Asteroid lightcurves: Can't tell a contact binary from a brick

Abstract Over the last couple decades, numerous papers based on lightcurve observations of asteroids have derived shapes that were claimed to represent “contact binary” or fluid equilibrium figures. From these shapes, the authors then presumed to derive densities, or density limits, on such bodies. In this paper we remind that it is well established, theoretically as well as observationally, that lightcurve analysis alone, under practical limitations of phase angle and aspect range, in most cases cannot establish concavities of a figure, i.e., the resultant shape inversion is a convex hull, not an actual shape. Therefore, it is generally not possible to infer “bilobed” or “contact binary” shapes from lightcurve analysis alone, let alone infer densities from such analysis. We also show that even “rubble pile” strength overrides fluid equilibrium for all but the very largest asteroids or TNOs. As a result, supposing Jacobi ellipsoidal or Roche near-contact figures and inferring densities therefrom is not justified.