Orbital Evolution, Activity, and Mass Loss of Comet C/1995 O1 (Hale-Bopp). I. Close Encounter with Jupiter in Third Millennium BCE and Effects of Outgassing on the Comet's Motion and Physical Properties

Abstract

This comprehensive study of comet C/1995 O1 focuses first on investigating its orbital motion over a period of 17.6 yr (1993-2010). The comet is suggested to have approached Jupiter to 0.005 AU on -2251 November 7, in general conformity with Marsden's (1999) proposal of a Jovian encounter nearly 4300 yr ago. The variations of sizable nongravitational effects with heliocentric distance correlate with the evolution of outgassing, asymmetric relative to perihelion. The future orbital period will shorten to ~1000 yr because of orbital-cascade resonance effects. We find that the sublimation curves of parent molecules are fitted with the type of a law used for the nongravitational acceleration, determine their orbit-integrated mass loss, and conclude that the share of water ice was at most 57%, and possibly less than 50%, of the total outgassed mass. Even though organic parent molecules (many still unidentified) had very low abundances relative to water individually, their high molar mass and sheer number made them, summarily, important potential mass contributors to the total production of gas. The mass loss of dust per orbit exceeded that of water ice by a factor of ~12, a dust loading high enough to imply a major role for heavy organic molecules of low volatility in accelerating the minuscule dust particles in the expanding halos to terminal velocities as high as 0.7 km s^{-1}. In Part II, the comet's nucleus will be modeled as a compact cluster of massive fragments to conform to the integrated nongravitational effect.