THE PROGRAM TO LINK THE HIPPARCOS REFERENCE FRAME TO AN EXTRAGALACTICREFERENCE SYSTEM USING THE FINE GUIDANCE SENSORS OF THE HUBBLE-SPACE-TELESCOPE

The Hipparcos Satellite Mission has produced positions, proper motions , and parallaxes of unprecedented accuracy for 118,000 stars. For the positions and proper motions to represent true directions and physical motions in space, they must refer to an accurate realization of a non rotating (inertial) reference frame. One of the major problems confron ting the Hipparcos Project was the determination of the relationship o f the satellite coordinate system to the best available celestial coor dinate system, hopefully at an accuracy approaching the systematic acc uracy of the Hipparcos satellite itself. In 1978 we recognized the pos sibility of using the Fine Guidance Sensors of the Hubble Space Telesc ope (HST) to measure the Hipparcos coordinate offset and its time vari ation with respect to extragalactic objects, and we undertook that pro ject. The first HST observation for the project was made in 1993 April , and the last observation used in the solution was made in 1995 Octob er. The HST data consist of accurate angular separations of Extragalac tic Objects from Hipparcos stars. Ultimately the HST solution was inco rporated with other techniques to form the final link between Hipparco s and the International Celestial Reference System. This link determin ed the final catalogue system for the published Hipparcos data. We des cribe the problem, the instrumentation of the two space missions, the limitations imposed by the missions on the observations, the reduction and analysis procedures, and the final results with respect to the fi nal Hipparcos solution. We determine the relation of the Hipparcos coo rdinate axes with respect to the ICRS coordinate axes from the HST, Hi pparcos, and VLBI data alone. The rms uncertainties are about 2 millia rcseconds for the angular offsets and about 2.5 milliarcseconds/yr for the angular rotations. (C) 1997 American Astronomical Society. [S0004 -6256(97)03612-1].