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Published online by Cambridge University Press: 30 March 2016
The link of the Hipparcos Catalogue to an extragalactic reference system will be carried out on two time-scales: the first, coinciding with the duration of the mission, to provide data for immediate use by the data reduction consortia, and the second over a longer term to provide for subsequent reappraisal of the final Hipparcos catalogue. The first will be based on radio stars and on an indirect link to extragalactic objects by means of observations with the Hubble Space Telescope; for the second we hope for longer term programmes with the Hubble Space Telescope, together with a variety of ground-based links.