|
|
Comets
|
|
Like asteroids,
comets are pieces of interplanetary
rubble remaining from the formation of the planets. The difference is
that comets formed in the outermost fringes of the solar system, far
away from the warmth of the Sun. The solid part of a comet is called
its nucleus. Anywhere from 2 to 50 km across, a nucleus is too small
to see even with the largest of telescopes. A comet's nucleus is
essentially a giant dirty snowball, mostly water and ice, coated with
a dark dust.
As a comet moves
toward the inner solar system, the dark nucleus undergoes and
incredible change. The Sun warms it, turning exposed icy surfaces
directly into a gas. The jets become stronger as the comet nears the
Sun, the dark crust cracks and exposes more ice. A growing cloud of
gas and dust completely hides the nucleus. Sunlight excites the gas
and it glows with a faint bluish light. A thin stream of gas rushing
away from the Sun pushes back the comet's gas cloud, forming one part
of the tail. Dust particles take a slightly different route, fanning
out along the comet's orbit. They reflect light form the Sun, so the
comet's dust tail glows with a yellow light. If the comet's path
brushes the Earth's, the dust particles create a meteor shower when
they run into the atmosphere.
Comet Halebopp
Halley's Comet (shown
below) is perhaps the most known in the world. It is visible to us
without aid of a telescope every 76 years. It was last near Earth in
1986 and space probes were sent to take a closer look. One space
probe passed within 600 km of the comet's nucleus and the photographs
it sent back show violent jets of gas and dust erupting from a dark
potato-shaped nucleus just 15 km long and 8 km wide. Haley's Comet
will be back in 2061. A comet can only last for several thousand
circuits of the Sun before its store of energy is exhausted and it
fades away.
Halley's Comet
in 1986
Astronomers believe
that millions of comets orbit the Sun in an area called the Oort
Cloud, some thousands of times more distant than Pluto. Only the
feeble gravity of a passing star is needed to send some of these
toward the inner solar system.