Geoscience Reference
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in situ
investigations, we know still so little about the most fundamental
physical properties of these objects, including masses, sizes, and average
densities.
The chances to improve significantly and in reasonably short times our
overall knowledge of the physical properties of asteroids by means of remote
observations are not so high, in spite of the imminent development of new
instruments which are expected to produce large amounts of spectrophoto-
metric data (Pan-STARRS, the Large-Aperture Synoptic Survey Telescope,
etc.). These new observing facilities will be very important to achieve an
improved knowledge of the overall inventory of Main Belt asteroids and
near-Earth objects (NEOs). However, the capabilities of these surveys will
be forcedly limited in their scopes (for instance, asteroid sizes cannot be
measured by means of spectrophotometry) and will be affected by intrin-
sic limitations due to the simple fact of being ground-based. To make an
example, several years of observations are usually needed to derive detailed
information upon properties like the spin axis orientation by means of disk-
integrated photometry, a task which is particularly suited to dedicated
asteroid surveys. The reason is that spin axis orientation can be derived
from photometric observations only when these cover a suciently large
interval of observing geometries (aspect angles), which are usually obtained
from observations corresponding to different oppositions of the same object.
Moreover, as mentioned above, none of new observing facilities will be able
to make direct measurements of fundamental physical parameters like size
and albedo (surface reflectivity), not to mention asteroid masses, which
will remain also essentially unknown in the future, but in cases of flybys by
space missions, and/or of discovered binary systems.
For the above reasons, a primary objective of asteroid science today is
to develop new tools to derive, possibly in relatively short times, reliable
measurements of masses, sizes (and hence, average densities), in addition to
reflectance and rotational properties, for a statistically significant sample
of the whole population. The development of such facility(ies) would mark
a real milestone in asteroid science, and would produce a real revolution in
this field. In this paper, we show that Gaia, one of the cornerstone missions
of the European Space Agency, can be this tool.
Gaia will be primarily an astrometric mission, reaching a level of
unprecedented accuracy in the measurement of positions and proper
motions of celestial bodies. In addition, Gaia will also have powerful pho-
tometric and spectroscopic capabilities. A large number of major advances
in practically all fields of modern Astrophysics are expected to come from
