Asteroid Bennu Is So Soft You Could Punch Through It: Strength Derived From a 121.6-Gram Sample
A 490-meter-wide asteroid with a surface strength of less than 1 pascal. The most vivid conversion comes from author Paul Sánchez in a phys.org interview: compressed freshly-ground coffee forms a small cylinder with a strength of about 50 pascals—and Bennu's surface is 50 times softer than that. As he put it, "you could poke a hole in the coffee cylinder with one finger."
First, a timeline note—the paper itself was published in *Nature Communications* on July 8, but the September 4 phys.org interview is what sparked this wave of coverage. Still, the chain of numbers uncovered in that interview is worth retelling.
It All Started With a Botched Sampling
On October 20, 2020, the OSIRIS-REx spacecraft's robotic arm touched Bennu's surface at 10 cm/s, preparing to blast nitrogen gas to stir up samples. The expected surface was relatively firm rubble. In reality: the head sank several centimeters, the gas blasted a crater about 68 cm deep, and the whole arm sank in nearly half a meter. The University of Arizona described it as "a punch into a ball pit." The mission returned 121.6 grams of sample—double the 60-gram target. A lucky accident.
From Sample to Strength, Skipping No Steps
The paper's backbone links three puzzle pieces: laboratory-measured interparticle adhesion (0.5–4 nanonewtons, Jardine et al. 2025), the imaging-derived average particle size (1.2 mm, Ballouz et al. 2026), and 78 contact-dynamics simulations done in this study.
The simulations used LMGC90, an open-source platform developed at the University of Montpellier. The scenario was concrete: between two one-meter boulders sits a "particle bridge" made of 2–5 cm grains—how much force does it take to break it? 18 runs with spherical particles, 60 with polyhedra—ten aspect ratios, from round to elongated.
The results land at 0.001 to 0.01 pascals. The paper's own wording: "effectually zero strength." Prior upper bounds inferred from remote sensing and the sampling event were within 1 pascal—two independent lines of evidence agree.
The Funniest Part: It's Weak Because the Grains Are Too Sticky
You'd think this softness means grains don't stick together. It turns out that's half wrong. Bennu's individual grains are quite sticky—nanonewton-scale van der Waals forces were actually measured. The real reason for its weakness is that the surface lacks fine dust—without enough small particles to fill the gaps, there are few contact points and thus few adhesive bonds. The glue is good; it's just applied too thinly.
The simulations also reveal two counterintuitive knobs. Smaller grains make stronger particle bridges (strength drops from 0.005 Pa to 0.001 Pa as size grows); flatter grains are also stronger (as aspect ratio goes from 1 to 0.4, strength rises from 0.0008 to 0.0024 Pa)—flat grains interlock and create richer contact types. The two effects can even cancel out—large and flat can match small and round. That's the origin of the "universal scaling framework" in the title.
One honest caveat: the coordination number Zc, a key parameter, cannot be measured on the sample—the paper admits experimental methods fall short. So it delivers a scaling law paired with a measurable input.
Why This Level of Detail Matters
An asteroid's surface strength directly determines how humanity can push it. When DART struck Dimorphos in 2022, its momentum was amplified by ejecta—and the amplification factor depends on how soft the surface is. Rubble piles like Bennu make up the bulk of near-Earth asteroids, and their "push-back feel" is a core parameter of planetary defense.
There's also a timing coincidence: ESA's Hera probe launched October 7 and arrives at the Didymos binary system this December to inspect DART's impact crater. Soon, this numerical chain extracted from 121.6 grams of sample will be checked against a real impact site.
References: Nature Communications paper | phys.org interview | Walsh et al. 2022 sampling event analysis | ESA Hera mission