Ten years ago, in June 1995, the Royal Greenwich Observatory and ESA organised a workshop on `Future possibilities for Astrometry in Space' in Cambridge, UK. This was partly in response to the recommendation to ESA, by an independent review body - the Horizon 2000+ Survey Committee - to `initiate a Cornerstone-level programme in interferometry for use as an observatory open to the wide community. The first aim is to perform astrometric observations at the 10 microarcsec level'.
The aim of the workshop was to provide a forum for discussion for people interested in the scientific and technical aspects of the design of such a future astrometric space mission. At that time the Gaia concept envisaged two or three small interferometers for measuring astrometric parameters of tens of millions of stars throughout the Galaxy, down to 15 mag. No provision for acquiring a number of basic stellar parameters (e.g. radial velocity, metallicity, stellar type,..) was explicitly foreseen. The workshop was one of the first opportunities for members of the scientific community to express their interest in Gaia, and their willingness to participate in its scientific preparation. Papers presented were published in the workshop proceedings (ESA SP-379, September 1995, M.A.C. Perryman & F. van Leeuwen, eds).
In the intervening ten years comprehensive studies by ESA, industry and the scientific community have provided remarkable progress, demonstrating that these highly ambitious goals are indeed technically feasible. The interferometric approach has been replaced by a simpler monolithic optical design, improvements in CCD technology have been achieved, and the aim of detecting tens of millions of stars to a limit of 15 mag has been extended to cover around one billions objects to a completeness limit of 20 mag, including solar system bodies and extragalactic objects. On-board acquisition of radial velocities, along with broad-band and medium-band photometry, is now an integral and essential part of the present mission. On the downside, the 10 microarcsec target accuracies at 15 mag have been replaced by a slightly less demanding 20 microarcsec. The overall progress is highlighted by a number of significant milestones that have recently been reached: the successful completion of the major system-level studies and associated technological development activities (News item: 2005-06-29, 2005-06-09), the decision by ESA to issue the invitation to tender for Phase B2 and beyond to industry (News item: 2005-07-01), and the initiation of the organisation for the Gaia data processing tasks under the auspices of the Data Analysis Coordination Committee (News item: 2005-6-27). Remarkably, the schedule since 1995 has also been maintained: a launch date of somewhere between 2010-15 was noted at the meeting, ESA's SPC envisaged a launch before 2012 when the mission was accepted in 2000, and the current Phase B2/C/D implementation schedule has adopted a target launch date of the second half of 2011.
The photograph above shows the attendees at the joint RGO-ESA workshop on `Future possibilities for Astrometry in Space', Cambridge, UK, 19-21 June 1995. A larger version of the photograph can be viewed here, and a guide to people in this photograph can be found here.