According to research published in Nature Communications, the surface strength of asteroid Bennu measures below 1 pascal, making it roughly 50 times weaker than freshly ground coffee due to a shortage of fine dust. University of Colorado Boulder senior research associate Paul Sánchez and his colleagues developed a universal scaling framework using contact dynamics simulations to evaluate how particle size and shape dictate the structural integrity of granular asteroids.
Understanding the Rubble-Pile Spin Barrier on Small Asteroids
Space missions have demonstrated that numerous compact asteroids—including Bennu, Ryugu, and Itokawa—fail to qualify as solid rocks.
Instead, observations referenced by Paul Sánchez indicate that these formations function as granular asteroids—unfixed aggregations of boulders, rock, and dust maintained via mutual gravity combined with modest cohesive interactions.
Observations show that small asteroids measuring less than 150 meters across can rotate with spin periods below 2.4 hours. Larger asteroids face a structural limit known as the rubble-pile spin barrier, which stems directly from self-gravity. A rubble pile spinning fast enough will fling itself apart. While larger bodies respect this threshold, smaller asteroids do not because self-gravity weakens rapidly as a body shrinks while cohesive forces do not. Cohesion adds to gravity rather than replacing it, becoming the dominant force holding smaller objects together.
Did you know? Van der Waals forces between fine regolith grains supply the hidden strength that prevents small, fast-spinning asteroids from flying apart, a mechanism first proposed by Sánchez and colleagues in 2010.
Simulating Granular Bridges to Measure Asteroid Tensile Strength
To measure tensile strength without simulating an entire celestial body, researchers modeled a small piece of an asteroid. According to the study, the team packed a matrix of cohesive grains between two one-meter boulders to form a granular bridge, running 78 simulations using the open-source contact dynamics code LMGC90.
The team tested spherical and polyhedral grains across 10 different shapes with sizes ranging from 2 to 5 centimeters. When the boulders were pulled apart by a slowly increasing force, stress in the bridge rose, plateaued, and dropped abruptly to zero upon rupture. The simulations demonstrated that tensile strength depends on grain size and sphericity. Smaller grains pack more contact points into the same space, while angular grains meet along edges or entire faces rather than single points.
| Grain Property | Structural Impact on Asteroid Material |
|---|---|
| Size (2–5 cm) | Smaller grains increase contact density, strengthening the granular bridge. |
| Shape (Sphericity) | Angular grains meet along edges or faces, offsetting the need for smaller sizes. |
Applying the Scaling Framework to Asteroid Bennu and Planetary Defense
Applying the new framework to Bennu using cohesive forces measured from samples returned by NASA’s OSIRIS-REx mission in 2023 yielded a surface strength below 1 pascal. This figure aligns with independent estimates from remote sensing and the sampling event itself, according to the research team. Paul Sánchez noted that while a small cylinder of freshly ground coffee has a strength of about 50 pascals, Bennu’s surface is 50 times weaker due to a shortage of fine dust needed to fill voids between larger boulders.
Understanding these mechanical properties is critical for planetary defense missions. When NASA tested that in 2022, when its DART spacecraft struck the asteroid Dimorphos, how far an asteroid moves when struck depends on properties that have to be assumed. The research team plans to extend this framework to broader grain size ranges and explore the unseen interiors of asteroids.
Frequently Asked Questions
Why are small asteroids not solid rocks?
According to space mission data, small asteroids like Bennu and Ryugu are rubble piles consisting of loosely bound dust, rock, and boulders held together by gravity and cohesive forces.
How strong is the surface of asteroid Bennu?
Data from sample analysis and simulations show Bennu’s surface strength is below 1 pascal, which is roughly 50 times weaker than freshly ground coffee.
What causes Bennu’s low surface strength?
Researchers attribute the low strength to a shortage of fine dust grains required to fill voids between larger rocks and create stable cohesive contacts.
Want to stay updated on the latest space exploration discoveries and asteroid mechanics research? Subscribe to our newsletter or explore our latest articles on planetary defense and sample return missions.
Worth a look