Geoscience Reference
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surveys, due to the above-mentioned capability to obtain accurate photo-
metric measurements also far from opposition, when the objects are located
at small solar elongation angles and are seen at different aspect angles.
The results of the simulations performed so far indicate that, whatever
choice will be made of the most suitable algorithm of inversion of pho-
tometric data, the Gaia data-set of disk-integrated photometry will be a
major resource to derive spin properties and overall shapes for a number
of objects that will not be smaller than 10,000, to be quite conservative.
Again, we stress that this will be possible by means of only 5 years of Gaia
observations.
4. Taxonomic Classification
The derivation of an asteroid taxonomy will be a direct by-product of Gaia's
spectrophotometric capability. According to current knowledge, there are
more than 200,000 main belt asteroids that will exhibit apparent magni-
tudes brighter than 20 in
V
light when detected by Gaia. A new taxonomic
classification will then be produced, as a fairly trivial exploitation the spec-
trophotometric data coming from the Gaia MBP, which will observe all
celestial objects in 11 bands across the electromagnetic spectrum between
about 3,000 and 9,500 A. In this respect, Gaia will have a couple of major
qualities: first, this huge spectrophotometric data-base will be obtained
using a unique, homogeneous photometric system, and not merging together
data coming from different instruments. Second, and equally important,
the spectral coverage of the MBP will certainly include also some bands
in the the blue region of the reflectance spectrum. This is quite important,
because the
U
and
B
regions of asteroid spectra, which were well covered
by classical UBV spectrophotometry based on photoelectric photometers
several years ago, tend now to be missed by the most recent spectroscopic
surveys, like SMASS and SMASS2.
7
,
8
This is a problem, because the blue
region is very useful to distinguish between several different sub-classes of
primitive objects. Among the hundreds of thousands of asteroids that are
expected to be taxonomically classified using the Gaia spectrophotometric
data-base, a large fraction will consist of primitive, dark objects belonging
to the so-called
C
-class complex, which dominate the asteroid inventory in
the outer region of the asteroid belt. In contrast to spectroscopic surveys like
SMASS and SMASS2 that were mostly limited to an interval of wavelengths
between 5,000 and 9,500 A,
8
Gaia spectrophotometric data are expected to
be better to discriminate among different subclasses of the big
C
complex,
