Comets:
Not long ago, many
people thought that comets were a sign that something bad was about to happen to them.
Long ago people didn't understand about how objects in the sky moved, so the sight of a
comet must have been very disturbing. There are many historical records and works of art
which record the appearance of comets and link them with terrible events such as wars or
plagues.
Now we know that comets are lumps of ice and rock that periodically come into the center
of the solar system from somewhere in its outer reaches, and that some comets make
repeated trips. When comets get close enough to the Sun, heat makes them start to
evaporate. Jets of gas and dust form long tails that we can see from Earth. These tails
can sometimes be millions of miles long.
In 1985-1986, a spacecraft
called Giotto visited the most famous comet, Halley, on Halley's most recent visit to the
inner solar system. In 1993, comet Shoemaker-Levy became trapped by the gravity of Jupiter
and plunged into Jupiter's atmosphere!
In 1996 and 1997 we saw comet Hyakutake, and comet Hale-Bopp. Hale-Bopp was one of the
brightest comets ever seen on Earth.
This is an actual comet.