Hipparcos was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions and distances of celestial objects on the sky. This was the first practical attempt at all-sky absolute parallax measurement, something not possible with groundside observatories, and thus represented a fundamental breakthrough in astronomy. The resulting high-precision measurements of the absolute positions, proper motions, and parallaxes of stars enabled better calculations of their distance and tangential velocity; when combined with radial velocity measurements from spectroscopy, astrophysicists were able to finally measure all six quantities needed to determine the motion of stars. The resulting Hipparcos Catalogue, a high-precision catalogue of more than 118,200 stars, was published in 1997. The lower-precision Tycho Catalogue of more than a million stars was published at the same time, while the enhanced Tycho-2 Catalogue of 2.5 million stars was published in 2000. Hipparcos's follow-up mission, Gaia, was launched in 2013. The word "Hipparcos" is an acronym for High Precision Parallax Collecting Satellite and also a reference to the ancient Greek astronomer Hipparchus of Nicaea, who is noted for applications of trigonometry to astronomy and his discovery of the precession of the equinoxes.
Background By the second half of the 20th century, the accurate measurement of star positions from the ground was running into essentially insurmountable barriers to improvements in accuracy, especially for large-angle measurements and systematic terms. Problems were dominated by the effects of the Earth's atmosphere, but were compounded by complex optical terms, thermal and gravitational instrument flexures, and the absence of all-sky visibility. A formal proposal to make these exacting observations from space was first put forward in 1967. Even other space telescopes including (but not limited to) Hubble, are only capable of measuring relative astrometry, measuring positions and parallaxes by reference to nearby background stars/sources. Compiling a variety of such local, relative parallaxes results in regional errors, making distance comparisons between distant sky regions quite difficult. Hipparcos was the first project to engage in absolute astrometry: in addition to being an all-sky survey, Hipparcos introduced the concept of using two telescopes, separated by a wide "basic" angle, being directed onto a single focal plane. As a result, all measurements made by either telescope are connected to simultaneous measurements by the other. With a variety of transit angles on each individual source, it then becomes connected to many other parts of the sky. The final data reduction can then use these myriad distant sky connections to deduce a single global rigid reference frame by which to anchor all position/parallax measurements. (This is somewhat similar to how Earth coordinate systems were anchored by a global network of triangles.) The mission was originally proposed to the French space agency CNES, which considered it too complex and expensive for a single national programme and recommended that it be proposed in a multinational context. Its acceptance within the European Space Agency's scientific programme, in 1980, was the result of a lengthy process of study and lobbying. The underlying scientific motivation was to determine the physical properties of the stars through the measurement of their distances and space motions, and thus to place theoretical studies of stellar structure and evolution, and studies of galactic structure and kinematics, on a more secure empirical basis. Observationally, the objective was to provide the positions, parallaxes, and annual proper motions for some 100,000 stars with an unprecedented accuracy of 0.002 arcseconds, a target in practice eventually surpassed by a factor of two. The name of the space telescope, "Hipparcos", was an acronym for High Precision Parallax Collecting Satellite, and it also reflected the name of the ancient Greek astronomer Hipparchus, who is considered the founder of trigonometry and the discoverer of the precession of the equinoxes (due to the Earth wobbling on its axis).
Satellite and payload
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