The nuclei of comets 26P/Grigg-Skjellerup and 73P/Schwassmann-Wachmamn 3

The target of the second GIOTTO fly-by, Comet 26P/Grigg-Skjellerup (26P/GS), and the back-up candidate for ESA's ROSETTA mission, Comet 73P/Schwassmann-Wachmann 3 (73P/SW3), were observed to support the interpretation of the in situ experiment results and the preparation of the forthcoming cometary mission, respectively. The main goal was the size estimation of the nuclei through broadband CCD imaging when the comets were far from the Sun (26P/GS: 3.8 AU in September 1993, 73P/SW3: 3.0 AU in December 1994). The nucleus of 26P/GS has an equivalent radius of 1.5 km (for an albedo of 0.04) with a body axis ratio of 0.9 or less (considering other observations possibly just 0.5 or less). The comet was inactive at 3.8 AU outbound and exhibited a slightly bluish V-R colour. Comet 73P/SW3 was smaller than 1.1 km radius (for an albedo of 0.04) before break-up in 1995 and showed solar V-R colour. The nucleus was weakly active at 3.0 AU inbound in December 1994. The gas production rates measured during the perihelion passages of the comets in 1992 (26P/GS) and 1995 (73P/SW3) together with our radius estimations suggest that the nuclei of both comets had a significant crust coverage on their surface.