Geoscience Reference
In-Depth Information
3. Determination of Spin Properties
Photometry at visible wavelengths has been extensively used since many
years to derive information on the rotational state of asteroids. Lightcurves
are obtained to determine the spin periods, and lightcurve morphology is
also analyzed in order to derive some estimates of the overall shapes of the
rotating bodies. Moreover, having at disposal lightcurves taken at differ-
ent oppositions of the same object, corresponding to a variety of aspect
angles (the aspect angle being defined as the angle between the direc-
tion of the spin axis and the direction of the observer as seen from the
object's barycenter), makes it possible to derive the direction of orientation
of the spin axis (“asteroid pole”). Different techniques have been devel-
oped for this purpose.
3
Predictions concerning asteroid shapes and spin
axis directions based on ground-based photometry have been found to be
fairly accurate, according to the results of
in situ
investigations carried out
by space probes.
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One major advantage of observing from an orbiting platform with
respect to conventional ground-based observations is that in principle from
space it is easier to observe the objects in a wide range of observing geome-
tries, not limited to those corresponding to a limited time “window” around
the epoch of opposition. In particular, from space the asteroids can be
seen at small solar elongation angles, which are hardly achievable from the
ground. In this respect, Gaia will not be an exception. Each Main Belt aster-
oid will be typically observed tens of times during 5 years of planned oper-
ational lifetime. The simulations indicate that each object will be detected
at a wide variety of ecliptic longitudes. In particular, Gaia will observe each
object over a large fraction of the interval of its possible aspect angles. The
same variety of aspect angles, which is strictly needed to derive the orienta-
tion of the spin axis, can be covered from the ground only over much longer
times. This
apriori
opens exciting perspectives concerning the possibility
to use Gaia disk-integrated photometry as a very ecient tool to derive the
poles of the asteroids, as well as the sidereal periods and the overall shapes.
The main difference with respect to the situation usually occurring in
traditional asteroid photometry, is that in the case of Gaia we will not
have at disposal full lightcurves, but only a number of sparse photomet-
ric measurements lasting a few seconds (corresponding to a transit across
the Gaia focal plane), obtained according to the law that determines the
scanning rate of the sky by the satellite. This would seem in principle a
crucial limitation, but it is more than compensated by the high number
