Simulations of Regolith Interactions in Microgravity Paul Sanchez and Daniel J. Scheeres, Col-orado Center for Astrodynamics Research, University of Colorado, Boulder, CO 80309-431 (diego.sanchez-lana@colorado.edu), E. Beau Bierhaus and Benton Clark, Lockheed Martin, Waterton Campus, Colorado

Interactions between a rigid sampling device andregolith settled in a micro-gravity environment areshown. The results are relevant for the OSIRIS-RExmission.

Introduction:NASA’s OSIRIS-REx mission will travel to the car-

bonaceous asteroid 1999 RQ36, acquire a sample ofregolith from its surface and return the sample to Earthin September 2023. A crucial aspect of the mission oc-curs when the spacecraft descends to touch and samplethe surface of the asteroid. One region of the aster-oid where the sampling may occur is in the equato-rial region of the body. This region is the geopotentiallow over the entire asteroid, and has a total surface ac-celeration (gravity minus centripetal) on the order ofmicro-Gs. The physical interaction between a space-craft sampling device and the regolith resident in thisenvironment occurs in a realm of physics that cannotbe reconstructed on Earth, and which would be verydifficult to reconstruct on-orbit. Thus the prime man-ner to evaluate the possible outcomes of these interac-tions is through direct, first-principles simulations of asampling head contacting a regolith in a micro-gravityregime.

This abstract reports the formulation and results ofa suite of simulations to model the interaction be-tween a sampling device and a micro-gravity regolithbed. These simulations were performed to provide in-sight into one possible regime of interaction for theOSIRIS-REx mission’s surface sampling. Results ofthese interactions show that low-speed contacts be-tween a sampling head and the regolith can introducea fluid-like response in the regolith. The forces arefound to act similar to an aerodynamic drag, and thusincreasing the contact speed can supply greater resis-tance from the regolith. In contrast, settling the grainsin a higher gravity field also increases the resistance ofthe regolith on the sampling mechanism. Understand-ing these interactions is important, as the forces be-tween regolith and a sampling device may play a rolein designing and predicting the sampling phase of anyfuture missions to asteroids in similar environments.

Simulation Method:The simulation program that is used for this re-

search applies a Soft-Sphere Discrete Element Method[1, 2, 3, 4] to simulate interactions between a rigidsampling head descending to the surface and the re-golith granular aggregates on the surface. The regolithparticles are modeled as spheres that follow a prede-termined, but randomized, size distribution and inter-

Figure 1: Time evolution of a 15 cm/s contact. The snap-shots correspond to t = 0.02, 1.0 and 2.0 s and show a cross-sectional cut of the granular bed. The sampling head hasbeen made semi-transparent.

act through a soft-repulsive potential when in contact.This method considers that two particles are in con-tact when they overlap. For each particle-particle con-tact, the code calculates normal and tangential contactforces as described in [5, 1]. Interactions between par-ticles and the rigid sampler head are handled followingthe same methodology.

Regolith and Sampling Head Models:We assume that the gravitational field is constant

and independent of the particle positions, as our studyfocuses only on a small volume located on the surfaceon the asteroid. Two sets of simulations were carriedout, one used a monodisperse-continuous size distribu-tion (10mm, 20% dispersion); the other used a polydis-perse, 1/D size distribution (5-22 mm). The samplinghead was modeled as being combined with a space-craft with a total mass of 1300 kg. The sampling headwas modeled as a right cylinder with diameter 32 cmand an inner “sampling” chamber cut symmetrically inthe bottom with a diameter of 21 cm and a depth of 3.5cm (see Fig. 1).

Results:Figure 1 shows the time evolution of a 15 cm/s con-

tact at 0.02, 1.0 and 2.0 seconds after initial contact. Aclose examination of the images and the velocity fieldsgenerated by the particles will show that the particlesin the first layers of the granular bed, and a short radialdistance beyond the edge of the sample head, can ac-quire vertical speed of the same order of magnitude ofthe contact speed. The simulation also shows that afterthe initial contact the regolith trapped in the chamberwill remain there for the entire simulation.

Figure 2 shows the time evolution of the net verti-cal force on the sampler at initial speeds of 10 cm/sand 15 cm/s for the monodisperse case. For both casesthe initial peak occurs when the outer annulus contacts

2271.pdf44th Lunar and Planetary Science Conference (2013)

Figure 2: Vertical force on the sampler head vs. time forcontact speeds of 10 and 15 cm/s into a monodisperse re-golith.

the regolith, and the second peak occurs when the in-ner chamber contacts the regolith. Following thesepeaks the force is approximately constant for a fewseconds until the sampler head starts to “feel” the bot-tom of the simulation chamber (see Fig. 1). The peaksin force are approximately proportional to the contactspeed, implying that the force is due to momentumtransport from the sampler head to the regolith. Theforce magnitude following the peaks is approximatelyproportional to the square of the speed of the samplerhead, implying that the force experienced by the sam-pler head is similar to a fluid drag acting on the body.

Figures 3 show the time evolution of the net verticalforce at an initial speed of 10 cm/s into a monodisperse(top) and polydisperse (bottom) regolith. Note that theforce variation is smoother for the polydisperse caseas compared to the monodisperse case, implying thata range of grain sizes acts to distribute the force loadsmore evenly throughout the regolith.

Implications:These simulations predict that for regolith settled in

a micro-gravity environment, a low speed interactionwith a rigid sampling head may behave more like afluid interaction than as a rigid material interaction.This is an unexpected result of the simulation, butfollows directly from the first-principles calculations.The model developed here is but one extreme that maybe encountered at asteroids, and continued refinementswill clarify the role of cohesion, friction, and coeffi-cients of restitution.

References: [1] P. Sanchez, et al. (2011) TheAstrophysical Journal 727(2):120. [2] P. Cundall(1971) in Proceedings of the International Symposiumon Rock Mechanics vol. 1 129–136 -, Nancy. [3] P. A.Cundall, et al. (1992) Engineering Computations9(2):101 http://dx.doi.org/10.1108/eb023851 doi.

Figure 3: Vertical force on the sampler head vs. time for ancontact on a monodisperse (top) and polydisperse (bottom)sample for a 10 cm/s contact.

[4] M. P. Allen, et al. (1989) Computer Simulation ofLiquids Oxford science publications OxfordUniversity Press, USA, New York ISBN 0198556454.[5] H. Herrmann, et al. (1998) Continuum Mechanicsand Thermodynamics 10:189 ISSN 0935-117510.1007/s001610050089. [6] D. Scheeres, et al.(2010) Icarus 210(2):968 ISSN 0019-1035http://dx.doi.org/DOI: 10.1016/j.icarus.2010.07.009doi. [7] L. E. Silbert, et al. (2001) Phys Rev E64(5):051302http://dx.doi.org/10.1103/PhysRevE.64.051302 doi.

Acknowledgements:The research reported here was supported by a grant

from Lockheed-Martin to the University of ColoradoBoulder.

2271.pdf44th Lunar and Planetary Science Conference (2013)