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.Brown 2021 "Microarcsecond Astrometry: Science Highlights from Gaia" https://www.annualreviews.org/content/journals/10.1146/annurev-astro-112320-035628 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. Hipparcoss 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
The spacecraft carried a single all-reflective, eccentric Schmidt telescope, with an aperture of . A special beam-combining mirror superimposed two fields of view, 58° apart, into the common focal plane. This complex mirror consisted of two mirrors tilted in opposite directions, each occupying half of the rectangular entrance pupil, and providing an unvignetted field of view of about 1° × 1°. The telescope used a system of grids, at the focal surface, composed of 2688 alternate opaque and transparent bands, with a period of 1.208 arc-sec (8.2 micrometre). Behind this grid system, an image dissector tube (photomultiplier type detector) with a sensitive field of view of about 38-arc-sec diameter converted the modulated light into a sequence of photon counts (with a sampling frequency of 1200 Hz) from which the phase of the entire pulse train from a star could be derived. The apparent angle between two stars in the combined fields of view, modulo the grid period, was obtained from the phase difference of the two star pulse trains. Originally targeting the observation of some 100,000 stars, with an astrometric accuracy of about 0.002 arc-sec, the final Hipparcos Catalogue comprised nearly 120,000 stars with a median accuracy of slightly better than 0.001 arc-sec (1 milliarc-sec).
An additional photomultiplier system viewed a beam splitter in the optical path and was used as a star mapper. Its purpose was to monitor and determine the satellite attitude, and in the process, to gather photometric and astrometric data of all stars down to about 11th magnitude. These measurements were made in two broad bands approximately corresponding to B and V in the (Johnson) UBV photometric system. The positions of these latter stars were to be determined to a precision of 0.03 arc-sec, which is a factor of 25 less than the main mission stars. Originally targeting the observation of around 400,000 stars, the resulting Tycho Catalogue comprised just over 1 million stars, with a subsequent analysis extending this to the Tycho-2 Catalogue of about 2.5 million stars.
The attitude of the spacecraft about its center of gravity was controlled to scan the celestial sphere in a regular precessional motion maintaining a constant inclination between the spin axis and the direction to the Sun. The spacecraft spun around its Z-axis at the rate of 11.25 revolutions/day (168.75 arc-sec/s) at an angle of 43° to the Sun. The Z-axis rotated about the Sun-satellite line at 6.4 revolutions/year.
The spacecraft consisted of two platforms and six vertical panels, all made of aluminum honeycomb. The solar array consisted of three deployable sections, generating around 300 W in total. Two S-band antennas were located on the top and bottom of the spacecraft, providing an omni-directional downlink data rate of 24 kbit/s. An attitude and orbit-control subsystem (comprising 5-newton hydrazine thrusters for course manoeuvres, 20-millinewton cold gas thrusters for attitude control, and gyroscopes for attitude determination) ensured correct dynamic attitude control and determination during the operational lifetime.
Principles Some key features of the observations were as follows: through observations from space, the effects of astronomical seeing due to the atmosphere, instrumental gravitational flexure and thermal distortions could be obviated or minimised; all-sky visibility permitted a direct linking of the stars observed all over the celestial sphere; the two viewing directions of the satellite, separated by a large and suitable angle (58°), resulted in a rigid connection between quasi-instantaneous one-dimensional observations in different parts of the sky. In turn, this led to parallax determinations which are absolute (rather than relative, with respect to some unknown zero-point); the continuous ecliptic-based scanning of the satellite resulted in an optimum use of the available observing time, with a resulting catalogue providing reasonably homogeneous sky density and uniform astrometric accuracy over the entire celestial sphere;
the various geometrical scan configurations for each star, at multiple epochs throughout the 3-year observation programme, resulted in a dense network of one-dimensional positions from which the barycentric coordinate direction, the parallax, and the object's proper motion, could be solved for in what was effectively a global least squares reduction of the totality of observations. The astrometric parameters as well as their standard errors and correlation coefficients were derived in the process; since the number of independent geometrical observations per object was large (typically of order 30) compared with the number of unknowns for the standard model (five astrometric unknowns per star), astrometric solutions not complying with this simple five-parameter model could be expanded to take into account the effects of double or multiple stars, or non-linear photocentric motions ascribed to unresolved astrometric binaries; a somewhat larger number of actual observations per object, of order 110, provided accurate and homogeneous photometric information for each star, from which mean magnitudes, variability amplitudes, and in many cases period and variability type classification could be undertaken.
Development, launch and operations
The Hipparcos satellite was financed and managed under the overall authority of the European Space Agency (ESA). The main industrial contractors were Matra Marconi Space (now EADS Astrium) and Alenia Spazio (now Thales Alenia Space).
Other hardware components were supplied as follows: the beam-combining mirror by REOSC in Saint-Pierre-du-Perray, France the spherical, folding, and relay mirrors by Carl Zeiss AG in Oberkochen, Germany the external straylight baffles by CASA in Madrid, Spain the modulating grid by CSEM in Neuchâtel, Switzerland the mechanism control system and the thermal control electronics by Dornier Satellite Systems in Friedrichshafen, Germany the optical filters, the experiment structures and the attitude and orbit control system by Matra Marconi Space in Vélizy, France the instrument switching mechanisms by Oerlikon-Contraves in Zürich, Switzerland the image dissector tube and photomultiplier detectors by the Dutch Space Research Organisation (SRON) in the Netherlands the refocusing assembly mechanism by TNO-TPD in Delft, Netherlands the electrical power subsystem by British Aerospace in Bristol, United Kingdom the structure and reaction control system by Daimler-Benz Aerospace in Bremen, Germany the solar arrays and thermal control system by Fokker Space System in Leiden, Netherlands the data handling and telecommunications system by Saab Ericsson Space in Gothenburg, Sweden the apogee kick motor by SEP in France
Groups from the Institut d'Astrophysique in Liège, Belgium and the Laboratoire d'Astronomie Spatiale in Marseille, France, contributed optical performance, calibration, and alignment test procedures; Captec in Dublin, Ireland, and Logica in London contributed to the on-board software and calibration.