Hipparcos

Hipparcos (High Precision Parallax Collecting Satellite ) is a satellite for the purpose of astrometry. He was named after the Greek astronomer Hipparchus of Nicaea, the first time created a star catalog with over 1000 stars and discovered the variability of the star positions.

Course of the mission

Hipparcos was launched August 8, 1989, together with the German television satellite TV satellite aboard an Ariane 44LP 2. The satellite reached as planned the intended transfer orbit Geostationary ( Geostationary Transfer Orbit, GTO), in its distance from Earth 223-35652 km varied. However, the MARGIN II apogee of Hipparcos did not ignite and the satellite remained in his GTO orbit, rather than as intended to achieve a geostationary orbit, would have made ​​of the reciprocal of measurements angular distances of about 120,000 stars with previously unattainable precision will be. As a position above the equator 12 ° West planned.

With the help of a developed on this occasion new observation program for the course, a longer measurement period was originally intended as needed to allow the satellite to make its measurements of the less favorable orbit of the GTO from managed. Previously, the orbit was slightly enlarged with the help of the originally intended only for minor course corrections hydrazine thrusters so that the satellite orbited the earth is now at a distance of 526-35900 km altitude. This correction was necessary because friction effects of the residual atmosphere would otherwise have braked too strongly the satellite in the erdnäheren regions of the web. In this way, could not be made until the end of operation in June 1993 measurements, which even exceeded the original budget.

Measurement

For the accurate determination of the positions of stars in the Hipparcos a reflecting telescope with a mirror diameter of 29 cm and 1.4 m focal length was installed; using an additional mirror two sky regions were imaged at intervals of 58 ° at the same time. In the focal plane, a grating ( 8.2 micron spacing, corresponding to 1.2 " ) is placed by which the star brightness modulated periodically in the slow rotation of the satellite; the transmitted light was measured. For the measurements of the main catalog was an image dissector tube, a special form of a photomultiplier with adjustable " sight " is used; so that only one star were each recognized other stars whose light fell on the grid, could be hidden. From the brightness modulation, the star positions could be determined with each other in the direction of rotation; for the eventual position data Ausgleichungsrechnungen complex and the connection to ground-based observatories position data were necessary.

Results of the mission

Overall, the satellite over 1 million star positions determined, 118,000 of which have coordinates and movements with an accuracy corresponding to the angular diameter of a golf ball from 5000 km distance. The Hipparcos data (300 gigabytes) initiated - in addition to the automated measurement methods with today's CCD sensors - the renaissance of astrometry, and gave in the year of publication material for hundreds of essays of more than 1,000 astronomers.

The primary outcome are therefore measured positions of the stars, the ( epochs) were determined at several measuring points in time. From far apart in time epochs proper motions can be derived from positions at a distance of half years, the parallax and therefore the distances of the stars. To find the candidate star Hipparcos needed already so precise positions that extensive preparatory work with terrestrial telescopes were needed.

Hipparcos astrometry was for a significant milestone: the loci, parallaxes and proper motions of 118,000 stars were with a previously unattained precision of about 0.001 ", so a milliarcsecond, measured; they are listed in the Hipparcos catalog and available on the internet. In addition, surveyed a second instrument on board over a million stars with still considerable ± 0.02 ", which can now be found in the Tycho catalog. These two catalogs are the best realization of the new reference coordinate system on the sky ICRF. Now also allow amateur astronomers to accurately and semi-automatically calibrate with a telescope and digital camera each celestial object.

Hipparcos could repeatedly observe asteroids during its three-year lifespan. They were combined with precise meridian circle measurements ( La Palma and Bordeaux) to railway regulations that achieve accuracies of 0.04 "or 75 m. Next also star brightnesses were determined from the Hipparcos satellite as part of the Tycho catalog.

As a successor to Hipparcos, ESA's Gaia probe was successfully launched on 19 December 2013. You should measure at least 40 -fold accuracy about one billion stars. The German DIVA project, which was planned as preparation for Gaia has been canceled for financial reasons.