Comet 19/P Borrelly in 2021–22: Change in the spin axis orientation versus previous apparitions

Abstract

Comet 19P/Borrelly is characterized by the presence of a prominent polar jet that is nearly aligned with its rotation axis. The polar jet model has therefore been used in the past by several authors to determine the orientation of the comet's spin axis. To verify whether the spin axis during the 2021–22 apparition was in the expected positions according to the published pole coordinates, the position angles (PA) of the polar jet were accurately measured, resulting in a spin axis orientation at RA = 206.1° and Dec = −5.0°. These coordinates are different from all those published by other authors during previous apparitions. The comparison of the measurements of the PAs of the jet made by other authors during earlier apparitions (1911–1932 and from 1994 to 2008) with the hypothetical PAs calculated according to the new pole coordinates showed significant differences, thus confirming that the comet's axis is currently pointing to a different position. The lack of suitable images of the last perihelion passage of 2015 does not allow to clarify whether the shift of the spin axis observed in 2021–22 occurred during this orbit or is the result of a slow, secular drift or of a precession.