Geoscience Reference
In-Depth Information
1. Introduction
In spite of the tremendous improvement in our understanding of the physi-
cal properties and overall histories of the asteroids achieved in recent years
through major theoretical advancements and detailed observational data,
from both remote and in situ investigations, several problems are still open.
The asteroids are bodies of fundamental importance for a number of rea-
sons. First, they are the outcomes of a process of partly aborted planetary
growth. As such, they are bodies in some sense intermediate between fully
grown major planets, and the original planetesimals from which planets
were originated. The asteroids are a strongly heterogeneous population,
and can be classified into a number of distinct taxonomic classes. At least
some of these classes are thought to include very primitive objects, likely
identical in mineralogy to the most primitive meteorites, the carbonaceous
chondrites, which are samples of the oldest material present today in the
Solar System. Other taxonomic classes are thought to correspond to bodies
that, during their histories, experienced different degrees of thermal evo-
lution, ranging from short episodes of mild heating, to complete melting
and differentiation. Given this large diversity, asteroids are very important
remnants of the early epochs of the Solar System history, and represent
a strong constraint for the theoretical models of formation of our plan-
etary system. In addition to purely theoretical considerations, asteroids
are also important from more pragmatic points of view. In particular, it is
now widely recognized that episodes of catastrophic impacts with asteroidal
bodies have likely determined big changes in the evolution of the Earth's
biosphere in the past. Understanding the inventory and size distribution of
the potentially hazardous objects is a task to whom large resources have
been allocated in recent years in several countries. It is clear, however, that
also the basic physical properties of the asteroids, including their overall
composition and their internal structure, must be understood in order to
be able to develop any credible strategy of mitigation of the impact hazard.
Finally, it should also be mentioned that asteroids may be in the future an
important source of useful materials for the development of space activities
and the survival of human settlements in the inner solar system.
The above considerations are sucient to explain why asteroids have
been and are extensively studied. In addition to theoretical investigations
and remote-sensing observations, in recent years a major breakthrough has
been obtained by means of direct in situ investigations by means of space
probes. The first data came from missions that had other bodies as their
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