Asteroids
Asteroids, formerly know as "minor planets" but now more correctly
called "small Solar System bodies",
were most likely created when our Solar
System was formed, approximately 4.6 billion years ago. There is another
theory that the asteroids are fragments left by the disintegration of
a planet that once existed between Mars and Jupiter. However, it is more
likely that they are debris left over from the time of the Solar System's
formation. The name "asteroid" means
"little star", which they definitely are not: the Sun is a star! Asteroids
are believed to be relatively small solid objects; some icy, some
stony, some metallic.
Most asteroids are in the region between the orbit of Mars and the orbit of
Jupiter. This region is the "asteroid belt". Some asteroids have odd orbits that
bring them near the Earth's orbit; two even go inside the orbit of Mercury. It is
possible that the Earth will some day collide with an asteroid - it has happened
in the past.
No one knows exactly how many asteroids there are, but it is thought that they number
many thousands. About 100,000 are known at present. Large asteroids now rarely come
within a million kilometres of each other, but collisions between asteroids must
have occurred in the past, to produce the scattered fragments known as meteoroids.
Asteroids can be hundreds of kilometres in diameter; and
the smallest ones observed so far may be only 30 metres or so across.
They aren't all round; many of them have very irregular shapes,
like big boulders. Each new asteroid is given a number until its
orbit becomes well-known; then the discoverer is allowed to name
it. Until recently defined as a dwarf planet,
"1 Ceres", discovered in 1801 by the Sicilian astronomer
Piazzi (who named it after the Roman goddess of the harvest), was the
largest asteroid. "2 Pallas" and "4 Vesta" are now the largest asteroids
with diameters of approximately 530 km.
Many scientists believe that it was an asteroid that caused the extinction
of the dinosaurs around 64 million years ago. There is evidence to suggest
that an object about 10 km across crashed into the Earth, throwing up
a vast cloud of dust or water vapour. This great cloud would have shut
out the sunlight and caused a rapid temperature drop, bringing on an ice
age. Also, if the cloud lasted for a long time, plants would have stopped
growing and food chains would have been destroyed.
Perth Observatory astronomers have discovered more than 100 new asteroids since
1970.
