Ronald-Louis Ballouz

Observations of near-Earth asteroids (NEAs) suggest that these objects are nearly strengthless rubble piles, held together by microgravity and small interparticle cohesive (van der Waals) forces. These forces could stabilize a rubble pile’s surface and global shape, explaining the existence of fast-spinning asteroids whose centrifugal accelerations would otherwise cause them to rotationally disrupt. Evaluating this theoretical explanation requires experimental measurements of particle-particle cohesion with pristine astromaterials that have not been subjected to alteration by Earth’s atmosphere. Here we show, via nanoscale atomic force microscopy, that micron-scale regolith particles returned by spacecraft from the carbonaceous asteroid (101955) Bennu exert a small average interparticle cohesive force of ~1.25 nN, which corresponds to a tensile strength of 0.001 Pa. These measured cohesive forces are a factor of a few smaller than values obtained for carbonaceous meteorites, and two orders of magnitude smaller than for stony ordinary chondrite and achondrite meteorites. We argue that Bennu’s small interparticle cohesive forces led to the paucity of millimeter-scale particles observed on its surface. This low cohesive strength also implies that carbonaceous NEAs are weakly resistant to rotational disruption, which may explain the observed scarcity of NEAs in proximity to the Sun and suggests that fast-spinning NEAs are not rubble piles.

Granular asteroids exhibit surprising structural stability despite low gravity and high spin rates. Understanding the mechanisms behind their strength is essential for predicting their evolution and informing deflection strategies. Using 3D particle-dynamics simulations, we develop a scaling framework for the tensile strength of cohesive, self-gravitating regolith, examining how particle size and shape jointly control macroscopic behavior. We show that tensile strength can be scaled using either an equivalent particle diameter or particle shape, the latter through sphericity and its reference to an equivalent spherical packing. We apply this model to asteroid Bennu using data from samples returned by the OSIRIS-REx mission and derive a cohesive surface strength consistent with estimates below 1 Pa from remote sensing and sampling-event reconstructions. Our results indicate that Bennu’s extremely low strength arises from a scarcity of fine dust in its regolith. This study demonstrates how particle-level properties govern large-scale stability and heterogeneity in granular asteroids.

The shape of boulders on planetary surfaces can be used to investigate the geological processes that influence them and to estimate key surface mechanical properties. Such studies are typically based on 2D images, although 2D projections of irregular particles are known to differ from their 3D geometry. To investigate how well 2D images represent 3D morphological parameters, we assess to compare particle shapes in both representations (2D and 3D). Three different granular samples were investigated (LHS-1 lunar highland simulant, Øysand sand, and glass grit) using 3D X-ray computed tomography (3DXRCT) to reconstruct 3D particle geometries within the samples. More than 2700 particles are analyzed using both their full 3D shapes and different 2D projections derived from the 3D reconstructions. Using correlations between 2D and 3D morphological analyses of the 3DXRCT scan data set, we establish statistical relationships, with associated confidence intervals, between the two representations. The outcome is a methodology for estimating 3D size and shape parameters from 2D measurements. We apply this approach to boulders observed on asteroid Bennu by the NASA OSIRIS-REx mission. The resulting 3D size and shape parameters, estimated from 2D images, closely match the true 3D parameters derived from LIDAR data. We conclude with a set of recommendations for performing reliable morphological analyses of boulders and particles in 2D planetary images, including considerations of image resolution and sample size.

Due to the extremely low gravity of small near-Earth asteroids (NEAs), it has been assumed that impact-generated rock fragments escape into space and thus do not contribute to the accumulation of regolith. However, centimeter-sized stones returned from the small NEA Bennu by NASA’s OSIRIS-REx mission exhibit impact craters up to a few millimeters wide, implying that impact fragments and impact-processed rocks are retained despite the microgravity environment. To understand how, we combined detailed physical analysis of Bennu samples, laboratory experiments of impacts into simulant rocks, and 3D numerical simulations of disruptive impacts into boulders. We find that the majority (∼85% by mass) of impact fragments eject toward and penetrate the asteroid’s weak, porous surface, leading to their retention. In addition, crater depth-to-diameter ratios ( d/D ) suggest that the Bennu samples (median crater d/D = 0.36 ± 0.1) are structurally representative of the asteroid’s large boulders (median crater d/D = 0.33 ± 0.08, measured previously). Our analyses indicate that most of Bennu’s surface rocks (those with diameters ≲20 m) could be products of in situ collisional disruption. This impact-driven mechanism of regolith production likely occurs on other small NEAs with highly porous surfaces.

Thermal inertia is used to infer physical properties of asteroid surfaces. The carbonaceous asteroid Bennu has low thermal inertia suggestive of a surface covered in sub-centimeter rock fragments. However, spacecraft observations revealed that Bennu is instead blanketed by boulders of differing physical properties, with the most abundant population displaying very low thermal inertia compared to carbonaceous chondritic meteorites. Here we show that morphologically distinct particles in samples returned from Bennu also possess distinct thermal and physical properties, consistent with their genetic connection to the boulders. Angular particles have higher thermal inertia, greater hardness, and fewer but longer cracks that lead to more efficient splitting, relative to the hummocky particles. A hummocky particle exhibits low thermal inertia at sub-millimeter scales due to fine pores. Tortuous crack networks in hummocky particles further reduce thermal inertia while resisting disaggregation. Samples from Ryugu, a carbonaceous asteroid with similarly low thermal inertia, have cracks like those in Bennu’s hummocky particles yet have bulk densities that indicate lower porosity. These observations imply that the low thermal inertia of both asteroids is driven by cracks in rocks resulting from geological processes within the parent body or, more recently, micrometeoroid impacts and thermal fatigue.

Planetary rings provide natural laboratories for studying the fundamental processes that govern the evolution of planetary systems. However, several key features, such as the sharp inner edges of Saturn’s rings, remain unresolved. In this Letter, we introduce and quantify the eclipse–Yarkovsky (EY) effect, a thermal torque arising from asymmetric thermal emission of particles during planetary eclipses, which is effective for particles larger than millimeters in size. We formulate this effect within a continuum framework appropriate for collisionally coupled planetary rings and derive the continuum evolution equation that includes the EY torque and viscous diffusion (Equation (26)), constraining its magnitude using ring particle spin distributions obtained from N -body simulations. We find that the EY effect systematically produces a positive angular momentum flux that could overcome the viscous torque, driving ring material outward and leading to long-term decretion. The total EY torque principally depends on the optical depth, in which we identify three dynamical regimes: dense, transitional, and tenuous, each exhibiting distinct evolutionary pathways. In the dense and transitional regimes, the EY torque can produce a sharp inner edge such as that of Saturn’s A ring. In the tenuous regime, it can drive an entire ring outward while preserving shape. This outward transport may also facilitate satellite formation beyond the Roche limit. We also quantitatively show that planetary thermal radiation exerts an opposing torque, namely the planetary–Yarkovsky effect, whose importance depends on planetary emissivity and ring-particle albedo, and may lead to inward transport in Saturn’s close-in rings.

The 2029 close approach between asteroid 99942 Apophis and Earth represents a unique opportunity to study the influence of tidal encounters on the deformation and rotational evolution of an S-complex near-Earth object in the 0.1–1 km size range—a class of object of particular interest to the planetary defense community. The encounter may give insight into the internal structure of the body, presenting an opportunity for our first direct probe of an asteroid’s interior. We present results from numerical simulations of the Apophis–Earth tidal encounter, modeling Apophis as a self-gravitating granular aggregate of meter-sized spherical particles, with the aim of constraining measurable rotation change, deformation, and seismicity induced in Apophis during the encounter, in support of potential rendezvous missions. We vary particle size distributions, particle shapes, packing configurations, and interior structures (with large embedded cores) to examine the effects that internal structure has on these measurable encounter outcomes. We find that the packing configuration with the lowest shear strength exhibits the most deformation by a factor of 4 over the most compact configuration, and the presence of internal cores in a low shear strength configuration can reduce the stress felt by the asteroid. We also find that short-period seismic events are likely to occur in the shallow subsurface layers of the body in the hours just following the time of perigee. These events have amplitudes on the order of 10 ⁻⁴ m s ⁻² Hz ^−1/2 in the 0.1–1 Hz range, which should be measurable with the latest seismic instruments developed for asteroid interior characterization.

Granular asteroids can exhibit unexpected structural stability despite their low gravity and rapid spin rates. Understanding the physical mechanisms underlying their strength is crucial for predicting asteroid evolution and designing deflection strategies. Here, using three-dimensional particle dynamics simulations, we develop a scaling framework for the tensile strength of cohesive, self-gravitating regolith matrices, focusing on the combined effects of particle size and shape. Surprisingly, we find that the tensile strength of granular systems can be effectively described by a single parameter—either equivalent diameter or sphericity—despite the geometric complexity of the constituent particles. We then apply our model to the granular asteroid Bennu by using physical data from regolith samples returned by the OSIRIS-REx mission. We find a cohesive surface strength that is consistent with estimates (< 1 Pa) based on remote sensing and reconstruction of the sampling event. Our analysis suggests that a scarcity of dust in Bennu’s regolith drives its low surface strength. This work provides broad insights into how particle-level properties influence macroscopic stability, shedding light on the structural integrity and heterogeneity of granular asteroids.