Asteroids - Extreme Events: A Physical Reconstruction and Risk Assessment - page 224

of the flux density of the reflected solar radiation received from that body (Jones,
1999). The flux density is the power of the electromagnetic radiation incident
on a unit area of a receiving surface.
Asteroid types
The spectral characteristics of asteroids allow them to be grouped into
four different classes; they can be further classified by their size and orbital path
(Taylor 2001). C-type asteroids are carbonaceous and are the darkest asteroids.
They are rich in hydrated silicates and in carbon. About 60% of known aster-
oids are of this type and are found in the outer portion of the main asteroid
belt (Encrenaz et al. 1990). S-type asteroids are siliceous with spectral charac-
teristics of rocky bodies that principally consist of pyroxene, olivine, as well as
ametallic phase (iron and nickel). They represent 30% of catalogued asteroids
and are abundant among the Apollo--Amor objects in the inner part of the main
belt (Encrenaz et al. , 1990). M-type asteroids are metallic consisting entirely of
iron and nickel (Encrenaz et al. , 1990). U-type are unclassified asteroids that do
not fit this classification. The asteroid Vesta is an unclassified asteroid; it has a
particularly high albedo and a spectrum dominated by pyroxene and feldspar
absorption bands (Encrenaz et al. , 1990). Smaller asteroids are referred to as mete-
oroids and micrometeoroids which are then further classified to meteors and
meteorites when they enter the Earth's atmosphere.
Ameteoroid is a small asteroid less then 1 km across (Jones, 1999). Smaller
particles are called micrometeoroids or cosmic dust grains, which includes any
interstellar material less then 0.01 m across that should happen to enter our
solar system (Encrenaz et al. , 1990; Jones, 1999). A meteorite is a meteoroid that
reaches the surface of the Earth without being completely vaporised. Much of
our understanding about asteroids comes from examining pieces of meteorites.
Usually, meteoroids are too small though to survive the passage through the
Earth's atmosphere. When a meteoroid strikes our atmosphere at high veloc-
ity, friction causes it to incinerate into a streak of light known as a meteor or
'shooting star' (NASA, 2004). The term meteor comes from the Greek word 'mete-
oron', meaning phenomenon in the sky. The temporary incandescence resulting
from atmospheric friction of this matter typically occurs at heights of 80--110
km above Earth's surface.
Asteroid impacts with Earth
The damage caused by the impact of an asteroid with the Earth's surface
depends upon the size of the asteroid, the landing location and the speed it