The strength of regolith and rubble pile asteroids

Sánchez, Paul; Scheeres, Daniel J.

doi:10.1111/maps.12293

Cited by 217 publications

(191 citation statements)

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“…In particular, lunar regolith has micron to centimeter sized grains described by an approximate d -3 size distribution 6,19 . Rubble pile asteroid (25143) Itokawa also has a d -3 grain size distribution but has boulders ranging up to ~40 m in size on the surface 20 , which is reflected in its much higher thermal inertia value of ~750 J m -2 K -1 s -1/2 (ref.…”

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confidence: 99%

“…To check whether internal cohesive forces are also required to prevent structural failure of 1950 DA, we applied the Drucker-Prager model for determining the failure stresses within a geological material 4,6 (Methods). In this model, the maximum spin-rate a rubble pile asteroid can adopt depends on its overall shape, degree of self-gravity, and internal strength.…”

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Cohesive forces prevent the rotational breakup of rubble-pile asteroid (29075) 1950 DA

Rozitis

MacLennan

Emery

2014

Nature

173

137

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Space missions 1 and ground-based observations 2 have shown that some asteroids are loose collections of rubble rather than solid bodies. The physical behavior of such 'rubble pile' asteroids has been traditionally described using only gravitational and frictional forces within a granular material 3 . Cohesive forces in the form of small van der Waals forces between constituent grains have been recently predicted to be important for small rubble piles (10-kilometer-sized or smaller), and can potentially explain fast rotation rates in the small asteroid population 4-6 . Hitherto, the strongest evidence came from an analysis of the rotational breakup of main belt comet P/2013 R3 (ref. 7), although that was indirect and poorly constrained by present observations. Here we report that the kilometer-sized asteroid (29075) 1950 DA 8 is a rubble pile that is rotating faster than that allowed by gravity and friction. We find that cohesive forces are required to prevent surface mass shedding and structural failure, and that the strength of the forces are comparable to, though somewhat less than, that of lunar regolith.It is possible to infer the existence of cohesive forces within an asteroid by determining if it is a rubble pile with insufficient self-gravity to prevent rotational breakup by centrifugal forces. One of the largest known candidates is near-Earth asteroid 1950 DA (mean diameter of 1.3 km; ref.8), as it has a rotation period of 2.1216 hr that is just beyond the critical spin limit of ~2.2 hr estimated for a cohesionless asteroid 9 . A rubble pile structure and the degree of self-gravity can be determined by a bulk density measurement, which can be acquired through model-tomeasurement comparisons of Yarkovsky orbital drift 10 . This drift arises on a rotating asteroid with non-zero thermal inertia, and is caused by the delayed thermal emission of absorbed sunlight, which applies a small propulsion force to the asteroid's afternoon side. Thermalinfrared observations can constrain the thermal inertia value 11 , and precise astrometric position measurements conducted over several years can constrain the degree of Yarkovsky orbital drift 2 . Recently, the orbital semimajor axis of 1950 DA has been observed to be decreasing at a rate of 44.1 ± 8.5 m yr -1 because of the Yarkovsky effect 12 , which indicates that the asteroid's sense of rotation must be retrograde. Using the Advanced Thermophysical Model 13,14 , in combination with the retrograde radar shape model 8 , archival WISE thermal-infrared data 15 (Extended Data Table 1, and Extended Data Figs 1 and 2), and orbital state 12 , we determine the thermal inertia and bulk density of 1950 DA (Methods). The thermal inertia value is found to be remarkably low at 24 +20 / -14 J m -2 K -1 s -1/2 , which gives a corresponding bulk density of 1.7 ± 0.7 g cm -3 ( Fig. 1 and Extended Data Fig. 3). This bulk density is much lower than the minimum value of 3.5 g cm -3 required to prevent loss of surface material by centrifugal forces (Fig. 2).Spectral observations of 1950...

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confidence: 99%

mentioning

confidence: 99%

Cohesive forces prevent the rotational breakup of rubble-pile asteroid (29075) 1950 DA

Rozitis

MacLennan

Emery

2014

Nature

173

137

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“…The boulders are pulled in opposite directions by a force increasingly stronger than their mutual gravitational attraction to measure the tensile strength of the bridge. This study [17] showed that the tensile strength of a cohesive aggregate is inversely proportional to the average particle size of the particles forming it (porosity, Hamaker constant, coordination number and others are considered constants). Basically, the smaller dust and regolith in an asteroid could be seen as a weak cement, a matrix in which the larger boulders are embedded.…”

Section: Granular Asteroids and Yorpmentioning

confidence: 99%

“…Time evolution of two selfgravitating granular bridge: (top) cohesionless and (bottom) cohesive. The latter needed a pulling force about 40 times greater than the one on top to fail catastrophically [17].…”

Section: Granular Asteroids and Yorpmentioning

confidence: 99%

Looking into the evolution of granular asteroids in the Solar System

et al. 2017

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Abstract. By now it has been accepted that most of the small asteroids in the Solar System are granular aggregates kept together by gravitational and possibly, cohesive forces. These aggregates can form, deform and disrupt over millennia subjected to different internal and external factors that would ultimately determine how they evolve over time. Parameters such as porosity, cohesive and tensile strength, angles of friction, particle size distributions, stress states, heterogeneity and yield criteria among others, determine how these granular systems will react when subjected to different, changing, external factors. These external factors include solar photon momentum, gravitational tides, micro-and macro-impacts and are believed to have produced and shaped the current asteroid population. In our research we use a combination of Soil Mechanics theory, Soft-Sphere Discrete Element Method (SSDEM) Simulations and Orbital Mechanics in order to understand how simulated, homogeneous and heterogeneous, ellipsoidal and spherical gravitational aggregates, a crude but useful representation of an asteroid, evolve when rotated to the point of disruption. Then, we compare our results to the shapes of observed asteroids as well as to the disruption patterns of a few active asteroids. Our results lead us to believe that the different shapes of observed asteroids as well as their unique disruption patterns could give us clues about their internal structure, strength and geophysical properties in general. Granular Asteroids and YORPIn the last decade, the study of asteroids as granular aggregates has gained importance. Even though it was originally assumed that the smallest of asteroids were monolithic rocks with a bare surface, the pictures obtained by NASA Galileo, Near-Showmaker (or just NEAR) and JAXA Hayabusa space missions revealed a substantial layer of unconsolidated rocky material and dust (regolith) covering the surface of (951) imply that not only asteroid surfaces are covered by regolith, but that their internal structure is not monolithic. This is what we define as a granular asteroid, a naturally occurring self-gravitating granular system.Of the many events that can disrupt a granular, nonmonolithic asteroid, here we will focus only on the disruption that could be caused by rotation. The angular velocity of this rotation, in the context of NEOs, can be accelerated by the Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect which results from an interplay between solar radiation pressure and the shape of an asteroid [5,6]. Very succinctly, when the photons that come from the Sun ime-mail: diego.sanchez-lana@colorado.edu e-mail: dscheeres@colorado.edu e-mail: thirabayashi@purdue.edu e-mail: simon.tardivel@colorado.edu pact on the surface of an asteroid, they will be reflected, absorbed and re-emitted from it anisotropically due to its irregular shape. This will produce a net torque that affects the angular velocity of the asteroid. Over millennia, this change in the angular velocity will produce the deformation or ...

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“…Note that the actual value is actually closer to 45 µN/W due to the fact that the geometrical losses are in fact closer to 13% than 30% for the range of possible combinations between power and beam diameter covered in this specific example. Sánchez and Scheeres (2014) In order to put the surface velocity requirement into context, one can look at the asteroid spinning rate distribution in Figure 20. For asteroids larger than 1km, the spin limit is usually dominated by the gravity stresses and equates…”

Section: Constraints On the Laser Systemmentioning

confidence: 99%

Theoretical peak performance and optical constraints for the deflection of an S-type asteroid with a continuous wave laser

Thiry

Vasile

2017

Advances in Space Research

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This paper presents a theoretical model to evaluate the thrust generated by a continuous wave (CW) laser, operating at moderate intensity(<100GW/m 2 ), ablating an S-type asteroid made of Forsterite. The key metric to assess the performance of the laser system is the thrust coupling coefficient which is given by the ratio between thrust and associated optical power. Three different models are developed in the paper: a one dimensional steady state model, a full 3D steady state model and a one dimensional model accounting for transient effects resulting from the tumbling motion of the asteroid. The results obtained with these models are used to derive key requirements and constraints on the laser system that allow approaching the ideal performance in a realistic case.

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The strength of regolith and rubble pile asteroids

Cited by 217 publications

References 48 publications

Cohesive forces prevent the rotational breakup of rubble-pile asteroid (29075) 1950 DA

Cohesive forces prevent the rotational breakup of rubble-pile asteroid (29075) 1950 DA

Looking into the evolution of granular asteroids in the Solar System

Theoretical peak performance and optical constraints for the deflection of an S-type asteroid with a continuous wave laser

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