BYORP Effect on True Rubble Pile Secondaries

Kilometer-sized binary asteroids usually consist of a fast-rotating spheroidal primary and a relatively large, elongated, synchronously-rotating secondary. It is widely accepted that small binary asteroid formation is driven by YORP-spinup of the primary, but there is no agreement on the exact mode of secondary’s formation (for example, rapid vs gradual). The longevity of small binaries is also controversial, as some calculations of the Binary YORP effect (i.e. radiative acceleration due to asymmetry between leading and trailing hemispheres) on the secondary found lifetimes of only 100,000 years or less. This led to the suggestion that the tidal dissipation within the primary may be strong enough to counteract BYORP. These very strong tides are in excess of previous theoretical expectations and would need to have a curious size dependence (i.e. be more efficient for small bodies). We would also expect tides in near-breakup rotators to depend on the rotation rate, but no such correlation is observed. Here we use the results of the DART mission to Didymos-Dimorphos pair to motivate a rethinking about the operation of BYORP is small binary asteroids. Our ideas about YORP and BYORP strengths that were formed in early 2000s were based on shapes of asteroids like Ida or Eros, which, while not solid bodies, probably contain very large solid blocks that make up a large fraction of their interiors. Such irregular shapes with relatively smooth surfaces covered with regolith were well described with randomized Gaussian spheroids. DART images of Dimorphos show an ellipsoidal pile of blocks that may be called a “true rubble pile”, as all the visible blocks appear to be much smaller than the asteroid, and this may also be true of the interior. At the meeting we will present results of numerical modeling of the action of BYORP on bodies consisting of blocks of various sizes. Our preliminary models indicate that BYORP on true rubble piles may be an order of magnitude weaker than on Gaussian spheroids, potentially reconciling the theory and observations of BYORP effect acting on small binary asteroids.