The connection of a catalogue of stars with an extragalactic reference frame

It is shown that the best quasi-inertial reference system in the near future will be based on positions of quasars determined by VLBI and that the positions and proper motions of the 100,000 stars to be observed by the Hipparcos astrometric satellite will not be a priori in a fixed system. Three methods of connecting the Hipparcos reference system to a VLBI extragalactic reference system are examined: (1) the use of radio stars observed by both VLBI and Hipparcos; (2) the use of VLBI quasars and of Hipparcos stars coupled with the quasars' optical counterparts by Space Telescope observations; and (3) the use of Hipparcos stars coupled by Space Telescope observations with extragalactic objects whose radio emission is too faint for accurate VLBI observations. It is concluded that method (1) is the most reliable technique and that accuracies of 0.001 arcsec/yr for rotation and of 0.002 arcsec for position at epoch can be obtained with about 20 radio stars if the VLBI accuracy matches the Hipparcos precision.