![\"\"](\"https:\/\/www.newsbeep.com\/ie\/wp-content\/uploads\/2026\/09\/Bennu\u00b71999_RQ36_anaglyph.gif\")
Anaglyph video of Bennu filmed by OSIRIS-REx. Credit: NASA<\/p>\nAnaglyph video of Bennu filmed by OSIRIS-REx. Credit: NASA<\/p>\n
Asteroid Bennu is astonishingly fragile for what you\u2019d normally expect for a huge rock hurling through the solar system. Its surface has a tensile strength several times weaker than a clump of freshly ground coffee.<\/p>\n
\u201cIf 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,\u201d Paul S\u00e1nchez, a senior research associate at the University of Colorado Boulder, said in a statement.<\/p>\n
The new study from S\u00e1nchez and colleagues reveals that Bennu<\/a> is not a solid chunk of rock but a vast aggregate of boulders, pebbles, and dust. They found that its strength depends heavily on the size and shape of the grains filling the spaces between larger pieces. Bennu appears to lack enough of the smallest grains to bind that rubble tightly together.<\/p>\n But how does a pile of asteroid rubble stop from simply falling apart?<\/p>\n Grainy Cement<\/p>\n Many asteroids are not solid rocks. They are \u201crubble piles\u201d made of boulders, pebbles and dust held together mainly by their own gravity. That works reasonably well for larger asteroids, where gravity is strong enough to keep the loose material from drifting away.<\/p>\n One clue comes from how fast asteroids spin. A 2000 study of asteroid rotation<\/a> found that larger rubble-pile asteroids rarely complete a rotation in less than about 2.2 hours. If they spun much faster, their weak gravity would no longer be enough to keep loose material on the surface, and the asteroid could begin shedding debris or even break apart. Astronomers call this the rubble-pile spin barrier.<\/p>\n But some much smaller asteroids spin faster than that and remain intact. That told researchers that gravity could not be the whole story. In a 2010 Icarus study<\/a>, S\u00e1nchez and his colleagues proposed another source of structural strength: tiny grains of dust can cling to one another through van der Waals forces, very weak attractions that act when particles are extremely close together. On Earth, these forces are usually insignificant compared with gravity. On a tiny asteroid, however, gravity is so weak that they can help hold the surface material together.<\/p>\n A 2014 follow-up study<\/a> showed how fine grains could act like a weak cement between larger rocks. Another 2014 Nature study<\/a> found that this kind of cohesion could explain why the fast-spinning rubble-pile asteroid 1950 DA had not torn itself apart.<\/p>\n \u00d7<\/p>\n Thank you! One more thing…<\/p>\n Please check your inbox and confirm your subscription.<\/p>\n Those models, however, simplified the problem by treating the grains mostly as spheres. Real asteroid fragments can be jagged, flattened or elongated, and those shapes change how tightly the particles pack and how many points of contact they make with one another. That is the gap the new study set out to address.<\/p>\n Researchers simulated 78 \u201cgranular bridges\u201d: masses of smaller particles squeezed between two meter-wide boulders. The grains ranged from 2 to 5 centimeters across and included spheres as well as 10 varieties of angular or elongated shapes. The researchers gradually pulled the boulders apart and measured how much stress the bridge survived before breaking.<\/p>\n Smaller grains made stronger bridges because more particles could occupy the same volume, creating more points of contact. Angular grains also strengthened the material by interlocking and making multiple contacts instead of merely touching at one point.<\/p>\n (a) The basic simulation setup, with smaller asteroid grains packed between two large boulders. (b) How the granular bridge changes as the particle size increases. (c) The bridge progressively breaking apart as the boulders are pulled away from each other. (d) The different grain shapes tested, from more rounded to more elongated or flattened. Credit: Nature Communications<\/p>\n Barely Cohesive<\/p>\n The researchers then applied their framework to Bennu using measurements from material that NASA\u2019s OSIRIS-REx<\/a> spacecraft returned to Earth in 2023.<\/p>\n The dataset contained actual measurements of the asteroid\u2019s particles, including their sizes, shapes, packing, and the forces between them. Earlier laboratory analysis found Bennu\u2019s returned material ranges from submicron dust to rocks several centimeters across. The initial analysis of the returned Bennu samples<\/a> also revealed abundant angular and irregular particles.<\/p>\n The model puts the tensile strength of Bennu\u2019s sampling site below one pascal and potentially around 0.001 to 0.01 pascal under assumptions close to the measured average particle size. That squares with what OSIRIS-REx experienced firsthand. When its sampling head touched Bennu in 2020, it plunged far more easily into the surface than mission scientists had expected. A subsequent Science Advances analysis of the touchdown<\/a> described the subsurface as having \u201cnear-zero cohesion.\u201d<\/p>\n Bennu is not exactly dust-free. Instead, its particle-size distribution contains too few fine grains to efficiently fill the gaps between larger ones. That leaves fewer microscopic contacts through which cohesive forces can act. The study concludes that this dust scarcity is probably the main reason the sampling site was so surprisingly weak.<\/p>\n Know Thy Asteroids<\/p>\n Knowing whether an asteroid behaves like a rock or a loosely packed gravel heap becomes especially important when scientists try to hit one, such as for planetary defense research purposes. <\/p>\n NASA demonstrated the idea in 2022 when DART deliberately crashed into Dimorphos<\/a>. The impact successfully shortened the small asteroid\u2019s orbit around Didymos by about 33 minutes, proving that a kinetic impactor can alter an asteroid\u2019s motion<\/a>. NASA\u2019s DART results also showed that material blasted from the asteroid added substantially to the push<\/a>.<\/p>\n Predicting such a response requires knowing how easily the target fractures, deforms, and ejects material. Those properties remain notoriously difficult to measure from Earth.<\/p>\n \u201cOur theory is linked to particle size, particle size distribution and porosity. It could provide an initial approximation to asteroid tensile and cohesive strengths,\u201d S\u00e1nchez explained. \u201cThese are basic material parameters for the codes that simulate impacts on asteroids, the only tested method for asteroid deflection.\u201d<\/p>\n In other words, something as small as the grains wedged between two rocks could help determine what happens if humanity ever needs to deflect a dangerous asteroid away from Earth.<\/p>\n The study was published in the journal Nature Communications<\/a>.<\/p>\n![\"\"](\"https:\/\/www.newsbeep.com\/ie\/wp-content\/uploads\/2026\/09\/particle-shape-and-siz-1024x366.jpg\")
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<\/a>The Dimorphos post-impact debris. Credit: NASA<\/p>\n
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