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Astron. Astrophys. 341, 912-917 (1999)

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1. The programme and the targets

With the advent of space missions to comets, short-period comets became the focus point of scientific interest since only this object class is potentially accessible for spaceprobes (within the constraints of a limited financial budget and of a mission profile of less than two decades duration). Although frequently monitored during the perihelion arcs of their orbits a few key parameters of these comets are still widely unknown: the size, shape and rotation, the surface colour and crust, the activity onset of the nucleus. Apart from their importance for the interpretation of the scientific results from in situ experiments of the spacecraft and for the planning of future cometary missions, the good knowledge of these nucleus parameters is also relevant for our scientific picture of comets per se. Only a handful of reliable measurements of the size and the rotation period of comets are published (for reviews see Meech 1998, Jewitt 1998). These observations indicate (but do not prove) that short-period comets have very small nuclei (Meech 1998) of the order of a few kilometers or even less than one kilometer (for example: radius of 0.7 km for 46P/Wirtanen, Boehnhardt et al. 1996; 0.4 km for 46P/Honda-Mrkos-Pajdusakova, Lamy et al. 1996 and Meech & Hainaut 1998). About surface properties like crust coverage and activity onset, our observations and knowledge are even more sparse.

It is therefore not a surprise that all target comets of past and future spacecraft missions were selected without good and accurate knowledge of these nucleus properties and even after the spacecraft visits some of these basic parameters of the objects are still unknown. This holds also for two targets of cometary missions of the European Space Agency (ESA), i.e. Comet 26P/Grigg- Skjellerup (26P/GS), the target for the second comet fly-by of the GIOTTO spacecraft in 1992, and Comet 73P/Schwassmann-Wachmann 3 (73P/SW3) which was the former prime target of ESA's new ROSETTA mission and is now still listed as a back-up object for later launch windows of the mission.

26P/GS: the nucleus radius (between 0.4-3 km) is constrained by radar observations (lower limit radius of 0.4 km; Kamoun 1983 and Kamoun et al. 1996) and ground-based photometry (Roemer 1966, Birkle & Boehnhardt 1992 and references therein, Meech 1998). Sitarski (1992) tried to estimate the body shape and axis ratio of the comet from modelling the non-gravitational forces by a nucleus rotation model with forced precession (prolate ellipsoid rotating around its longer axis, axis ratio of 0.7). The nucleus rotation period is unknown. Only marginal indications exist from ground-based observations for the presence of active regions on the nucleus (Birkle & Boehnhardt 1992, Fulle et al. 1993). Unfortunately, it was not possible to investigate these nucleus properties with GIOTTO since the adequate onbord experiments could not be operated during the fly-by. Gas and dust production rates at the time of the GIOTTO encounter can be found in Jockers et al. (1993), Fulle et al. (1993) and from GIOTTO experimenters (Neubauer et al. 1993, Johnstone et al. 1993). A single set of production rates is also available from the 1987 apparition (Osip et al. 1992). It is noteworthy that OPE data of the coma passage of 26P/GS allow an interesting, but controversial interpretation by multiple jets or nucleus fragments (McBride et al. 1997, Le Duin et al. 1996). Coma lightcurve parameters are given by Muraoka (1996, 1997).

73P/SW3: this comet is much less observed than the GIOTTO target 26P/GS. Lightcurve parameters are given by Marsden (1994), a size estimation (1.2 km radius; Sekanina et al. 1998) of the nucleus is based upon the visual brightness of the central coma condensation given by Baldet (1930a,b) from the 1930 apparition. The comet broke into at least 3 pieces in autumn 1995 (Boehnhardt & Käufl 1995; for a detailed analysis see Sekanina et al. 1998) after it underwent a major outburst in late August and September 1995 as concluded from OH radio observations (Crovisier et al. 1996).

Information on the orbits of both comets (both in the past and for the future) can be found in Belyaev et al. (1986).

Because of their relevance for spacecraft missions and for cometary science we have selected both comets as targets for ground-based CCD imaging campaigns. The goal of the observations was to constrain the size and rotation period, to measure the broadband colours and to assess the activity status of the nucleus while far from the Sun. In the following, we describe the results obtained through our imaging campaigns for 26P/GS in 1993 and for 73P/SW3 in 1994.

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© European Southern Observatory (ESO) 1999

Online publication: December 16, 1998