Hipparcos

Hipparcos (sometimes written HIPPARCOS as it is an acronym for High Precision Parallax Collecting Satellite) was a space observatory operated by European Space Agency (ESA) from 1989 to 1993 to perform astrometric surveys. The Hipparcos Input Catalog (HIC) provided around 120,000 target stars for the survey. Results were the Hipparcos Catalog of these stars, and through comparison with other observation data and further analysis, additional catalogs including many more stars: the Tycho Catalog (or Tycho-1 Catalog), and based on that, the ACT Reference Catalog (for Astrographic Catalog / Tycho, aka ACT or ACT-RC), the Tycho Input Catalog (TIC, cross-referencing Hipparcos data with GSC data), the Tycho Reference Catalog (TRC), and the Tycho-2 Catalog. All the latter are generally referenced/cited by the "TYC" designators. All these are concerned with the exact positions of many stars at any given time, useful as guide stars, e.g., for very accurate determination of where a telescope is pointing.

Gaia is a newer space observatory to carry out the same work. Combining Hipparcos and Gaia data yields a time-baseline of over 25 years, very useful for determining proper motions, orbits of resolvable binary stars, and identifying stellar associations.