N° 37–1993: Hipparcos: mission accomplished

During the last few months of its life, as the high radiation environment to which the satellite was exposed took its toll on the on-board system, Hipparcos was operated with only two of the three gyroscopes normally required for such a satellite, following an ambitious redesign of the on-board and on-ground systems. Plans were in hand to operate the satellite without gyroscopes at all, and the first such "gyro- less" data had been acquired, when communication failure with the on-board computers on 24 June 1993 put an end to the relentless flow of 24000 bits of data that have been sent down from the satellite each second, since launch. Further attempts to continue operations proved unsuccessful, and after a short series of sub-systems tests, operations were terminated four years and a week after launch.

An enormous wealth of scientific data was gathered by Hipparcos. Even though data analysis by the scientific teams involved in the programme is not yet completed, it is clear that the mission has been an overwhelming success. "The ESA advisory bodies took a calculated risk in selecting this complex but fundamental programme" said Dr. Roger Bonnet, ESA's Director of Science, "and we are delighted to have been able to bring it to a highly successful conclusion, and to have contributed unique information that will take a prominent place in the history and development of astrophysics".

Extremely accurate positions of more than one hundred thousand stars, precise distance measurements (in most cases for the first time), and accurate determinations of the stars' velocity through space have been derived. The resulting HIPPARCOS Star Catalogue, expected to be completed in 1996, will be of unprecedented accuracy, achieving results some 10-100 times more accurate than those routinely determined from ground-based astronomical observatories. A further star catalogue, the Thyco Star Catalogue of more than a million stars, is being compiled from additional data accumulated by the satellite. These catalogues will be of enormous value in astronomers' attempts to understand and describe the properties and evolution of stars, and the dynamical motion of these stars within our Galaxy. In the process, HIPPARCOS has discovered many thousands of new binary star systems, measured the precise light variations of many hundreds of thousands of stars over its operational lifetime, and has provided an accurate and independent validation of the predictions of General Relativity.

Scientists working with ESA on the HIPPARCOS programme, were at ESOC on 13-14 July to review the progress of the data processing, and to examine whether any further efforts might allow the satellite to continue operating. "All of us are sorry to see the end of this remarkable satellite" said Dr. Michael Perryman, ESA scientist responsible for HIPPARCOS, "On the other end, we are delighted that it has delivered substantially more than it had been originally designed for. When our final results are published, some very interesting new insights into the nature of our Galaxy, its structure and its evolution, will emerge" he added.

A large team of scientists from the various ESA member states are responsible for the analysis and interpretation of the vast amount of data that has been generated by the HIPPARCOS satellite, in what is considered to be the largest single data processing challenge ever undertaken in astronomy. Working with ESA since the time of the mission acceptance in 1980, their immediate work will only end with the publication of the HIPPARCOS and Tycho Star Catalogues later this decade. Only then will an astrophysical exploitation of the results commence. Proposals have already been submitted to ESA to follow up its successful breakthrough into space astrometry with new missions proposed for launch early in the next millennium.

Note for Editors :

The Hipparcos mission was accepted within the ESA mandatory scientific programme in 1980. With overall management by ESA, the system prime contractor was Matra Marconi Space (France), also responsible for the payload and the on-board software development: with Alenia (Italy) responsible for the procurement of the spacecraft, as well as integration and testing of the complete satellite. A total of 35 European firms were involved in the satellite construction. The satellite was launched by Ariane 4 on 8 August 1989.
Hipparcos was named after the pioneer Greek astronomer Hipparchus who compiled a detailed star map in around 120 BC and, by comparing it with observations made by his predecessors, established that the Earth's rotation axis slowly changed its direction in space.

The Hipparcos satellite carried out its measurements in a highly elliptical 10-hour orbit, ranging between 500 km and 36,000 km above the Earth's surface, resulting from non-functioning of the satellite's apogee boost motor shortly after launch. A redesign of the on-board attitude control system, and the addition of two more ground stations into the control network, nevertheless allowed ESA's operations team at ESOC (Darmstadt, Germany) to operate the satellite with close to full efficiency.

For the 20-strong operations team at ESOC and the ground station teams in Perth (Australia), Goldstone (USA) and Kourou (French Guiana) the mission has been highly interesting, challenging and very demanding. "After more than three years of excellent performance, underlining the remarkable quality of the satellite's design and construction, it was showing its age in several areas" said Dietmar Heger, ESOC's Spacecraft Operations Manager for Hipparcos. "The orbit was subjected to very significant levels of high energy electron and proton radiation, much higher than those expected in its intended geostationary orbit. We cannot be surprised or disappointed that the satellite has finally ceased to function" he said.

The scientific activities associated with the Hipparcos mission are under the responsibility of four European scientific teams, together comprising about 100 scientists from universities and scientific institutes, and led by Professor Erik Hoeg (Copenhagen University Observatory, Denmark), Professor Jean Kovalevsky (Observatoire de la C.te d'Azur, France), Dr. Lennart Lindegren (Lund Observatory, Sweden) and Dr. Catherine Turon (Observatoire de Meudon, France).

Final accuracies on the stellar positions, distances and annual motions measured by Hipparcos, are in the range 1-2 milli-arcsec (i.e. 1-2 thousandths of a second of arc, equivalent to the angular size of a golf ball viewed from the other side of the Atlantic Ocean). Distances of stars many millions of times further away than our own Sun are being measured, and their velocities are being measured with accuracies of a few hundred metres per second. A further million stars will be contained in the accompanying Tycho Catalogue, with positions not as accurate as those derived from the main experiment, although still considerably more accurate than those measurable from the ground.

At the heart of the satellite was an extremely accurate optical telescope, which measured the separations between stars in different parts of the sky as the satellite turned slowly around its axis. The telescope mirrors on Hipparcos were so accurately polished that, if scaled up to the size of the Atlantic Ocean, deviations of the surface would nowhere exceed 10 cm in height. The global positional accuracies determined by Hipparcos are impossible to achieve from the Earth's surface, where the atmosphere, the temperature variations, instrument flexure, and the absence of all- sky visibility provide insurmountable barriers to ground-based observers.

The Hipparcos results will represent a milestone in mankind's understanding of the structure and evolution of our Galaxy, and an invaluable legacy to future generations of astronomers.