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To find anything to rival the new results on star positions and motions from the Hipparcos satellite, the European Space Agency's director of science has to look back 400 years. Commenting on the Hipparcos Symposium which commences in Venice on 13 May, Roger Bonnet compares it to astronomy in Denmark at the end of the 16th Century.
"ESA's Hipparcos brings the greatest step forward in star measurements since Tycho Brahe," Bonnet says. "When the Danish astronomer died in 1601, the German astronomer Johannes Kepler inherited his careful observations. Kepler used them to discover the laws of the motions of planets, and paved the way for Isaac Newton's gravitational theory. Now we have another multinational success story from European astronomy".
"Hipparcos began as an imaginative French concept to chart the stars by satellite," Bonnet continues. "ESA adopted the idea and many astronomers in our member states collaborated in the mission. A hundred-fold improvement in the accuracy of star positions may already alter the size of the Universe and the ages of stars. So don't be surprised if the results from Hipparcos are as revolutionary as Tycho Brahe's, in their impact on our knowledge of the cosmos."
The study of the Earth itself will benefit from the new star data. Wobbles of the Earth and variations in its rate of rotation can now be measured far more accurately. The ozone layer will be monitored by ESA's Envisat environmental mission, by looking for chemical alterations in the light from 1000 Hipparcos stars, when seen on lines of sight slanting through the atmosphere.
Even the erratic evolution of life on Earth may make more sense, as Hipparcos picks out stars that passed close enough to cause trouble here. Reliable identifications of stars heading towards or away from our vicinity were impossible before Hipparcos. The satellite measured shifts in the directions of stars in the sky with such high precision that astronomers can now pick out those few stars that scarcely change their bearings. Such stars are probably moving almost directly towards or away from us. A US-European team, led by Robert Preston at the Jet Propulsion Laboratory in California, used Hipparcos to search for nearby stars with very small shifts in position. They were, or will be, passers-by.
Gliese 710, an inconspicuous star in the constellation Ophiuchus, is currently 63 light-years away and approaching at about 14 kilometres per second. From the Hipparcos data, it will pass within about one light-year, one million years from now. Joan Garcia-Sanchez, a doctoral student in Preston's team, identifies Gliese 710 in one of the scientific posters that display Hipparcos results in Venice. Garcia-Sanchez has found evidence that Gliese 710 is today moving more slowly towards the Sun than it was several decades ago. That may mean it is orbiting around another star, so far unidentified. If so, the closest distance to which Gliese 710 will approach may be nearer or farther than in the team's initial estimate.
The stars of the Alpha Centauri system, at 4 light-years, are the nearest at present. Several stars investigated by Preston and his colleagues will come within 3 light-years during the next 8,500,000 years. Others have already passed by during a similar time-span and are now travelling away from us.
"A star coming too near could put the Earth at risk," Bob Preston explains. "It might dislodge comets from a swarm that surrounds the Sun in the Oort Cloud, and send them into the inner Solar System. Some comets could then collide with our planet. The fossils tell us of past disasters, in extinctions of many species, and we hope to identify culprits among stars now hurrying away from the scene. The theory isn't new, but only now can we check it, thanks to the amazing precision of Hipparcos."
Uncertainty about the timing of the stellar visits arises from inadequate information about the speed of approach or recession. That is measured from ground-based observatories, by shifts in the wavelengths of light (blueshifts and redshifts). A team led by Dave Latham at the Center for Astrophysics in Cambridge, Massachusetts, is busy making fresh observations to improve the ground-based data on the visitors, past and future.
The Hipparcos Catalogue and the wider Universe
Hipparcos results fill large catalogues due for release to the world's astronomers via the Internet and in CD-ROMs early in June, and in 16 printed volumes soon afterwards. These are results of four years of space observations, 1989-93, and a further three years of intensive computation. The Hipparcos Catalogue echoes the name of Hipparchus, who founded astrometry in ancient Greece. It gives precise data on 118,000 stars. A million stars, logged with lesser but still unprecedented ccuracy, comprise the Tycho Catalogue, named after Tycho Brahe.
The role of Albert Einstein's theory of gravity illustrates the extraordinary accuracy of the Hipparcos Catalogue. General Relativity predicts a deflection of starlight by the Sun's gravity. Hitherto this was confirmed only with objects seen within a degree or two of the edge of the Sun, where the effect is strongest. Hipparcos was sensitive to the bending of light-rays even in directions at right angles to the Sun.
If the computations had ignored General Relativity the star-fixing would be less accurate. Conversely, Einstein's predictions are found to be correct to within one part in a thousand.
As well as the positions, distances and motions of many stars, the Hipparcos Catalogue includes unprecedented surveys of double and variable stars. Hipparcos observed 24,000 double stars in the Hipparcos Catalogue, of which 10,000 were not previously known to be double.
Frequent observations, over the four years of the space operations, monitored the changes in brightness of 12,000 known variable stars, and discovered 8000 more.
The stars charted by Hipparcos are relatively close, compared with the galaxies. Eleven teams of astronomers have cooperated in linking them to the wider cosmos, defined in the International Celestial Reference Frame. They matched Hipparcos stars to galaxies near the lines of sight, to the major photographic surveys of the sky, and to the sharp observations in long-range astronomy that come from widely spaced radio instruments and the Hubble Space Telescope. Radio astronomers will henceforward be able to correct any mismatch, by comparing nearby radio sources in the Hipparcos Catalogue with the positions of distant quasars.
Jean Kovalevsky of the CJte d'Azur Observatory in Grasse, France, coordinated this work of uniting Hipparcos results with the cosmos in general. He is also the leader of the consortium called FAST (Fundamental Astronomy by Space Techniques) which is one of two multinational teams that worked in parallel to generate the basic Hipparcos Catalogue.
"We have related the stars of Hipparcos to the whole Universe of galaxies and quasars, to better than two tenths of a millionth of a degree," Kovalevsky reports. "We have also got rid of a possible rotation of the system of Hipparcos stars, to more than twice that precision. And the wide Universe gives us a more reliable reference frame for the sky and the stars than our spinning, wobbling and orbiting planet has ever provided."
Among the astronomical space missions already adopting Hipparcos data for better aiming is ESA's Infrared Space Observatory ISO. The forthcoming X-ray astronomy missions, NASA's AXAF and ESA's XMM, will rely on Hipparcos when searching the sky. Hipparcos stars will also guide deep space probes, including the NASA-ESA Cassini/Huygens mission to Saturn and Titan, and ESA's Rosetta mission to Comet Wirtanen.
The Tycho project progresses from 1,000,000 to 3,000,000 stars
The Tycho Catalogue gives the positions of many more stars, so although its accuracy is about one-tenth as good as the Hipparcos Catalogue's, it is the more generous source of greatly improved data for the world's astronomers. Plotting and characterizing eight times as many stars as the Hipparcos Catalogue, the Tycho Catalogue offers a comprehensive survey of the stars around the Sun. It includes 99.9 per cent of stars down to magnitude 10, which means stars 100 times fainter than the unaided human eye can normally see. Russian and German astronomers are now comparing the Tycho results on a million stars with positions of the same stars observed from the ground during the past 100 years, to measure their motions across the sky.
The Danish astronomer Erik Hog is a worthy heir of Tycho Brahe. He adopted a star mapper, installed in Hipparcos for checking the telescope's aim, as an additional source of astrometric data. From Copenhagen University Observatory he led the multinational Tycho Data Analysis Consortium (TDAC) which produced the Tycho Catalogue.
"A million million bits of data came from our star mapper in Hipparcos," Hog remarks. "We nearly drowned in the torrent when we started. So we limited the Tycho Catalogue to one million stars, in order to keep up with the work on the Hipparcos Catalogue. With more experience and faster computers, and based on the Hipparcos and Tycho Catalogues, we've started work on a Second Tycho Catalogue of three million stars. We hope to have it ready by 1999."