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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 as, for 20000 objects as faint as 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.
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
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.
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Next: 2. The GAIA Mission
Up: 1. The Historical Context
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Mattia Vaccari
2000-12-05