Geoscience Reference
In-Depth Information
main targets, as in the case of the first asteroid flybys performed by the
Galileo space probe. More recently, a number of space missions have been
and are designed specifically for investigations of asteroidal targets. The
most recent fully successful mission of this kind has been accomplished
by the NEAR-Shoemaker probe,
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which has performed an extensive explo-
ration of the near-Earth asteroid 433 Eros. Another mission which is cur-
rently active and has produced spectacular results is the Japanese Hayabusa
mission, which touched down on the near-Earth asteroid 25143 Itokawa, but
is not thought to have succeeded in its final goal of collecting a surface sam-
ple, to be brought back to Earth. In this paper, we briefly describe another
space mission devoted to asteroids, that is currently under development,
and is supposed to be launched in 2006. This mission has been named
Dawn, and will have as specific targets, for the first time, two of the largest
main-belt asteroids, (1) Ceres and (4) Vesta. The main goal of this mission
will be to solve the “Ceres-Vesta paradox”, but more in general it will be of
primary importance to obtain a better understanding of the early phases of
planetary accretion in our Solar System. The rest of this chapter is devoted
to give a brief explanation of the “Ceres-Vesta paradox”, to explain how
Dawn will be important to solve it, and to give a general description of
the scientific payload of the probe, and its planned trajectory from the
Earth to its targets, as well as the current status of the mission preparation
activities.
2. The Ceres-Vesta Paradox
What we call the “Ceres-Vesta paradox” can be easily understood if we
first make a few preliminary assumptions concerning the mechanisms of
planetary growth and early heating of planetary bodies. In the first part of
this section we will describe the theoretical scenario as it has been until a few
months ago, when some new important developments have been proposed
by some authors. These new ideas will be discussed at the end of this section.
Let us then assume that (1) in the process of planetary growth the biggest
planetary embryos grow faster; (2) the fundamental sources of heating in
early planetary bodies were impact heating and the decay of short-lived
radiogenic nuclei like Al
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; (3) The largest bodies produced by planetary
growth, specially in the inner Solar System, likely preserved their relative
heliocentric distances over the age of the solar system.
All the above assumptions appear to be very reasonable according to
currently accepted models of Solar System formation, based on a large
