Hipparcos [ESA]

The scientific goals of theHipparcos (High Precision Paralax Collecting Satellite)mission were the accurate measurement of the trigonometric parallaxes, proper motions, andpositions of 100,000 selected stars, mostly fainter than 10th magnitude.

The spacecraft consisted of two platforms and six vertical panels, all made of aluminumhoneycomb. The solar array consisted of three deployable sections. Antennas were locatedon the top and bottom of the spacecraft. An attitude and orbit-control subsystem ensuredcorrect dynamic attitude control and determination during the 2.5 year planned lifetime.The spacecraft spun around its Z-axis at the rate of 12 rev/day at an angle of 43 degreesto the sun. The Z-axis rotated about the sun-satellite line at 6.4 rev/year. Thespacecraft carried a single telescope which, in the focal plane, superimposed two fieldsof view 58 degrees apart. The attitude of the spacecraft about its center of gravity wascontrolled to scan the celestial sphere in a regular movement. The telescope used a systemof grids, at the focal surface, composed of alternate opaque and transparent bands. Behindthese grids, an image-dissector tube converted the modulated light into a sequence ofphoton counts 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 was obtained from thephase difference of the two star pulse trains. The telescope was an all-reflectiveeccentric Schmidt system. A complex mirror was employed which consisted of two mirrorstilted in opposite directions, each occupying half of the rectangular entrance pupil. Theunvignetted field of view was 94 arc-min by 54 arc-min. An additional photomultipliersystem, known as Tycho, viewed a beam splitter in the optical path and was used to gatherphotometric and astrometric data of 400,000 stars down to 11th magnitude. Measurementswere made in two broad bands corresponding to B and V in the Johnson BUV system. Thepositions of these latter stars were to be determined to a precision of 0.05 arc-s, whichis a factor of 25 less than the main mission stars. The mission was a facility type inwhich guest investigators propose particular research programs, and selected stars wereincorporated into the overall observing strategy.

The Mage-2 apogee motor failed and left the satellite in GTO, but the mission wasnevertheless successfully completed. On 15 August 1993, after more than three years of successful operations, communications with ESA's scientific satellite Hipparcos were terminated.

Launch Sites:

• Ko =   Centre Spatial Guyanais (CSG), Kourou, French Guiana, France   

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