Non-gravitational motion of the Jupiter-family comet 81P/Wild 2

II. The active regions on the surface

¹ Space Research Centre of the Polish Academy of Sciences, Bartycka 18A, 00-716 Warsaw, Poland e-mail: slawka@cbk.waw.pl
² Astronomical Observatory, Box 515, 75120 Uppsala, Sweden

Received: 1 July 2008
Accepted: 30 July 2008

Abstract

Aims. The non-gravitational perturbations in the orbital motion of comet 81P/Wild 2 are investigated using a two-source model of outgassing.

Methods. In the process of orbit improvement the nucleus orientation and the location of two active regions are found from numerical fitting of the non-gravitational acceleration model to positional observations of the comet. Two different approaches to the lag angle of the outgassing behind the subsolar meridian are considered, assuming this angle to be either constant or varying with heliocentric distance.

Results. The derived spin axis orientation (, ) as well as the location of two active regions, the northern () and the southern one (), agree with determinations based on different observations by other authors. Orbital linkages of two and three successive apparitions of 81P/Wild 2 indicate an evolution of the change in the orbital period, possible time variations of the spin axis of the comet and an increase of the source areas during the last five revolutions. The non-gravitational perturbations were also used to constrain the sizes of the two active areas as well as the mass and bulk density (∼400 kg m^-3) of the comet nucleus. The modeled two-source water production curve is compared with the activity data represented by the observed water production curve and the brightness curve.