Comments on the rotational state and non-gravitational forces of comet 46P Wirtanen

Experience of modeling the rotational state and non-gravitational forces of comet 1P/Halley and other comets is applied to comet 46P/Wirtanen. While the paucity of physical data of 46P/Wirtanen makes this process somewhat speculative, this comet's place as a target for the important Rosetta mission gives significance to such a study. The arguments are based on the summary of observational data provided by Jorda and Rickman (Planet. Space Sci. 43, 575, 1995) and a comparative study of the behavior of other periodic comets. It is found that 46P/Wirtanen has a level of surface activity relative to its mass that is dynamically more akin to that found in comet 1P/Halley than in a typical periodic comet. It is shown through an illustrative numerical example that this apparent fact should likely lead to an excited spin state for this comet and that significant changes in the spin period could occur in a single pass through perihelion. It is argued that the available observations are not sufficient to substantiate the claim of Jorda and Rickman (Planet. Space sci. 43, 575, 1995) that the nucleus is undergoing retrograde rotation and it is possible that the rotation is prograde as well as retrograde. The substantial requirements that must be placed on any future observing program necessary to determine the precise rotational state are outlined. An extended (similar to 2 month) southern hemisphere observing campaign is advocated to determine the nuclear rotational state in 1996 if possible before activity turns on.