Comet Bowell at record heliocentric distance

Comets probably contain the most primordial material in the Solar System, and those which are passing through the inner Solar System for the first time are likely to be the most pristine of all. Comet Bowell (1982 I = 1980b) was discovered in 1980 at the unusually large heliocentric distance of R = 7.3 AU (ref. 1). The small reciprocal of the original orbital semi-major axis, (1/a) = 30 × 10^-6 AU^-1, suggests that this comet is dynamically new in the sense that it has made few, if any, previous passages through the inner Solar System². Because prolonged solar irradiation may alter the properties of short-period comets, it is of great interest to see if 1982 I exhibits properties which are significantly different from those of the periodic comets. We present new observations of comet Bowell at the record distance R = 13.6 AU. An extended coma is present, the size of which is consistent with the same slow expansion rate v ~ 1 m s^-1 detected around perihelion (March 1982 and R ~ 3.36 AU). The cross-section of the solid grains within the central 10 arc s of the coma has decreased by over an order of magnitude since 1980-1984, which indicates that the coma production is declining. The decline began near R ~ 10 AU, the same distance at which production began on the pre-perihelion leg. The coma at R <= 10 AU may be formed by sublimation of CO₂ or an ice of similar volatility from the nucleus.