Techniques d'observation spectroscopique d'astéroïdes

32 CHAPTER 1. WHY ASTEROIDS?
tel-00785991, version 1 - 7 Feb 2013
of years [Yin et al., 2002]. After this period strong solar winds begin to clear dust from the Solar
System, leaving the minor bodies and planets with a paucity of new material to accumulate.
The asteroids are remnants of the planetesimal population that once formed the planets.
Even if some of the asteroids were affected by thermal and dynamical evolution and by collisions,
most of them did not suffer a significant geological evolution preserving the physical
evidences related to the first 200 million years of the Solar System history .
Currently, there is a wide diversity of bodies in the Solar System, thus in order to facilitate
their studies and the discussions some definitions are required for different categories. The definitions
of a planet, dwarf planet, and of a small body given below were assigned in Resolution
5 and 6 of the IAU (International Astronomical Union) 2006 General Assembly 1 .
According to this resolution, a planet is a a celestial body that: a) is in orbit around the
Sun, b) has sufficient mass for its self-gravity to overcome rigid body forces so that is assumes
a hydrostatic equilibrium - nearly round shape, and c) has cleared the neighborhood around its
orbit.
A dwarf planet is a celestial body that a) is in orbit around the Sun, b) has sufficient mass
for its self-gravity to overcome rigid body forces so that is assumes a hydrostatic equilibrium -
nearly round shape, c) has not cleared the neighborhood around its orbit, and d) is not a satellite
of a planet. Plutoids are dwarf planets with a semi-major axis greater than that of Neptune. All
other objects except satellites orbiting the planets shall be referred collectively as small bodies
of Solar System (are also called minor planets).
Following these definitions, there are eight planets in our Solar System: Mercury, Venus,
Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto is a dwarf planet, also as Ceres,
Haumea, Makemake, and Eris. Ceres is located in the asteroid belt, while all others have
semimajor axes greater than that of Neptune.
The categories of small bodies of Solar System include asteroids, trans-neptunian objects
(denoted TNOs), comets and other small bodies. The size of the objects ranges from dust grains
and small coherent rocks up to hundreds of kilometers size boulders. These categories are
briefly presented bellow. The statistic data is taken from Minor Planet Center (MPC) website
(http://www.minorplanetcenter.org/iau/mpc.html).
Asteroids are referred to being rocky minor planets that orbit the Sun at distances ranging
from interior to Earth’s orbit up to Jupiter’s orbit [de Pater & Lissauer, 2010].
Comets are ice-rich bodies for which the volatile constituents sublimate during close approaches
to the Sun. They are characterized by a nucleus - the inner part of the body, the coma
- the spherical halo of sublimated material surrounding the nucleus, and two tails: a dust tail
trailing opposite the comet’s trajectory and an ion tail in the anti-Sun direction.
Centaurs are icy bodies orbiting between (and in some cases also cross) the orbits of Jupiter
and Neptune. There are 64 known Centaurs as of August 12, 2012. Centaurs are on chaotic
1 http://www.iau.org/