Active regions on the surface of Comet 43P/Wolf-Harrington determined from its nongravitational effects

The nongravitational perturbations in the motion of the periodic comet Wolf-Harrington are investigated during its nine observable apparitions in the period 1924–1998. To explain the irregular variations in the nongravitational acceleration, two different models are considered and successfully used to link all the apparitions: a) Model of nucleus with the activation and deactivation of discrete outgassing sources on the surface; b) Forced precession model of the spin axis of the nucleus with an activity described by nonlinear changes of the perihelion shift of the gas production curve. The first model is represented by two slightly different orbital solutions in which the northern active region is persistent and the initiations and disappearances of two southern regions are responsible for the observed variability of the nongravitational acceleration. Profiles of the modelled gas production rates are compared with observed light curves of the comet and used for an estimation of the effective outgassing area and the activity level of the cometary nucleus. According to all employed models of the nongravitational acceleration similar shifts of the maximum of the comet activity with respect to successive perihelion passages over the whole examined interval of the motion have been detected. The model parameters describing physical properties of the comet nucleus such as the nucleus orientation, the localization and the size of the active regions or the oblateness of the nucleus are derived from numerical fitting of the models to positional observations of the comet.