Double ellipsoidal harmonic gravity field models for bilobed bodies: Example of comet 67P/Churyumov-Gerasimenko

Bilobed bodies represent a significant class of small extraterrestrial objects in the Solar System. We present a double harmonic-series approach to model the gravity of the lobes separately, thereby allowing their mass distributions to be constrained independently. We study an exemplary candidate contact binary, comet 67P/Churyumov-Gerasimenko, and establish two ellipsoidal harmonic series referred to the closest-bounding, non-overlapping ellipsoids of the respective lobes. We show that the model is more robust near the irregular body shape than a global spherical harmonic series, which is a customary product determined from spacecraft tracking. While they can be estimated directly from measurements, we demonstrate here that the double series can also be transformed from an existing spherical harmonic model by means of decomposition. We use simulations to analyze model solutions for a few heterogeneous mass distributions.