Particle shape and size predict asteroid strength, revealing Bennu's surface is 50 times weaker than ground coffee

A new study in *Nature Communications* reveals that the shape and size of particles in granular asteroids determine their tensile strength. Researchers used simulations to model how cohesive forces between grains affect the stability of asteroid surfaces. The study found that smaller, irregular particles can create stronger bridges when packed together, and that the strength of an asteroid depends on both grain size and shape. The framework allows predicting surface strength based on measurements of particle size and sphericity.

The study used simulations with a matrix of cohesive grains between two boulders, measuring the tensile strength by pulling the boulders apart until the bridge breaks. The team ran 78 simulations, using different grain shapes and sizes, and found that smaller grains produced stronger bridges. The framework is general: either measurement alone—size or sphericity—is enough to predict the strength. Applied to Bennu, the framework put the surface strength below 1 pascal, matching independent estimates from remote sensing and sampling.

The cause is a shortage of fine dust. "If there are not enough of the small grains to fill in the voids between the larger grains, there are not enough cohesive contacts to strengthen the structure of the asteroid," said Sánchez. The study highlights the importance of particle size and shape in determining asteroid strength, and suggests that future work should extend the framework to wider ranges of grain sizes and asteroid interiors.

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