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clear if the same processes can alter also the reflectance properties of objects
belonging to other taxonomic classes (i.e., having different surface compo-
sitions). Gaia will give the correct answers to these major open questions.
The determination of the rotational state for a set of more than 10,000
main belt asteroids will make it possible to use the spin properties of main
belt objects as a new powerful constraint to the models of the collisional evo-
lution of this population. So far, the models have been mainly constrained
by the size distribution, but since collisions strongly affect the spins, Gaia
data will add a new full dimension to the problem.
The multi-band observations carried out by Gaia for several tens of thou-
sands of objects will be useful not only to derive a new taxonomy, but also
to add spectral reflectance data as a further constraint to identify asteroid
families. These are groupings of objects having similar orbital properties,
which constitute the outcome of catastrophic events of collisional disrup-
tions of single parent bodies. Families have been so far identified mostly on
the basis of similarities in the orbital proper elements of their members,
11
but it has become evident that spectral properties can be added as an addi-
tional element to identify objects having a common collisional origin.
11
At
the same time, the spin properties derived for family objects will allow us to
test the possible existence of preferential alignments in the spin properties,
which have been recently found to likely exist in the case of the Koro-
nis family
12
and might be interpreted as a direct evidence of the so-called
YORP effect, a kind of thermal torque that is believed to be important in
the evolution of asteroidal spins.
13
If current plans will be respected, Gaia will be launched in 2011. The
present generation of planetary scientists will have a chance to participate
in this predicted revolution in our knowledge of minor planets.
Acknowledgments
We thank F. Mignard for putting at disposal of the Gaia Solar System
working group the results of his simulations of Gaia asteroid observa-
tions. We would like to thank him, as well as K. Muinonen, D. Hestroffer,
J. Virtanen, M. Kaasalainen, C.-I. Lagerkvist and V. Zappala for very useful
discussions.
References
1. P. Tanga, in
Proceedings of the Symposium
“
The Three Dimensional Universe
with Gaia
”, eds. C. Turon, K. S. O'Flaherty and M. A. C. Perryman (ESA
Publication Division SP-576, ESTEC, Netherland, 2005), p. 243.
