1.4 GAIA and the Near Future of Space Astrometry

We now live in what may be called the Golden Age of Space Astronomy. Many spacecraft are being launched every year, and most of these provide us with the deepest and highest-resolution images ever, thus allowing the investigation in greater detail of long-standing problems and the discovery of completely new phenomena. Even if, due to budgetary reasons, the majority of astronomical observations will still be carried out from the ground, the future prospects of space astronomy are very bright, since the development of new kinds of space telescopes is underway. In particular, even if conventional imaging and spectroscopic telescopes will still play an important role, the launch of interferometric telescopes is likely to prove the most significant achievement of observational astronomy in the next twenty years or so.

As for astrometry, some tasks could be performed with large space telescopes like the HST, but most will require, if only for the premium on HST-like telescopes' observing time, a smaller dedicated satellite. The missions that are currently being planned for the near future show a keen interest in both imaging and interferometric telescopes. For instance, NASA has already approved two astrometric missions, FAME and SIM, whose launch is expected around the middle of the upcoming decade. FAME is an imaging mission, namely a small Hipparcos-like scanning satellite superposing the images from two fields of view on a CCD-covered focal plane, and will perform an all-sky astrometric and 4-color photometric survey of the 40 million brightest stars. On the other hand, SIM will be the first space mission to make use of optical interferometry, and its main goal is to perform astonishingly accurate astrometric observations, at the level of 3 $\mu$as, for 20000 objects as faint as $V=20$ mag. These two missions are remarkably complementary in both design and scientific goals, and illustrate the twofold need of modern observational astronomers: small datasets of superb accuracy and large, uniform databases of somewhat lower accuracy. In its present design, the GAIA mission would satisfy, at least to a certain degree, both these demands with an astrometric, photometric and spectroscopic all-sky survey of the highest accuracy. In Table 1.2, GAIA performance is compared to that of the other mentioned missions. Note that the astrometric accuracy is a strong function of the observed object's magnitude, and that only the accuracy at the limiting magnitude is given here for all missions, while the accuracy expected for GAIA at different magnitudes is given in Section 2.7. Figure 1.1 illustrates the history of astrometric accuracy from Hipparchus to GAIA. The dramatic potential of space observations is shown by the jump in accuracy achieved by Hipparcos over ground-based observations and the still greater progress achievable by GAIA with respect to Hipparcos.

Table 1.2: GAIA measurement capabilities with respect to Hipparcos and future astrometric missions approved to date. Name of the mission, funding space agency, expected year of launch, expected limiting magnitude in the $V$ band, total number of observed stars and astrometric accuracy at the limiting magnitude expressed in mas. USNO stands for the United States Naval Observatory . See text for details about the quoted accuracy.

Mission	Agency	Launch	$V_{lim}$	Stars	Accuracy at $V_{lim}$
Hipparcos	ESA	1989	12	120000	2
FAME	USNO/NASA	2004	15	40 million	0.220
SIM	NASA	2005	20	$>$ 20000	0.003
GAIA	ESA	2009	20	$>$ 1 billion	0.200

Figure 1.1: Errors of best star positions and parallaxes in history. Accuracy and number of measured stars are indicated. A conservative number of 50 million stars at an accuracy of $10~\mu\textrm{as}$ is here indicated for GAIA for both positions and parallaxes, but the number of measured stars at this level of accuracy may be substantially larger (see Table 2.2). Courtesy of Erik Høg, Copenhagen University Observatory.