Seismology of rubble-pile asteroids in binary systems

The mutual gravitational interaction of binary asteroids, which make up approximately 15 per cent of the near-Earth asteroid (NEA) population, provides a continuous tidal force, creating ground motion. We explore the potential of kilometre-sized binary asteroids as targets for seismological studies of their interior structure. We use a numerical model wherein each body is constructed of discrete particles interacting via gravity and contact forces. The system's orbital properties are modelled based on those of typical binary NEAs: a secondary body orbits a primary body at a distance of a few to 10 primary radii, resulting in orbital periods of a few tens of hours. We varied the elastic moduli (stiffness) of the constituent particles and measured a strain of a few micrometres caused by the orbiting satellite. Over eight orbital periods, the acceleration of the strain vector along the primary body's equatorial axis indicates that tidally induced ground motion generated by a binary asteroid system is detectable by modern seismometers, like the instruments deployed on the InSight mission to Mars. Owing to the relatively short orbital period of the satellite - a mean of 25.8 h for known binary NEAs - only a modest mission lifetime would be required for a seismometer to adequately characterize an asteroid's interior through tidally induced deformation. Future deployment of seismometers on binary asteroids will allow for a detailed characterization of the structure of these objects.