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and to determine the relative abundance of these different subclasses, also
as a function of heliocentric distances. This will be important for studies of
the compositional gradient of the solid matter in the Solar System.
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Other
important applications of taxonomy will be discussed in Sec. 5. Here, we
stress that a new taxonomy is something that can be conceivably obtained
in the near future also by means of dedicated ground-based surveys like the
planned Pan-STARRS and/or the Large-Aperture Synoptic Survey Tele-
scope.
Per se
, the new taxonomic classification that will be produced by
Gaia is not a result that, alone, would be sucient to justify the costs of
a space mission. The mentioned coverage of the
B
region of the spectrum,
however, and the general link with other, unique results that will be pro-
duced by Gaia asteroid observations, make Gaia taxonomy a very welcome
and extremely useful addition to the already very rich scientific output of
this mission in the field of asteroid science.
5. Conclusions
Based on the considerations developed in this paper, we can expect that
in the future we will speak of two distinct eras in the history of asteroid
science: a pre-Gaia and a post-Gaia era. The post-Gaia era will be much
more advanced in many fundamental respects. Gaia astrometry will provide
us reliable measurements of mass for about 100 objects. For these same
objects we will know also the size and the overall shape, again obtained
from Gaia data, then we will have at disposal reliable estimates of average
densities. These objects will belong to different taxonomic classes, then
it will be possible to assess the relation between density and taxonomy,
to be possibly interpreted in terms of overall composition and structural
properties. This kind of knowledge cannot be realistically expected to be
achieved by other means in the next two decades.
The direct measurement of sizes will allow us to have an improved knowl-
edge of the asteroid size distribution down to 20 km in diameter. Today,
the vast majority of size data at our disposal come from indirect measure-
ments (thermal radiometry, polarimetry) and are subject to considerable
uncertainties. Direct size measurements, on the other hand, will allow us to
trivially derive the albedos of the same objects, being known their distances
at the epochs of the observations. In this way, we will check whether there
are dependencies of albedo upon size, as old results of IRAS radiometric
observations seem to indicate.
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An albedo variation due to space weather-
ing processes has been convincingly shown to exist for
S
-type objects by
the Galileo close-range images of asteroid 243 Ida,
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but it is not completely
