Friday, 16 May 1997

Beyond Hipparcos: Europe's astronomers want to pinpoint more stars, more exactly

The results of the European Space Agency's star-fixing satellite Hipparcos have enthralled the experts meeting in Venice this week. In the final sessions today (16 May) they discussed plans for a super-Hipparcos for the 21st Century. Hipparcos has improved on the precision of ground-based star measurements by a factor of a hundred. The new space mission would be at least a hundred times better still. Two eyes are better than one, and new ideas for space astrometry envisage the use of combinations of telescopes for visible light, exploiting the technique of interferometry long favoured by radio astronomers.

GAIA, an acronym for Global Astrometric Interferometer for Astrophysics, is the leading concept under study in the European Space Agency. It would carry two or possibly three pairs of mirrors spaced a few metres apart and orientated to look in different directions in the sky. The intended accuracy of star-fixing, to within about a billionth of a degree, makes GAIA an engineering challenge. Creating a stable optical arrangement may require active control of the spacing of mirrors and alignments of instruments. The spacecraft would fit in the nose-cone of an Ariane 5 launcher.

Other missions mooted in Venice include a German proposal DIVA, for a small-scale interferometer which could be a step along the road to GAIA's development. DIVA could be launched within a few years, and in 15 months in space it could chart a million stars with an accuracy three times better than that of Hipparcos. A Japanese concept called LIGHT would be more accurate than DIVA but less accurate than GAIA. NASA is considering a large interferometer, SIM, which would gain exceptional accuracy with mirrors spaced up to 10 metres apart, but it would point at chosen objects of interest rather than surveying the whole sky in the manner of Hipparcos and GAIA. An important motive for the SIM proposal is the detection of planets in the vicinity of other stars.

The enthusiasm of the Hipparcos astronomers for GAIA is not dampened by uncertainties in ESA's long-term science programme. These have been aggravated by cuts in the science budget since 1995 and by the need to replace the Cluster solar-terrestrial mission lost in a launch failure in 1996. GAIA would need much technological innovation, and it would rank as a Cornerstone mission.

The prospects for GAIA have therefore to be seen in the context of present efforts by ESA's planners to secure a steady sequence of Cornerstone missions, interspersed with medium-scale and small science missions. Four Cornerstone projects are already allocated, up to the FIRST far-infrared astronomy mission due at the end of 2005. The earliest launch for a fifth Cornerstone would be in the year 2009. A GAIA-type mission may be a candidate.

European scientists reviewing future mission opportunities in 1993-94 warmly commended the idea of an interferometer for astrometry using visible light. They made no final choice between that and another enchanting idea -- the use of an interferometer to make infrared observations of cosmic objects as sharp as those of the Hubble Space Telescope in visible light. For the time being, rival teams of scientists are honing their proposals for both interferometric concepts. Astrometrists believe that the success of Hipparcos will count in favour of a GAIA-type mission.

"Hipparcos is a highly original contribution from Europe to space astronomy," comments Michael Perryman, ESA's project scientist and himself a leading advocate for GAIA. "Much of the case for consolidating and extending it, in a new mission giving a dramatic stereoscopic view of the Galaxy, has been made in Venice this week. Astronomers associated with Hipparcos have presented initial findings from their first sight of the results. They have already reaped an amazing harvest of new knowledge. When astronomers all around the world begin to work with the Hipparcos data, starting next month, the payoff will multiply and the pressure will grow for an even better star-fixing mission."

The prime contractor and co-prime contractor for the Hipparcos satellite, Matra Marconi Space and Alenia Aerospazio, have been co-sponsors of this week's symposium. Their representatives in Venice made clear the enthusiasm of the aerospace industry to face the engineering problems of GAIA in forthcoming feasibility studies. "The most challenging task will be to fit an instrument of the required performance into the limited dimensions and mass of the satellite," commented Bruno Strim of Alenia Aerospazio.

Planet hunter par excellence

Even to astronomers newly accustomed to the astounding precision attained wth Hipparcos in charting our corner of the Milky Way Galaxy, the prospectus for GAIA is awesome. Perhaps hundreds of millions of stars could be mapped and characterized across most of the Galaxy, and direct measurements of distances to globular star clusters and the Magellanic Clouds would be feasible. The distribution of the mysterious Dark Matter which makes up most of the mass of the Galaxy could be inferred with high accuracy. As a bonus, any flaw in Einstein's theory of gravity, even as slight as one part in a million, would be revealed by GAIA.

The new star-fixer would be a planet hunter par excellence. Computer simulations reported by Mario Lattanzi of Turin Observatory and his colleagues indicate that GAIA might detect perturbations of a star by a planet no bigger than the Earth, out to a range of 65 light-years, and Jupiter-like planets at more than 300 light-years. Thus GAIA could check 100,000 stars or more, for the presence of planets.

Various designs for telescopes and instruments in GAIA were offered at the symposium. Fabio Favata of ESA's Astrophysics Division suggested how a small auxiliary instrument might add greatly to the value of the mission. By analysing the starlight with high accuracy, a spectrograph of advanced design could measure the speeds of stars towards or away from the Earth (blueshifts and redshifts) to an accuracy of 1 kilometre per second. At the same time, Favata noted, the spectrograph would give basic data on the chemical composition of each star.

Hipparcos star of the day: Beta Hydri and the fate of the Sun

Meanwhile the impact of Hipparcos results on astronomy, evident in the large number of talks and posters at the Venice Symposium, is only just beginning. Better knowledge of stars resembling the Sun is a case in point. In a two-way exchange of information, intimate knowledge of the star on which human life depends helps the astrophysicists to evolve their theories of other stars. The behaviour of sun-like stars, including changes in their magnetic cycles, gives clues to possible fluctuations in solar activity.

Beta Hydri in the southern constellation Hydrus (not to be confused with Hydra) is a star just 10 per cent more massive than the Sun. It is nearing the end of its normal life as it exhausts the hydrogen fuel in its thermonuclear core, and it therefore gives a strong impression of what will happen to the Sun in its old age. Hipparcos has altered the distance of Beta Hydri from 21 to 24 light-years. This correction has a drastic effect on the age of Beta Hydri, reducing it from 9.5 to 6.6 billion years, according to calculations by the Canadian theorist Don VandenBerg.

Dainis Dravins of Lund Observatory, Sweden, presented the new findings about Beta Hydri in Venice. He has studied the star for many years, using also ESA's X-ray observatory EXOSAT and the International Ultraviolet Explorer, IUE. He notes that the ageing seen in Beta Hydri is a very gradual process, and makes scenarios for the death of the Sun less shocking.

"Some billions of years from now," Dravins says, "the Sun will indeed age and swell and grow more luminous. But it will also lose mass, and the planets will move outwards into wider orbits. With a more luminous Sun, the outer planets and moons of the Solar System will become warmer. A spacefaring civilization might therefore endure on Mars, or on the moons of Jupiter and Saturn, long after the Earth becomes uninhabitable. Beta Hydri tells us that the period of benign senility could last for several billion years."

Background summary of Hipparcos and the Venice symposium (repeated from previous releases)

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.

ESA will release the catalogues to the world's astronomers in June. Meanwhile astronomers who helped in the mission have had privileged access to the data. At the Hipparcos Symposium in Venice 13-17 May they gave their early conclusions in 190 reports, in the form of talks or scientific posters. The results affect many branches of astronomy from asteroids to cosmology, but especially the theories of stellar physics and evolution, because Hipparcos provides, for the first time, accurate distances to many thousands of stars.