In a new Nature Communications study, researchers developed a universal scaling framework for the strength of granular asteroids, showing that their tensile strength can be predicted from the size and shape of their constituent particles.
Space missions have revealed that many small asteroids, such as Bennu, Ryugu and Itokawa, are not solid rocks but granular asteroids: loosely bound collections of dust, rock and boulders held together by their own gravity and weak cohesive forces.
Researchers have studied how these bodies are held together for more than a decade. However, previous simulations could only treat their fine grains as perfect spheres, while real grains are angular and irregular. The current study found a way to incorporate realistic particle shapes into the simulations.
Phys.org spoke to first author Paul Sánchez, a senior research associate at the University of Colorado Boulder. “The original idea was suggested by Dr. Michael R. Swift (University of Nottingham, U.K.), who was my Ph.D. supervisor and came to Boulder for a visit,” said Sánchez.
“We needed to find the effect that small cohesive particles could have in an asteroid, but simulating all the particles that are needed was (and still is) impossible. So the suggestion was to take some inspiration from the liquid bridges that water on Earth forms between tiny grains of sand.”
Rubble-pile spin barrier
“It has been observed that small asteroids [less than 150 meters (490 feet) across] can rotate with spin periods below 2.4 hours, whereas for larger asteroids this is a barrier (‘rubble-pile spin barrier’). This barrier is a basic result of the self-gravity of the asteroids,” said Sánchez.
A rubble pile spinning fast enough will fling itself apart, and for a body held together by gravity alone, that limit does not depend on size. Larger asteroids respect it, which is itself evidence that they are rubble piles rather than solid rock.
Smaller asteroids do not. Self-gravity weakens rapidly as a body shrinks, while cohesive forces between grains do not. Below that size, cohesion can become the dominant force holding an asteroid together. Cohesion does not replace gravity, however—it adds to it.
In 2010, Sánchez and his colleagues proposed that van der Waals forces between fine grains could supply that strength. These grains make up an asteroid’s regolith, the loose dust and rock covering its surface.
The team tested the idea in 2014 with spherical particles, and the results suggested small grains could act as a weak cement between larger boulders.
Two things were still missing. Simulations could not yet handle realistic grain shapes. The material properties these calculations depend on, such as the cohesive force between particles, also had to be assumed rather than measured.
That changed when NASA’s OSIRIS-REx mission returned samples from Bennu in 2023, allowing those properties to be measured for the first time.
Granular bridges
Rather than simulate an entire asteroid, the researchers modeled a small piece of one. They packed a matrix of cohesive grains, the fine material that binds larger pieces together like cement in concrete, between two boulders, each a meter (3.3 feet) across, forming a bridge. Pulling the boulders apart until the bridge breaks gives a measure of tensile strength.
The team ran 78 of these simulations using the contact dynamics method, implemented in the open-source code LMGC90.
In some, the grains were perfect spheres. In others, they were polyhedra, stretched or flattened into 10 different shapes ranging from nearly round to highly elongated. Grain sizes ranged from 2 to 5 centimeters (0.8 to 2 inches), in both uniform and mixed batches. Their shape was measured using sphericity.
“Sphericity is just a measure of how close the shape of a particle is to its spherical equivalent,” said Sánchez.
Both forces that hold a real asteroid together were built in: van der Waals attraction as a fixed pull wherever two grains touch and gravity calculated directly between every particle.
The boulders were then drawn apart by a force that increased slowly in small steps. Stress in the bridge rose, plateaued, then dropped abruptly to zero as the bridge ruptured. That peak value is the tensile strength.
Grain size, shape and Bennu’s surface
The simulations showed that strength depended on two things: how big the grains were and how far they departed from spherical. Smaller grains produced stronger bridges. Shrink the grains and more fit into the same space, which means more contact points holding it together.
Shape mattered too. Two spheres touch at a single point, while two angular grains can meet along an edge or an entire face.
“Particle shape is important because non-spherical particles can have multiple contacts, they can also be packed together very tightly, thereby increasing the number of contacts and reducing porosity (the empty space between particles),” explained Sánchez.
The two effects can offset one another, so a bridge of large angular grains can match one of small rounded grains. That is what makes the framework general: Either measurement alone—size or sphericity—is enough to predict the strength.
Applied to Bennu, using cohesive forces measured from the returned samples, the framework put the surface strength below 1 pascal, matching independent estimates from remote sensing and from the sampling event itself.
“If you form a small cylinder with freshly ground coffee, its strength is about 50 Pa and you can still poke a hole into it with a single finger. The surface of Bennu is 50 times weaker than that,” said Sánchez.
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,” he explained.
Future work
Almost everything known about asteroids comes from their visible surfaces. Their strength is different, revealing itself only when something disturbs them.
“Imagine you have a bunch of pebbles and dust in a box. You can tilt the box and find out the angle at which the particles inside begin to slide down (the disruptive event), but how can you know this angle without tilting the box?” said Sánchez.
That uncertainty matters for planetary defense, since how far an asteroid moves when struck depends on properties that have to be assumed. NASA tested that in 2022, when its DART spacecraft struck the asteroid Dimorphos.
“Our theory is linked to particle size, particle size distribution and porosity. It could provide an initial approximation to asteroid tensile and cohesive strengths. These are basic material parameters for the codes that simulate impacts on asteroids, the only tested method for asteroid deflection,” explained Sánchez.
The team next plans to extend the framework to wider ranges of grain sizes and to the interiors of asteroids—the part no mission has ever seen.
Written for you by our author Tejasri Gururaj, edited by Sadie Harley, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive.
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Publication details
Paul Sánchez et al, A universal scaling framework for granular asteroid strength and its application to the surface of asteroid Bennu, Nature Communications (2026). DOI: 10.1038/s41467-026-75169-4.
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Particle shape and size predict asteroid strength, revealing Bennu’s surface is 50 times weaker than ground coffee (2026, September 4)
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