Comet P/Encke's Nongravitational Force

Abstract

A nearly direct measurement of the nongravitational force on a cometary nucleus is presented. IUE spectra of comet P/Encke show significant Greenstein effects even when centered on the nucleus. These imply net heliocentric components of outflow of 1.6 and 0.3 km/s for the OH radicals before and after perihelion near r = 0.8 AU. This implies a large asymmetry in the radial nongravitational acceleration (A1) about perihelion, which must be taken into account in order to derive the mass of the nucleus. The asymmetry is consistent with Sekanina's (1988) latest model for the Encke nucleus, but the measurements suggest an outflow velocity near 2 km/s, significantly greater than generally accepted values. A tentative value for the mass of the Encke nucleus is proposed, although improved modeling of the nongravitational acceleration is required to obtain a definitive value.