Non-gravitational force modeling of Comet 81P/Wild 2: I. A nucleus bulk density estimate

The nucleus of Comet 81P/Wild 2 is modeled by assuming various smooth triaxial ellipsoidal or irregular body shapes, having different rotational periods, spin axis orientations, and thermophysical properties. For these model nuclei, a large number of surface activity patterns (e.g., maps of active and inactive areas) are studied, and in each case the resulting water production rate and non-gravitational force vector versus time are calculated. By requiring that the model nuclei simultaneously reproduce certain properties of the empirical water production curve and non-gravitational changes of the orbit (focusing on changes of the orbital period and in the longitude of perihelion), constraints are placed on several properties of the nucleus. The simulations suggest that the mass of Comet 81P/Wild 2 is M≲2.3×1013kg, resulting in a rather low bulk density, ρbulk≲600–800kgm−3 (depending on the assumed nucleus volume), and that the nucleus rotation is prograde rather than retrograde. The active area fraction is difficult to constrain, but at most 60% of the nucleus is likely to have near-surface ice.