The ballistic sorting effect has been proposed to be a driver behind the observed size sorting on the rubble pile asteroid Itokawa. This effect depends on the inelasticity of slow collisions with granular materials. The inelasticity of a collision with a granular material, in turn, depends on grain size. Here we argue that determining the inelasticity of such collisions in an asteroid-like environment is a nontrivial task. We show non-monotonic dependency of the coefficient of restitution (COR) on target particle size using experiments in microgravity. Employing numerical simulations, we explain these results with the growing influence of adhesion for smaller-sized particles. We conclude that there exists an optimum impactor to target particle size ratio for ballistic sorting.

We present a newly developed experiment for the examination of low-speed impacts under asteroid conditions. More specifically, our experimental setup enables us to simulate a very clean milligravity environment under vacuum, in which projectiles are shot at a granular bed at several cm/s. This granular bed consists of irregularly formed basalt particles with different size distributions. The experiment is carried out in the Bremen drop tower in its catapult mode, granting more than 9 s microgravity. Here, we discuss the setup and assess its performance.

<p>Rubble pile asteroids such as Itokawa, Ryugu, and Bennu are covered by regolith of various sizes. In some instances like on Itokawa the regolith is not distributed evenly but large boulders congregate in some areas whereas other are dominated by finer material [1].<span class="Apple-converted-space">  </span>Several mechanisms have been proposed to explain this size sorting, among them the so called ballistic sorting effect (BSE) [2]. The BSE depends on impacting particles rebounding more elastically from large targets (boulders) than when hitting a bed of fine grains. This mechanism of course not only applies to the primary impactor hitting an asteroid surface but also to secondary impacts from material ejected by the primary impact.<span class="Apple-converted-space"> </span>In order to fully understand the BSE on asteroids, it is therefore important to quantify the mass- and velocity distribution of ejecta generated by impacts into asteroid surfaces. In particular from impacts with low velocities that will then generate ejecta that is slower than the escape velocity of the asteroid.</p> <p>To conduct realistic experiments under asteroid conditions we use the ERICA (Experiments on Rebounding Impacts and Charging on Asteroids) platform [3] under the microgravity provided by the ZARM Bremen drop tower and the new GTB-Pro, also at ZARM. To provide a low but directed gravity level, i.e. asteroid gravity, ERICA consists of a vacuum chamber that contains the sample material which is mounted to a linear stage that - once the whole setup is in microgravity - provides a linear acceleration to simulate asteroid-level gravity. Using the linear stage eliminates any Coriolis forces a centrifuge would create. Due to the low g-jitter and the long microgravity time of 9.2 s in the drop tower and 2.5 s in the GTB-Pro these experiments are able to focus on ultra low velocity impacts in the cm/s rage, complementing earlier experiments by Brisset et al. [4].</p> <p>In the sample chamber we place granular beds (regolith analog) of various sizes and a launcher mechanism that impacts a basaltic projectile at the simulated asteroid surface. Using a stereo pair of cameras, as well as a high speed camera we then record the ejecta plume created by the impact. From the resulting image data we extract ejecta velocities using particle tracking (for larger ejecta particles) and digital image correlation (for smaller ejecta).<span class="Apple-converted-space"> </span></p> <p><span class="Apple-converted-space"><img src="" alt="" width="503" height="452" /></span></p> <p><em>Fig.1. Ejecta Plume generated by impacting a basaltic projectile (v = 65 cm/s)<span class="Apple-converted-space">  </span>in a bed of 0-300 μm sized particles at a gravity level of 2 • 10<sup>-4</sup> m/s^2</em></p> <p> </p> <p>[1] A. Fujiwara, J. Kawaguchi, D.K. Yeomans, M. Abe, T. Mukai, T. Okada, J. Saito, H. Yano, M. Yoshikawa, D.J. Scheeres et al., Science 312,1330 (2006)<span class="Apple-converted-space"> </span></p> <p>[2] T. Shinbrot, T. Sabuwala, T. Siu, M.V. Lazo,P. Chakraborty, Phys. Rev. Lett.118 (2017)<span class="Apple-converted-space"> </span></p> <p>[3] K. Joeris, L. Schönau, L. Schmidt, M. Keulen, V. De-sai, P. Born, J. Kollmer, EPJ Web of Conferences 249, 13003 (2021)</p> <p>[4] J. Brisset, C. Cox, S. Anderson, J. Hatchitt, A. Madison, M. Mendonca, A. Partida, D. Remie, Astron. Astrophys (2020)</p> <p> </p>

<p>As seen by the Hayabusa spacecraft the regolith on the surface of asteroid Itokawa shows strong segregation by particle size [1].</p> <p> </p> <p>Approaches to explain this segregation are either focused on the bulk material, like the Brazil Nut Effect, as obeserved in agitated granular media [2, 3]. Or the explanations stems from the idea of impact driven segregation, as proposed by Sinbrot with the Ballistic Sorting Effect [4]. Both kinds of effect may even contribute concurrently [5].</p> <p> </p> <p>We designed an experiment to investigate impact driven segregation. To experimentally recreate the surface of an asteroid [6], we utilize the microgravity of the Bremen drop tower. Inside the 10^-6 m/s² microgravity environment in the drop tower capsule, we use a linear stage to accellerate a vacuum chamber containing a granular bed (e.g. our asteroid surface) at a constant acceleration of 2*10^-4m/s². A launcher mechanism then hauls a basalt impactor onto this surface. The outcome of the impact is tracked using three cameras, enabling us to determine the coefficient of restitution (COR), defined as the ratio of the impactor's absolute velocities before and after the impact.</p> <p> </p> <p>This ratio is a good measurement for the energy disspiation happening during the impact and therefore the relative mobility of the impactor after rebounding. The COR measured in our experiments shows an interesting behaviour, especially for finely powdered beds. While the BSE proposed by Shinbrot only requires a decline of the COR with rising bed particle size, we observe an increased COR for very fine particles as well. Using numerical simulations we find this effect to be caused by inter-partilce cohesion. In detail, we show that without cohesion no such non-monotonic behavior is possible.</p> <p>From our experimental and numerical results we conclude that in a low gravity environment like asteroids cohesion is important for the size-depended COR and furthermore that its non-monotonic behavior should enhance size segregation for certain particle sizes.</p> <p> </p> <p>[1] A. Fujiwara, J. Kawaguchi, D.K. Yeomans, M. Abe, T. Mukai, T. Okada, J. Saito, H. Yano, M.Yoshikawa, D.J. Scheeres et al., Science 312, 1330 (2006)</p> <p>[2] S. Matsumura, D.C. Richardson, P. Michel, S.R. Schwartz, R.L. Ballouz, Mon. Not. R. Astron. Soc. 443, 3368 (2014)</p> <p>[3] C. Maurel, R.L. Ballouz, D.C. Richardson, P. Michel, S.R. Schwartz, Mon. Not. R. Astron. Soc. 464, 2866 (2016)</p> <p>[4] T. Shinbrot, T. Sabuwala, T. Siu, M.V. Lazo, P. Chakraborty, Phys. Rev. Lett. 118 (2017)</p> <p>[5] E. Wright, A.C. Quillen, J. South, R.C. Nelson, P. Sanchez, J. Siu, H. Askari, M. Nakajima, S.R. Schwartz, Icarus 351 (2020)</p> <p>[6] K. Joeris, L. Schönau, L. Schmidt, M. Keulen, V. Desai, P. Born, J. Kollmer, EPJ Web of Conferences 249, 13003 (2021)</p>

<p class="p1"><span class="s1">The<span class="Apple-converted-space">  </span>surfaces<span class="Apple-converted-space">  </span>of<span class="Apple-converted-space">  </span>rubble-pile<span class="Apple-converted-space">  </span>asteroids<span class="Apple-converted-space">  </span>are<span class="Apple-converted-space">  </span>covered<span class="Apple-converted-space">  </span>in<span class="Apple-converted-space">  </span>regolith of a variety of sizes.<span class="Apple-converted-space">  </span>In some cases like for the asteroid Itokawa, the size distribution of regolith is not uniform across the surface [1]. Some areas are dominated by finer grains, while other areas are covered by larger rocks.<span class="Apple-converted-space">  </span>There are a number of competing explanations for this observed size segregation [2–4]. One approach is the so called ballistic-sorting-effect [2], where impacting particles sort themselves through different rebound behavior.</span></p> <p class="p1"><span class="s1">In our work we want to set practical limits on the role ballistic sorting can play in shaping an asteroids surface. To this end we conduct a series of drop tower experiments examining the impact kinetics of slow (cm/s)<span class="Apple-converted-space">  </span>3 mm sized projectiles into a regolith surface under conditions realistic for asteroid surfaces, i.e. vacuum and low gravity. We track the impactor with high-speed cameras and determine its velocity in 3 dimensions before and after the impact. From these velocities, we can then compute a coefficient of restitution (COR).<span class="Apple-converted-space">  </span>We then repeat the experiment for surfaces composed of differently sized material.<span class="Apple-converted-space">  </span>We find that for a regolith bed made from particles of similar size as the impactor we get a lower COR (0,1) than for beds made up of significantly larger (0,5) or smaller particles (0,8). The more elastic collisions for larger sized targets follows from conservation of momentum. For the finer material we suggest that the higher COR is a function of interparticle adhesion.</span></p> <p class="p1"><span class="s1">[1] A. Fujiwara, J. Kawaguchi, D.K. Yeomans, M. Abe, T. Mukai, T. Okada, J. Saito, H. Yano, M. Yoshikawa, D.J. Scheeres et al., Science 312, 1330 (2006)</span></p> <p class="p1"><span class="s1">[2] T. Shinbrot, T. Sabuwala, T. Siu, M.V. Lazo, P. Chakraborty, Phys. Rev. Lett. 118, 111101 (2017)</span></p> <p class="p1"><span class="s1">[3] S. Matsumura, D.C. Richardson, P. Michel, S.R. Schwartz, R.L. Ballouz, Mon. Not. the R. Astron. Soc. 443, 3368 (2014)</span></p> <p class="p1"><span class="s1">[4] A.J. Dombard, O.S. Barnouin, L.M. Prockter, P.C. Thomas, Icarus 210, 713 (2010)</span></p>