Low-speed oblique impacts under low gravity with implications of asteroid surfaces and spacecraft landings

Recent exploration missions to asteroids have demonstrated that low-energy material mobility occur in asteroid environments. Low energy in this case may be defined as the energy level below that of the escape speed of the asteroid, allowing take off from the surface, orbital motion and re-impact. The episodes of ejection and reimpact of particles observed on Bennu by OSIRIS-REx mission [1], the evidence of material transport between binary companions in Didymos system [2] and potential sesquinary impacts following the DART impact on Dimorphos [3] suggest that such activity may be more common than previously thought, shaping asteroid surfaces incessantly. This would complicate the interpretations of asteroid surface evolution. It was indeed previously suggested that small craters of Bennu may have formed as a result of low-speed impact cratering through a study of low-energy impacts with respect to surface normal [4]. However, most re-impacting material would impact the surface at an angle due to the uncontrolled initial ejection conditions and irregular surfaces of asteroids.

This study therefore investigates the low-energy oblique impact cratering under low gravity. The impacts are simulated in a discrete element method code GRAINS, which can handle the contacts between non-spherical particles with a non-smooth contact model [5]. A gravity level (~9.81 x 10^-4 m/s²) representative to asteroid Didymos is selected. The granular bed is prepared with polygonal particles sized 2 to 7 cm equivalent radii and densities (3.3 g/cm³) similar to Didymos. The impact of a 20-cm diameter spherical impactor at the same density as the particle is then simulated on a granular bed with impact speeds of 10 and 20 cm/s and at angles between 30 and 90 degrees from local horizontal. Normal impacts are simulated for comparison with the authors’ previous work [4,6]. Qualitatively, the results show shallow non-circular craters and asymmetric ejecta field except of normal impacts. The impactor remains near the crater wall in the direction of lateral motion, which may be used as an indication of a low-speed impact structure in asteroids. The craters are quantitatively analysed with crater-scaling relationships [7] to extract scaling coefficients by including the non-circularity of the craters via the impact angle as a parameter the relationships. The presented results will have direct implications to understand low-speed activity on asteroids. It will particularly be relevant to identify sesquinary impacts on Didymos and to reveal the surface mechanical properties as observed by the Hera mission. The implications of the results can also be used to predict the outcomes of ballistic CubeSat landings during Hera missions, which will occur at a similar energy level, offering a further opportunity to understand small body surfaces.

References

[1] D. 1047 Lauretta, C. Hergenrother, S. Chesley, J. Leonard, J. Pelgrift, C. Adam, M. Al Asad, P. Antreasian, R.-L. Ballouz, K. Becker, et al., 2019. Episodes of particle ejection from the surface of the active asteroid (101955) Bennu, Science 366 (6470), doi:10.1126/science.aay3544.

[2] Sunshine, J.M., Rizos, J.L., Barnouin, O.S., Daly, R.T., Ernst, C.M., Farnham, T.L., Agrusa, H.F., Wright, E., Wiggins, S.E., Bruck Syal, M. and Stickle, A.M., 2026. Evidence of Recent Material Transport within a Binary Asteroid System. The Planetary Science Journal, 7(3), p.56.

[3] Langner, K., Marzari, F., Rossi, A., Zanotti, G., 2024. Long-term dynamics around theDidymos–Dimorphos binary asteroid of boulders ejected after the DART impact. Astron. Astrophys. 684, A151. http://dx.doi.org/10.1051/0004-6361/202348675.

[4] Çelik, O., Ballouz, R.L., Scheeres, D.J. and Kawakatsu, Y., 2026. Material Dependency in the Scaling of Low-Speed Craters under Microgravity, Icarus, 450, 116981.

[5] Ferrari, F., Lavagna, M. and Blazquez, E., 2020. A parallel-GPU code for asteroid aggregation problems with angular particles. Monthly notices of the Royal Astronomical society, 492(1), pp.749-761.

[6] Çelik, O., Ballouz, R.L., Scheeres, D.J. and Kawakatsu, Y., 2022. A numerical simulation approach to the crater-scaling relationships in low-speed impacts under microgravity. Icarus, 377, 114882. doi: 10.1016/j.icarus.2022.114882

[7] Holsapple, K.A., 1993. The scaling of impact processes in planetary sciences. Annu.Rev. Earth Planet. Sci. 21, 333–373.