Asteroids - Extreme Events: A Physical Reconstruction and Risk Assessment - page 223

likely that most of the asteroids greater than 100 km have been discovered, but
only a tiny proportion of the smaller asteroids have so far been observed. It is
estimated that there are in the order of 10 9 asteroids greater than 1 km across
(Jones, 1999;Taylor,2001). Asteroids are classified by their surface composition
and, in particular, their albedo (Encrenaz et al. , 1990;NASA, 2004). An asteroid
is different from a comet, as a comet develops a thin atmosphere when it is
within 10 AU of the Sun whereas asteroids do not (Jones, 1999).
Cosmic origins of asteroids
Asteroids are thought to have been derived from the planetesimals and
embryonic systems that were in the space between Mars and Jupiter during the
early formation of the Solar System. Asteroids are small condensations from
theprimitive solar nebula that were unable to form into a single body due to
thegravitational instability caused by the presence of Jupiter (Encrenaz et al. ,
1990). Interactions between asteroids and Jupiter, and the asteroids themselves,
have resulted in collisional fragmentation of the asteroid population over the
last 4600 million years (Jones, 1999). These collisions have also resulted in a
substantial loss of material from the asteroid belt over time; there was about
10 3 times more mass there about 4600 million years BP compared to today (Jones,
1999).
Asteroids display a wide range of orbital characteristics including values for
their semi-major axes, eccentricities and inclinations to the ecliptic (Encrenaz
et al. , 1990). Most asteroids found in the main asteroid belt have a semi-major
axis lying between 2 and 3.5 AU. Beyond 3.5 AU there are two distinct families
of asteroids; Hildas at4AUand Trojans at 5 AU. There are also a number of small
asteroids that have orbits that intersect with or closely approach the Earth's orbit
(Taylor, 2001). These near-Earth asteroids are otherwise known as the Apollo--Amor
family. They consist of asteroids originally derived from the main belt but which
were ejected from the 'Kirkwood gaps' by Jupiter's gravitational perturbations
(Encrenaz et al. , 1990;Taylor,2001). There are a few thousand near-Earth asteroids
that have diameters greater than 1 km. The Apollo asteroids are the current
suppliers of meteorites to Earth (Taylor, 2001)
The size of asteroids is determined mainly through indirect measurements,
with the exception of some of the larger asteroids such as Ceres ,afewofthe
Apollos ,and those that have been imaged from spacecraft enabling direct mea-
surement (Jones, 1999). Most asteroids are too small and too far away to be seen as
anything more than a point of light in the night sky (Jones, 1999). Indirect mea-
surements are made, therefore, and this is achieved through the determination