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Astron. Astrophys. 335, L50-L55 (1998)

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1. Introduction

Dozens of comets are observable every year, yet, for various reasons, most are never studied in much detail. Until recently, comet Wirtanen had been one of these objects: an inherently faint comet with unfavorable recent apparitions. In 1994, however, the European Space Agency announced that comet Wirtanen was the target for the ROSETTA spacecraft mission, which prompted numerous observing campaigns during the 1997 apparition. Characteristics of the nucleus and cometary environment that will impact the mission design and objectives include the size and composition of the nucleus and its activity level with respect to heliocentric distance. Furthermore, the safety of the spacecraft is a concern, so evaluation of the comet's dust production rates and dust-to-gas ratio is a priority for ground-based studies. Ultimately, comparing Wirtanen to other comets will help establish how results from the ROSETTA mission can be extended to comets in general.

Narrowband photometric measurements of Wirtanen were obtained at Lowell Observatory to determine the comet's basic composition and activity levels. As part of a larger comet database study (A'Hearn et al. 1995) a single measurement was obtained in 1991, well before Wirtanen was chosen as the ROSETTA target. During the 1997 apparition, an effort was made to obtain data in support of the spacecraft mission. Throughout both of these apparitions, however, the observing geometry was poor, and, during the months around perihelion, Wirtanen was always within 50o of the sun. This put the comet at high airmass near twilight, with only a short amount of time for observations. (Competition with comet Hale-Bopp for observing time further limited the number of measurements.) Overall, the observing geometry was similar, but reversed, in the two apparitions, with the Earth on opposite sides of the sun.

This paper presents our combined data set. The production rates of dust and different molecular species are given, and, where sufficient data exist, heliocentric distance relations are discussed. Abundance ratios, water production, and the dust-to-gas ratio are also calculated, and these results are compared to those of the overall database compiled by A'Hearn et al. (1995). A taxonomic classification of the comet is determined, along with an estimate of the active surface area. Finally, Wirtanen is compared to comet Halley to put it into context with previous spacecraft measurements.

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© European Southern Observatory (ESO) 1998

Online publication: June 18, 1998