Hipparcos - Background

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 proposal to make these exacting observations from space was first put forward in 1967.

Although originally proposed to the French space agency CNES, it was considered too complex and expensive for a single national programme. Its acceptance within the European Space Agency's scientific programme in 1980 was the result of a lengthy process of study and lobby. 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 also reflected the name of the Greek astronomer Hipparchus.