Kepler's Laws were shown by Newton to explain the motion of
comets. They orbit the Sun in highly elliptical orbits coming close to the Sun for a while and
then moving off to the outer reaches of the Solar System to return many years later (see
Figure 1). The orbit period of a comet may vary from a few years (Encke's comet has a
period of 3.28 years) to many thousands or even millions of year for very long period
comets.
Comet are huge balls of grit and ice
many kilometers in diameter. (Halley's comet has a nucleus about 16 km x 8 km across).
When they come near the Sun the heat melts some of the ice and
this produces a huge cloud of gas and dust that we see as the tail of the comet. These tails,
millions of kilometers in length, become visible as they near the Sun. The tail always points
away from the Sun because of the pressure of the solar wind on its tiny particles (Figure
1).
We only see a comet for a short time because they move fast when they are
close to the Sun but very slowly in the outer parts of their orbit, see Kepler's second law.
One of the most famous comets, Halley's comet returns every 75 years.
In recent
years comets Hyukutake and Hale-Bopp have been visible with the naked eye and have
made a fine show in the night sky. Comet Shoemaker-Levy was memorable because it was
pulled into many fragments by the gravitational pull of Jupiter. These fragments then
crashed into the surface of the planet, the impacts being visible through a telescope from
Earth. I saw comet Hyukutake from the roof of a small hotel on the edge of the Sahara
desert – a truly fantastic site – the comet's tail stretched across the sky a distance equal to
the length of the constellation Ursa Major (The Plough).
Comets are thought to
originate in the Oort cloud (where there may be up to 6x1012) and in the Kuiper belt. The
Oort cloud is a spherical region of space from about 6000 AU from the Sun to roughly 2 light
years away. The greatest concentration of comets is thought to be around 40 000 AU from
the Sun.
If a comet were ever to crash into the Earth
the effect would be catastrophic. The enormous effect of an impact of a large comet on the
Earth would change life as we know it on the planet. The impact of a comet thousands of
years ago probably caused the extinction of the dinosaurs.
It is thought that
the gravitational pull of Jupiter has helped deflect comets from orbits where they might
collide with the Earth.
