Asteroids - Extreme Events: A Physical Reconstruction and Risk Assessment - page 222

9
Asteroids
The Titius--Bode rule states that the radii of the planetary orbits in our
solar system increase geometrically with increasing distance from the Sun. This
rule suggests that there is a planetary gap between Mars and Jupiter (Encrenaz
et al. , 1990), or in other words that a planet should have once existed here. On
the1st January 1801, the missing planet was discovered by Italian astronomer
Giuseppe Piazzi. He named it Ceres (Jones, 1999). It was soon discovered though
that Ceres was notaplanetasitistoosmall; Ceres is a minor planet or what is
now known as an asteroid. Over the following years a number of other asteroids
were discovered in the gap between Mars and Jupiter. This gap is now called 'the
main asteroid belt' (Encrenaz et al. , 1990).
The word asteroid means 'resembling a star' (Jones, 1999). They are metallic,
rocky bodies without atmospheres that orbit the Sun. They are too small to be
classified as planets, hence the name 'minor planets'. They are largely confined
to themain 'doughnut-shaped ring' asteroid belt between Mars and Jupiter from
approximately 2 to 4 AU (AU = Astronomical Unit which is the distance from the
Sun to the Earth and in this case 2--4 AU = 300--600 million km) (Jones, 1999;
NASA, 2004). Tens of thousands of asteroids congregate in the main asteroid
belt, but not all are confined to this belt for some pass much closer to the
Earth; these are known as the near-Earth asteroids (Jones, 1999). Asteroids have
adiameterofgreaterthan1kmandindeed can be larger than 100 km across.
The largest asteroids to be discovered are Ceres, Pallas, Juno and Vesta ,respectively
(Encrenaz et al. , 1990). Ceres has a radius of 457 km, amounting to about one-
third ofthetotalmass of the entire main asteroid belt (Taylor, 2001). About 8000
asteroids have been observed to date and their orbits have been determined and
categorised. Of these, 238 are greater then 100 km across (Jones, 1999). It is
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