Geoscience Reference
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of single photometric measurements for each object, by the fact to have
data belonging to one single, homogeneous photometric system, and by the
good accuracy of Gaia photometry. The latter depends, in turn, on the
brightness of the target and varies for different detectors. It is expected
to be better than 0.01 mag for single detections of objects as faint as at
least
V
=18
.
5, using the Gaia astrometric field detectors. The photometric
accuracy is worse for the multi-band photometer (MBP), therefore MBP
data will not be used for the faintest asteroids, whereas they will be very
useful for brighter objects.
The magnitudes of the objects detected by Gaia at different epochs will
depend on several parameters: the most important ones are the sidereal
period, the shape and the orientation of the spin axis, and the illumina-
tion circumstances, described by the phase angle. Additional variations can
come in principle also from possible albedo variegation of the surfaces, but
this is not expected to be very relevant for the majority of the objects. The
possible existence of a non-negligible fraction of binary systems must also
be taken into account, but for the moment we will neglect this particular
complication of the problem. Gaia will obviously measure apparent magni-
tudes that will be immediately converted to absolute magnitudes (i.e., after
reducing apparent brightness to unit distance from Sun and Earth). More
precisely, it will be convenient to work in terms of
differences
of absolute
magnitude with respect to a reference observation of each object.
Assuming for sake of simplicity an object having a triaxial ellipsoid
shape, orbiting around the Sun along a typical main belt asteroid orbit, one
can plot how the absolute magnitudes are expected to vary as a function
of time, depending on the coordinates of its pole. An example is given in
Fig. 4. In this figure, we consider a simulated object having the same orbit
as the one of the main belt asteroid 39 Laetitia (which has a typical Main
Belt orbit). We assume a given triaxial shape (
b/a
=0
.
7and
c/a
=0
.
5) and
two slightly different choices for the spin axis orientation (ecliptic longitude
of the pole at 30
◦
, and ecliptic latitude of the pole at +60
◦
, left, and +30
◦
,
right). The plot shows the region in the time versus magnitude-difference
plane in which the object can be measured during a time span of 5 years,
in the two cases. We remind that we plot here the absolute magnitude
difference with respect to an arbitrary first observation obtained at a given
epoch. As a consequence, the shape of the domain in which the asteroid
can be observed during 5 years of mission depends not only on the assumed
asteroid pole, axial ratios and sidereal period, but also on the rotational
phase of the object at the epoch of the assumed first detection.
