ESA Science & Technology - INFO 22-1995: Hipparcos to deliver its final results catalogue

The Hipparcos project was recommended by the leading scientific advisors to the European Space Agency back in 1980. Its sole objective was to tackle he most rudimentary yet one of the trickiest questions in the whole of astronomy: in what directions do the various stars comprising our Milky Way lie? How far away are they? And how fast are they moving through the enormous expanses of space?

One million million bits of data were acquired by the satellite during its thee-year operational lifetime. After a flawless launch by an Ariane 4 rocket in August 1989, he boost motor on he satellite failed to ignite, and the satellite was destined to conduct its scientific observations from he wrong orbit. Ingenious ESA and industry engineers, assisted by scientists, a NASA ground- station, and strong political and scientific support, resolutely declined to forfeit he mission, and designed ever more complex control systems to keep he data flowing. In this way, contact between he satellite and the European Space Operations Centre (ESOC) - and a relentless stream of high quality scientific data - was maintained until August 1993. And since shortly after launch, number-crunching computers across Europe have been digesting he huge data stream, and piecing together the information in the largest computational jigsaw in the history of astronomy.

For thousands of years, astronomers have defined the sky's shape as an imaginary celestial sphere, with the direction to any single star given by angles like he longitude and latitude used by geographers. But the stars move, and the Earth is a moving, wobbling platform. Its atmosphere makes star images shimmer, and its gravity causes telescopes on its surface to droop as they scan the heavens. Observatories on the ground are limited to studying parts of he sky, and trying to piece together a celestial survey from the ground has run into insurmountable problems. The ingenuity of scientist involved in positional astronomy, or astrometry, led them, almost 30 years ago, to propose carrying out these observations from the relatively benign environment of space.

Hipparcos is, by present standards, a medium-sized satellite, with a 30 cm telescope sensing simply ordinary light. But it has been described as the most imaginative in the short history of space astronomy. This foresight has been amply repaid. In the long history of stargazing it ranks with the surveys by Hipparchus the Greek in the 2nd Century BC and by Tichy Brahe the Dane in the 16th Century AD, both of which transformed human perceptions of the Universe. Positions derived from the Hipparcos satellite are better than a millionth of a degree, and newly a thousand times more accurate than star positions routinely determined from he ground. This accuracy makes it possible to measure directly the distances to the stars. While it took 250 years between astronomers first setting out on the exacting task of measuring the distance to a star, and a stellar distance being measured for the first time, ESA's Hipparcos mission has revolutionised this long, painstaking, and fundamental task by measuring accurate distances and movements of more than one hundred thousand.

The measurement concept involved he satellite triangulating its way between he stars all wound the sky, building up a celestial map in much the same way as land surveyors use triangulation between hill-tops to measure distances accurately. Only the angles involved are much smaller : the accuracy that has been achieved with the Hipparcos Catalogue is such that he two edges of a coin, viewed from he other side of the Atlantic Ocean, could be distinguished. The results from Hipparcos will deliver scientists with long-awaited details of our place in he Milky Way Galaxy. Most of he stars visible to the naked eye are, to a large extent, companions of the Sun, in a great orbital march around the centre of the Galaxy, a journey so long that it takes individual stars 250 million years to complete, in itself a time so inconceivably long that only a few revolutions of our Galaxy have been completed since its formation - dinosaurs ruling our planet less than one quarter of a revolution ago. Hipparcos has measured the details of this cosmic procession, and will allow a clear disentangling of the motions of the individual stars; which lie relatively close to our Sun? Which are massive highly-luminous stars at great distances from it? Which are moving in an orbit close to our Sun's? and which are racing away from it hundreds of times faster than the speed of sound?

After eight years in the planning and construction stages, Matra Marconi Space, Alenia Spazio, and their 30 or so high-technology industrial partners delivered the Hipparcos satellite to ESA, on schedule and within cost. With the exception of its boost motor, the satellite and all its instruments functioned flawlessly, and indeed surpassed all expectations - in spite of he harsh environment that it was subjected to in its unintended orbit. The Matra Marconi Space Project Manager, Michel Bouffard, expressed his own satisfaction with the Hipparcos Catalogue completion, and remarked that "The Hipparcos project is definitely one of the best examples of intense and fruitful collaboration between ESA, industry and scientist".

A few more months work will be required before the Hipparcos Catalogue, and its companion Tycho Catalogue of more than a million stars, can be released by the European Space Agency. "We have to fit into the completed catalogue all of the double stars, and all of the magnitude measurements, that have been acquired," explained Dr Michael Perryman, ESA's scientist working with the European scientific teams, "But the schedule for this has now been drawn up," he added. Scientists who formulated parts of the satellite's observing programme, and who were involved in the analysis of the satellite data, will be given the first opportunity to try to understand what the data mean, from an astronomical perspective, when the results are delivered to them at the end of March 1996. The final product of the Hipparcos mission, a monumental 14-volume catalogue, with page upon page of astronomical results of unprecedented accuracy, will be published by ESA and the scientific teams at the end of March 1997.

The four leaders of he European scientific teams, who have been involved in the project since its earliest days, announced hat he Hipparcos and Tycho Catalogues have passed all he test that they have been able to devise to assess he quality of he data. Professor Jean Kovalevsky (Observatoire de la Cote d'Azur, Grasse, France), Dr Lennart Lindegren (Lund Observatory, Sweden), Professor Erik Hog (Copenhagen Observatory, Denmark), and Dr Catherine Turon (Observatoire de Paris-Meudon), and other scientific representatives from the ESA member states, are delighted with he latest developments. "There will be thousands of direct and indirect users of the Hipparcos results over the coming years," said Catherine Turon who lead the team responsible for putting together the satellite's observing programme, "And they will not only have access to a magnificent catalogue, but one which has been finalised somewhat more rapidly than our expectations before launch." Some ten thousand known double star systems have been observed by Hipparcos, and nearly ten thousand more have been discovered for the first time. In these binary systems, two stars orbit around each other, and Hipparcos promises to provide measurements of the masses of he stars in such systems, information which is impossible to determine in any other way. "Many of our scientific colleagues who have waited patiently for the results of this mission will be astonished with what the final catalogue contains," said Professor Hog, originator of the Tycho Catalogue concept, and one of the driving forces behind the mission.

In thousands of cases, the stars are seen to be variable by Hipparcos. In these cases, the light output from he star changes with time, and these changes have been accurately measured by the satellite. Astronomers rely on a subset of variable stars called Cepheids to measure the distances to galaxies beyond our own. Hipparcos measurement of variability and distances will underpin the use of Cepheids in estimating the scale and age of the Universe.

Completion of the Hipparcos Catalogue in December, when the present catalogue will have been supplemented with the information on double and variable stars, is timed to coincide with publication of the first scientific papers (in the European journal Astronomy & Astrophysics) which explain rigorously what has been done, and what these results are likely to mean. Generally, precise distances give precise luminosities for all kinds of stars. So theories of how stars burn, evolve chemically, change colour, and alter heir mass and diameter in the process, will be tested far more rigorously. The Hipparcos survey includes X-ray stars, giant stars throwing out clouds of gas, dying stars, and stars recently born ; the distances and energy emission of many of these objects have been a matter of guesswork until now. On top of his, motions of stars detected by Hipparcos animate he familiar stellar scenery. Each of the stars hat looks so still in he night sky is engaged in its own private odyssey through the Milky Way, which reflects its origins, and he architecture and dynamics of the Galaxy.

Jean Kovalevsky has been involved throughout the conception and execution of he mission. "Some people said Hipparcos was too difficult for Europe," he recalls. "Others thought it boring compared with more obvious astrophysical missions. Critics of the first kind are already put to shame. I can promise that the rest will be confounded too, when they see how our results impact on every branch of astrophysics, from planetary science to cosmology," he added.

As Hipparcos has now assessed the majority of the most luminous stars within 250 light-years of our Sun, it will give a vivid picture of our stellar surroundings. Experts will look at he catalogue contents to see whether any stars seem set to pass close to the Sun. Stars are born in clusters and retain a memory of their common origin in a shared motion through the Galaxy. In open clusters like the Pleiades, related stars have stayed quite close together. From the motions of stars in dozens of open clusters, Hipparcos is now set to establish their ages, and their course and speed through the Galaxy.

Looking at stars towards and away from the Galactic centre, the catalogue contains he relative speeds of stars in their orbits at different distances. It has also looked up and down at stars making temporary excursions outside the flat disc of the Milky Way, and at halo stars on orbits that keep them independent of the disc. Clues about the hidden mass of our Galaxy should be scattered liberally throughout the Hipparcos Catalogue.

The scientific papers which will be published in three months time already contain some surprising new results. The measurements provide one of he best tests of Einsteins's General Theory of Relativity carried out to date. By measuring the light of stars deflected from their straight line paths by he presence of our massive Sun, the Hipparcos scientists have been able to demonstrate that General Relativity is good to at least one part in a thousand. Other scientists have already been able to line up he astronomical reference framework established in he radio wavelength range with the new reference framework provided by Hipparcos. They can be aligned and superimposed with great precision, and for the first time answers to how radio and optical emission are related in different sources can be accurately studied. But the biggest surprise so far comes from a census of our Sun's very closest neighbours. many of he stars previously thought to lie only a few light years away from our Sun have been shown, through the remarkable accuracy of he Hipparcos measurements, to be much further away.

"The Hipparcos Catalogue is ready to lead fundamental positional astronomy into the 21st Century," said Dr Lennart Lindegren, leader of one of the data reduction teams, and one of the masterminds behind a follow-up astrometry mission which is currently being assessed by ESA's scientific advisors. The rnillionths of a degree accuracy delivered by Hipparcos could be upstaged by an experiment which seems capable of providing billionths of a degree accuracy, but this time on tens of millions of stars. It is a scientific field where creativity is ripe for harnessing. Such a mission would not only lay to rest any remaining elements of uncertainty in our understanding of distances and dynamics within our Galaxy.

By examining the occurrence of extra-solar planets around all of the tens of thousands of stars within a few hundred light years of our Sun it would provide a remarkable advance in our knowledge of planets around other stars and their formation process. And while unseen dark matter in space is now believed to account for 90 per cent of the total mass of the Universe, little is known about is physical nature: is it in black holes, planets, or exotic elementary particles? This is one of the most important question of astronomy for the coming decades, and his is where a future space astrometry mission would most probably make a great impact, since the patterns of motion of stars reveal the distribution of mass in space.

While the scientific community is eager to lay its hands on the riches of the Hipparcos mission, he astonishing potential of space astrometry at even higher accuracy levels is mobilising interest amongst he younger generations of astronomers throughout the world.