Hipparcos is the first space mission dedicated to measuring the positions of the stars
measurements of star positions was the main activity of astronomers for several hundred years, until the early part of this century, during which time they tried to measure the motions, then later the distances, of stars
stars are so far away that even the nearest appears to 'wobble' (due to the Earth's motion around the Sun) by only about a second of arc (1/3600 of a degree)
the angles that have been measured by the Hipparcos satellite are very much smaller still - about one thousandth of a second of arc (0.001 arcsec)
this accuracy corresponds to the angular size of a golf ball viewed from the other side of the Atlantic Ocean; to the size of a person standing on the Moon as seen from the Earth; and to the growth of a human hair in 10 seconds viewed from a distance of about 10 metres
this remarkable accuracy has allowed astronomers to measure, for the first time, distances of tens of thousands of stars in our Galaxy, and the motions of these stars through space
the mirror of the Hipparcos telescope had to be manufactured so accurately, that if it was scaled up to the size of the Atlantic Ocean, the bumps in its surface would be only 10 cm high
1000 Gbit of data were returned from the Hipparcos satellite in its four years of operations. All of the data had to be combined together, making the production of the Hipparcos and Tycho Catalogues the largest data analysis problem ever undertaken in astronomy at the time.