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 Jupi ter. The last one occurred in 1984, and placed the comet on an orbit w ith reduced perihelion distance. Since no observations with electronic detectors are yet available, the visual lightcurves of the last perih elion passages in 1986 and 1991 are studied. This yields a production rate of 4 x 10(28) H2O melec s(-1) at perihelion acco to a simple corr elation law between heliocetric magnitude and production rate. The mag nitudes at recoveries on photographic plates indicate a nuclear absolu te magnitude of about 16.5, corresponding to a nucleus of radius 1.8 k m, assuming an albedo of 0.03. In order to reconcile the nuclear radiu s with the gas production curve, it is found that the fraction of the active area of P/Wirtanen should be as high 0.25. This appears to offe r the best explanation to the large nongravitational acceleration suff ered by this comet.