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

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5. Concluding remarks

Our observations have demonstrated that the second target of the GIOTTO spacecraft, Comet 26P/Grigg-Skjellerup, has a 1.5 km radius nucleus of slightly bluish colour. They clarified the question of the nucleus size of this comet which represented a so far unknown parameter in the interpretation of results from the GIOTTO in situ experiments. Comet 73P/Schwassmann-Wachmann 3, although only a back-up target for ESA's ROSETTA mission, may have gained scientific attractiveness, since its nucleus is split and one can expect that the new surface areas exhibit fresh and widely unprocessed cometary material to sunlight. This could be an enormous advantage and gain for the surface science experiments of a spaceprobe which focuses on the exploration of the pristine cometary matter from the origin of the solar system. On the other side, such a mission could have a higher risk for hazardous encounters with large (blocksize or larger) bodies which remain for a very long time in the neighbourhood of the split nuclei completely undectable from ground. As a striking fact and in contrast with our general expectation of reddened cometary nuclei both comets show solar or slightly bluish V-R colours although their nuclei were covered by a large surface crust. Fresh layers of dust fall-out from the latest perihelion activity cycles could be responsible for the neutral (i.e. solar) colour of both nuclei.

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© European Southern Observatory (ESO) 1999

Online publication: December 16, 1998