Comet P/Wirtanen, summary of observational data

P/Wirtanen is now the main target of the Rosetta mission. The orbit of this Jupiter family comet is perturbed by frequent approaches to Jupiter. The last one occurred in 1984, and placed the comet on an orbit with reduced perihelion distance. Since no observations with electronic detectors are yet available, the visual lightcurves of the last perihelion passages in 1986 and 1991 are studied. This yields a production rate of 4 × 1028 H2O molec s−1 at perihelion according to a simple correlation law between heliocentric magnitude and production rate. The magnitudes at recoveries on photographic plates indicate a nuclear absolute magnitude of about 16.5, corresponding to a nucleus of radius 1.8 km, assuming an albedo of 0.03. In order to reconcile the nuclear radius with the gas production curve, it is found that the fraction of the active area of P/Wirtanen should be as high as 0.25. This appears to offer the best explanation to the large nongravitational acceleration suffered by this comet.