Many small asteroids, such as Dimorphos, Ryugu and Bennu, have quite low densities compared to moons or terrestrial planets. Known as ‘rubble-pile’ asteroids, they appear to be composed of loose conglomerations of rocks, small grains and dust, and are not ‘solid’ at all. Planetary scientists believe that rubble-pile asteroids are formed by material coalescing after the destruction of larger asteroids.

The Japanese spacecraft Hayabusa2 visited asteroid Ryugu in 2018 and revealed that about 50 per cent of its volume is empty space! Meanwhile, measurements have revealed that Bennu has a density only slightly greater than that of water. It is likely that Bennu has hollow cavities in its interior, some of which are probably filled with water. NASA’s OSIRIS-REx probe, which collected a sample from Bennu in 2020, would almost certainly have sunk into Bennu’s surface had it not fired its rockets on contact.

Just like other celestial bodies, it is gravity that keeps these asteroids together – as long as the centrifugal force due to their rotation is not strong enough to overcome gravity. Research has shown that, for many asteroids, ‘cohesive’ and ‘adhesive’ forces – the tendency of materials to attract each other and ‘stick together’– are also required to prevent rotation from tearing these asteroids apart. In fact, the ratio of the gravitational to centrifugal forces on a rubble-pile asteroid determines not only the maximum size of boulders on its surface, but also the minimum mass at which the asteroid can remain intact.

So a boulder on the surface of Dimorphos is easily held there by gravity because the asteroid’s rotation is not sufficient to overcome gravity and throw it off into space.

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Asked by: Roger Jordan, via email

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