Hipparcos

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Hipparcos
Hipparcos
Type: Space telescope
Operator: European space agencyESA ESA
COSPAR-ID : 1989-062B
Mission dates
Dimensions: 500 kg
Begin: August 8, 1989, 23:25 UTC
Starting place: CSG , ELA-2
Launcher: Ariane 44LP V33
Status: in orbit, out of order
Orbit data
Rotation time : 637.2 min
Orbit inclination : 6.5 °
Apogee height :  35755 km
Perigee height :  544 km

Hipparcos ( Hi gh P recision Par allax Co llecting S atellite ) is a satellite for the purpose of astrometry . It was named after the Greek astronomer Hipparch of Nicaea, who was the first to compile a star catalog with over 1000 stars and who discovered the variability of the star locations .

Course of the mission

Hipparcos was launched on August 8, 1989 together with the German television satellite TV-SAT 2 on board an Ariane 4 4LP. The satellite reached the planned Geostationary Transfer Orbit (GTO) on schedule , in which its distance from the earth varied between 223 and 35,652 km. However, the lit MARGIN II - apogee of Hipparcos not and the satellite remained in his GTO orbit, rather than as provided a geostationary orbit to achieve the mutual from measurements angular distances of about 120,000 stars have made with hitherto unequaled precision to be. The planned position above the equator was 12 ° west.

The original plan was for the satellite to be operated geostationary and to be able to exchange data with the Odenwald ground station almost continuously for 24 hours a day. Due to the now elliptical orbit, the satellite was temporarily in the Van Allen Belt and could not collect any useful data during this time. In total, contact was only possible for less than 10 hours per day. In contrast to the planning, the ESA stations Odenwald , Kourou (French Guyana), Perth and Goldstone of NASA had to be used for contact with the satellite . The station in Villafranca served as a backup for the Odenwald station. The stations in Perth and Goldstone had to be upgraded with additional reception technology for this purpose.

With the help of a new observation program developed for this occasion, which of course required a longer measurement phase than originally planned, it was possible for the satellite to take its measurements from the less favorable orbit of the GTO. Previously, the orbit had been slightly enlarged with the help of the hydrazine correction engines, which were actually only intended for minor course corrections , so that the satellite now orbited the earth at a distance of between 526 and 35,900 km. This correction was necessary because the frictional effects of the remaining atmosphere in the regions of the orbit closer to the earth would otherwise have slowed the satellite down too much. In this way, measurements could be made by the end of operation in June 1993, which even exceeded the originally set goals. A total of 1000 Gbits of data was received.

measuring technology

A Schmidt telescope with a 29 cm mirror diameter and 1.4 m focal length and a field of view of 0.9 ° × 0.9 ° was installed in Hipparcos for the precise determination of the star positions ; With the help of an additional mirror, two regions of the sky were simultaneously imaged at a distance of 58 °. A grid (8.2 μm line spacing; corresponds to 1.2 ″ ) was placed in the focal plane through which the star brightness was periodically modulated as the satellite slowly rotated; the transmitted light was measured. The satellite should rotate once around its own axis within two hours and continuously carry out measurements. The rotation axis should be slowly changed so that the entire sky was covered. An image dissector tube , a special form of a photomultiplier with an adjustable “field of view”, was used for the measurements in the main catalog ; this meant that only one star was recorded at a time, other stars whose light also fell on the grid could be masked out. The star positions relative to each other in the direction of rotation could be determined from the brightness modulation; For the final position data, complex adjustment calculations and the connection to position data from terrestrial observatories were necessary. The data from the star sensor was used for the Tycho catalog; it was a by-product of the mission.

Results of the mission

In total, the satellite determined over 2.5 million star locations , 118,000 of them with coordinates and movements with an accuracy that corresponds to the angular diameter of a golf ball from a distance of 5000 km. The Hipparcos data (300 gigabytes ) initiated - in addition to the automatable measurement methods with today's CCD sensors - the renaissance of astrometry and provided material for hundreds of articles by more than 1,000 astronomers in the year of publication .

The primary result are the positions of the measured stars that were determined at several measurement times ( epochs ). Self-movements can be derived from epochs that are far apart in time , the parallaxes and thus the distances of the stars from positions at intervals of half a year . To find the candidate stars, Hipparcos already required such precise positions that extensive preparatory work with terrestrial telescopes was necessary.

Hipparcos was an important milestone for astrometry : The locations, parallaxes and proper motions of 118,000 stars were measured with a previously unattainable precision of about 0.001 ″, ie one milli-arcsecond (mas); they are listed in the Hipparcos catalog and are available on the Internet . In addition, a second instrument on board measured over 2.5 million stars. More than a million objects can be found in the Tycho 1 catalog from 1997 with a still considerable ± 0.02 ″ . The Tycho catalog was revised with an improved data reduction of the collected data. In this way, the accuracy could be increased and the number of objects in the Tycho 2 catalog grew to over 2.5 million.

The Hipparcos catalog and the two Tycho catalogs were the best implementation of the new reference coordinate system in the sky ICRF until the results of the Gaia mission were published . The positions of 100,000 bright stars up to a magnitude of 11.5 formed a new frame of reference, the Hipparcos Celestial Reference Frame (HCRF) with an accuracy of 1 mas. They now also allow amateur astronomers to accurately and semi-automatically each celestial object with telescope and digital camera to measure. Hipparcos was able to observe asteroids several times during its three-year tenure. They were combined with precise meridian circle measurements (La Palma and Bordeaux) to determine orbits that achieve accuracies of 0.04 ″ or 75 m. Star brightnesses were also determined by the Hipparcos satellite as part of the Tycho catalog.

The ESA Gaia probe was successfully launched on December 19, 2013 as the successor to Hipparcos . For Gaia DR1 , the first results of the Gaia mission were combined with the data from the Hipparcos mission and thus Tycho-Gaia Astrometric Solution (TGAS) with position, parallax (distance) and angular velocity for more than 2 million stars was realized. An accuracy of 0.3 mas was achieved for the position. Gaia DR2 no longer contains data from the Hipparcos mission and has published 1.7 billion objects with an accuracy between 0.02 and 2 milli-arcseconds. The later publications should, depending on the magnitudes and spectral classes, achieve accuracies of up to 25 micro-arcseconds. In addition, the Gaia mission can record spectra and use the Doppler effect to determine the radial velocities of the lighter objects , which was not yet possible with Hipparcos. The German DIVA project, which was planned in preparation for Gaia, was canceled for financial reasons.

literature

Web links

Commons : Hipparcos  - collection of images, videos and audio files

swell

  1. ^ Orbit data according to Chris Peat: Hipparcos - Orbit. In: Heavens Above. May 9, 2012, accessed May 10, 2012 .
  2. ↑ Hipparcos. National Space Science Data Center, accessed May 10, 2012 .
  3. ↑ a b Hipparcos. (PDF of 1.2 MB) ESA, October 2, 2001, p. 104/105 (p. 17/18 in PDF file) , accessed on December 2, 2012 (English).