This simulation – the top view on the left and side view on the right – shows an aggregate (assumed to be the common structure of small asteroids) asteroid that is spun-up due to differential heating, through the so-called 'YORP' effect. The orange particles are those initially at the surface and the white particles are those initially below the surface. While spinning up, the particles that are at mid-latitude move to the equator where the centrifugal force is greatest. When this force exceeds the gravity of the body, these particles escape and can potentially collide together to form a secondary. This mass shedding process turns an initially more or less spheroidal body into what is known as a top shape body, i.e. a spheroid with an equatorial bulge, which seems to be a common shape based on radar models of some small asteroids, as well as direct images of asteroids Ryugu and Steins (and soon Bennu, the destination of NASA's OSIRIS-REx mission). Taken from Walsh, K.J., Richardson, D.C. & Michel, P. 2008. Rotational break up as the origin of binary asteroids. Nature 454, 188-191.