• Completed
• Launched 8 August 1989
• Ended August 1993
• Results updated 2007

Hipparcos was the first mission dedicated to measuring the positions and motions of stars. This branch of astronomy is known as astrometry. In the process, Hipparcos also measured the brightness and colours of the stars it mapped. It is the predecessor of ESA's Gaia mission, due for launch in 2011.

As well as giving a 3-D picture of the distances between stars close to our Solar System, data from the satellite was also used to confirm the value of a basic parameter in Einstein’s Theory of General Relativity, describing the bending of light around the Sun.

The data from Hipparcos was originally published in a catalogue released in May 1997. Thanks to recent advances in computational processing power and extensive investigations of the Hipparcos data, it has been possible to revisit the original data and significantly improve the accuracy of the derived catalogue.

The latest catalogue from the mission went online in January 2008.

Mission facts

• The satellite was named after Hipparchus of Rhodes, a Greek mathematician and astronomer who lived from 190 to 120 BC. Hipparchus is known as a ‘father’ of astronomy for his work in classifying stars into six categories of brightness known as Magnitudes.
  
  
• At its time of operation, the Hipparcos spacecraft gathered more data than any previous project in the history of astronomy. Its successor, the Gaia mission, is designed to produce data 10 to 100 times more accurate, and for 10,000 more objects, collecting altogether over one thousand times more data during its five year mission.

Technology

While in orbit, the 1.1 tonne satellite turned slowly on its axis to see every part of the sky at least twice every six months, scanning it in at least two different directions.

Each star was measured on average 115 times over the mission.

By observing the sky through two telescope apertures simultaneously, it was possible to derive accurate positions for more than 50,000 stars.

UK involvement

Representatives from the Royal Greenwich Observatory, now based within the Institute of Astronomy, part of the University of Cambridge, were an essential part of one of the two data processing consortia that worked on the original data and contributed to the Hipparcos data catalogue released in 1997.