Models of comets with strongly variable nongravitational effects

The nongravitational motion of five “erratic” short-period comets is studied on the basis of published astrometric observations. We present the precession models which successfully link all the observed apparitions of the comets: 21P/Giacobini-Zinner, 31P/Schwassmann-Wachmann 2, 32P/Comas Solá, 37P/Forbes, and 43P/Wolf-Harrington. We used the Sekanina's forced precession model of the rotating cometary nucleus to include the nongravitational terms into equations of the comet's motion. Values of six basic parameters (four connected with the rotating comet nucleus and two describing the precession of spin-axis of the nucleus) have been determined along the orbital elements from positional observations of the comets. The solutions were derived with additional assumptions which introduce instantaneous changes of modulus of reactive force,Aand of maximum of cometary activity with respect to perihelion time. The present precession models impose some contraints on sizes and rotational periods of cometary nuclei. According to our solutions the nucleus of 21P/Giacobini-Zinner with oblateness along the spin-axis of about 0.32 (equatorial to polar radius of 1.46) is the most oblate among five investigated comets.

What happened with the comet 71P/Clark?

Discovered in 1973, 71P/Clark had made only five revolutions around the Sun. The motion of the comet has been subjected to a constant small deceleration between 1973 and 1989 and then a dramatic jump in the nongravitational perturbations occured. To explain the discontinuity in the comet's nongravitational effects a change in the surface distribution of active areas and a nucleus orientation was considered. The model of discrete source of out gassing was employed in the orbital calculations. From numerical fitting of the model parameters to positional observations the spin-axis orientation, localizations and outgassing areas of two active regions have been derived and the orbit successfully improved. The nucleus radius was estimated as equal to 0.76 km.