In a special session on the history of the European Space Agency's Hipparcos mission, 14 of the scientists and engineers most closely responsible for it described their efforts over 17 years to conceive, build and fly the satellite, and translate its observations into catalogues of the stars. Built for ESA by Europe's aerospace industry, and sent into orbit by an Ariane 4 launcher, the Hipparcos satellite scanned the sky for nearly four years, 1989-93, measuring angles between stars. Protracted calculations by multinational teams then produced the Hipparcos Catalogue giving the positions and motions of 118,000 stars with 100 times the precision of previous surveys. The accompanying Tycho Catalogue contains a million stars charted with lesser but still unprecedented accuracy.
High drama punctuated the years of patient work when, after its launch, Hipparcos failed to reach its intended orbit. "It was a real shock," said Dietmar Heger of ESA's European Space Operations Centre (Darmstadt). The initial prediction was that Hipparcos would last for only 9 months and accomplish only 5-10 per cent of its science programme. Quick action by the multinational teams extended the satellite's life, and ground stations hurriedly recruited around the world enabled Hipparcos to fulfil its task of pinpointing the stars.
At the close of the celebration session the president of the International Astronomical Union, Lodewijk Woltjer of the Observatoire de Haute-Provence, France, hailed Hipparcos as "a triple success" -- for the scientific community, for European engineering, and for European cooperation. Now the science and technology exist for a star-mapping mission a hundred times more accurate than Hipparcos, but the ability of ESA to follow up its success is uncertain at a time when the budget for science is shrinking.
"Europe will soon have to decide," Woltjer said, "if it wishes to spend what is needed to remain an independent player in the top league of space research, or if it is content to fall back to the distant second place it had some decades ago."
Open clusters of stars, of which the Pleiades cluster in the constellation Taurus is the most famous, are the subject of several posters. The stars of an open cluster all formed at the same time but have different masses. Some cluster members are small and relatively faint, while others like the "Seven Sisters" seen in the Pleiades with the naked eye are massive and very luminous. Clusters therefore give astronomers invaluable clues to the behaviour and evolution of stars of different masses. The Pleiades have for long been the astronomers' favourite cluster for honing their theories, but Hipparcos now reveals that the stars of the Pleiades are far from typical.
Jean-Claude Mermilliod of the University of Lausanne, Switzerland, together with colleagues at the Observatoire de Paris-Meudon, has prepared a poster comparing the Pleiades with half a dozen other nearby clusters. The results are puzzling. Differences in chemical composition, for example between the well-known Coma Berenices and Beehive (Praesepe) clusters, have remarkably little influence on the luminosities of the stars of different mass. On the other hand Hipparcos brings the Pleiades substantially nearer than astronomers had supposed them to be, and as a result the stars are less luminous than they should be according to previous theories.
A co-author of this and other contributions concerning the Pleiades is Catherine Turon of the Paris-Meudon Observatory. As leader of the Input Catalogue Consortium she has been an outstanding figure in the Hipparcos mission, and is now well-placed to judge where the early results may be leading. To the question, "Why are the Pleiades so different?" she offers two possible answers.
"Perhaps there is something radically wrong with our theories of stellar evolution," Turon says. "Another and more intriguing possibility is that the Pleiades, and perhaps some other star clusters, represent a zone of the Galaxy which is chemically different from most of the disk, and younger in an evolutionary sense."
Hipparcos shows that the accepted distance of Beta Pictoris from the Earth, 53 light-years, seriously underestimated its true range. By fixing the distance of Beta Pictoris at a much more reliable figure of 63 light-years, the Hipparcos correction sharpens astronomers' knowledge of the star and its dust. The dust-ring is almost 20 per cent wider than they previously supposed. Technical uncertainties about the character of the star and the density of the dust are eliminated. But the star is no infant and may be up to 100 million years old. That aspect of the results lends support to the idea that, if Beta Pictoris is a planet-making star, its work may be largely complete. The new inferences from Hipparcos are reported to the Venice Symposium by Françoise Crifo of the Paris-Meudon Observatory and her colleagues.