The Program to Link the HIPPARCOS Reference Frame to an Extragalactic Reference 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 nonrotating (inertial) reference frame. One of the major problems confronting the Hipparcos Project was the determination of the relationship of the satellite coordinate system to the best available celestial coordinate system, hopefully at an accuracy approaching the systematic accuracy of the Hipparcos satellite itself. In 1978 we recognized the possibility of using the Fine Guidance Sensors of the Hubble Space Telescope (HST) to measure the Hipparcos coordinate offset and its time variation with respect to extragalactic objects, and we undertook that project. The first HST observation for the project was made in 1993 April, and the last observation used in the solution was made in 1995 October. The HST data consist of accurate angular separations of Extragalactic Objects from Hipparcos stars. Ultimately the HST solution was incorporated with other techniques to form the final link between Hipparcos and the International Celestial Reference System. This link determined the final catalogue system for the published Hipparcos data. We describe 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 final Hipparcos solution. We determine the relation of the Hipparcos coordinate axes with respect to the ICRS coordinate axes from the HST, Hipparcos, and VLBI data alone. The rms uncertainties are about 2 milliarcseconds for the angular offsets and about 2.5 milliarcseconds/yr for the angular rotations. (C) 1997 American Astronomical Society. [S0004-6256(97)03612-1].