidence indicate that most of the smaller asteroids (&lt; 1 km) consist of granular material loosely bound together primarily by self-gravity; these are commonly called rubble piles [1]. While the strength of these rubble piles is valuable information on their origin and fate, it is still debated in the literature [2]. Therefore, we have started a laboratory measurement campaign on simulated asteroid regolith, studying the impact of several factors on material strength, such as grain size, size mixtures, and surface properties. In the work presented here, we focus on fine-coarse mixtures and the influence of the fraction of fines on the sample strength. Computer simulations suggest that the increase in the ratio of fine grains to coarse grains will increase the strength of the sample in all configurations [3].&amp;#160; In a series of table-top measurements, we have determined sample compression and shear strengths for various fine-coarse mixtures. We used confined setups (less than 10cm in length) to measure the strength of the material in constricted environments such as an asteroid&amp;#8217;s core and unconfined setups (greater than 10cm in length) to simulate open environments such as the surface of an asteroid.&lt;/p&gt; &lt;p&gt;Using CI Orgeuil high fidelity asteroid soil simulant [4], we performed three measurement types to determine the strength of our samples. Samples of regolith were created by measuring percentage by volume amounts of both coarse and fine grains into the measurement container. We prepared coarse grains in two size distributions, mm-sized (Figure 1) and cm-sized. The fine fraction was composed of grains sieved between 100 and 250 &amp;#181;m. A shear box setup was used to obtain shear yield measurements which in turn provided values for the Angle of Internal Friction (AIF), bulk cohesion, and tensile strength of the samples. A compression setup was used to measure values for the Young&amp;#8217;s Modulus (YM) in both confined and unconfined samples. The third setup measured the Angle Of Repose (AOR), the steepest angle of descent relative to the horizontal plane to which a material can pile before collapse. From the AOR, we determined the coefficient of friction of each sample.&lt;/p&gt; &lt;p&gt;For compression and AOR measurements, we find that the strength of the coarse grain samples increases with the addition of a fine fraction (Figure 2, left). These findings are intuitive and support the results from computer simulations. However, we find that the increase of the fine fraction in a sample of coarse grains does not consistently increase the sample shear strength. With increasing fine fractions, the AIF and bulk cohesion (Figure 2, right) of the mixed samples decrease (until a point of saturation). This could be indicative of the fine grains acting as a lubricant as the larger grains move across each other, aiding rolling and reducing interlocking strength.&lt;/p&gt; &lt;p&gt;Our findings suggest that in the case of the surface of an asteroid, the presence of fine grains does indeed increase the strength of coarse regolith material. &amp;#160;However, fine grains in the regolith sublayers or the asteroid interior will reduce material strength due to grain interlocking and ease disruption. Therefore, rubble piles that are depleted in fine grains will have higher internal strength compared to those composed of grain size distributions that include sub-mm sized particles.&lt;/p&gt; &lt;p&gt;[1] Walsh, K.J., 2018. Rubble pile asteroids. Annual Review of Astronomy and Astrophysics, 56, pp.593-624.&lt;/p&gt; &lt;p&gt;[2] Holsapple, K., 2020. Main Belt Asteroid Histories: Simulations of erosion, cratering, catastrophic dispersions, spins, binaries and tumblers. arXiv preprint arXiv:2012.15300.&lt;/p&gt; &lt;p&gt;[3] S&amp;#225;nchez, P. and Scheeres, D.J., 2014. The strength of regolith and rubble pile asteroids. Meteoritics &amp; Planetary Science, 49(5), pp.788-811.&lt;/p&gt; &lt;p&gt;[4] Metzger, P.T., Britt, D.T., Covey, S., Schultz, C., Cannon, K.M., Grossman, K.D., Mantovani, J.G. and Mueller, R.P., 2019. Measuring the fidelity of asteroid regolith and cobble simulants. Icarus, 321, pp.632-646.&lt;/p&gt;