Comets
Comets are very different from asteroids and other space debris, both
in their composition and behaviour. They are mainly dirty ice blocks,
predominantly frozen water (H2O), with frozen carbon dioxide
(CO2), carbon monoxide (CO), and other ices, laced with rock
dust and a rich variety of organic compounds.
Typically, a comet is an irregularly shaped object with a nucleus a
few kilometres across. Most comets spend their time at immense distances
from the Sun, way outside the orbit of Pluto and about one-third of
the way to the nearest star in a region called the Oort comet cloud,
named after the Dutch astronomer Jan Oort. These comets have orbital
periods of millions of years and their orbits can be inclined in any
direction, that is, they can move around the Sun in any plane, not just
the planetary plane.
We can only observe these visitors from the outer reaches of the Solar
System when their highly elliptical orbits bring them close to the Sun
on their perihelion passage, the part of their orbit nearest the Sun.
Their presence in deep space is assumed from calculations of the orbits
of comets as they approach the Sun in the inner Solar System.
Not all comets spend most of their life in the dark reaches of space
in the Oort cloud. Halleys Comet has a period of only 76 years
and has a retrograde orbit around the Sun, that is, it goes backwards
compared to the planets, or clockwise around the Sun. Halleys
Comet is an example of a short-period comet that has been captured by
a close encounter with Jupiter, and thrown into a smaller orbit during
its long journey from the Oort cloud towards the Sun. The next expected
sitting of Halleys comet will be in 2062.
Observing comets
As a comet approaches the inner planets, its nucleus
is warmed by the Sun and it begins to release gases and dust that form
the comets coma and tail. These familiar characteristics only
occur when the comet is within the orbit of Jupiter, where the Sun's
light is warm enough to vaporise ice off the comet. Beyond Jupiter in
cold space, the comet is simply a lump of dirty frozen ice. Comets can
form the most spectacular astronomical shows with brilliant comas trailed
by incredibly long tails, and sometimes not only one tail!
The
tale of two tails
The nucleus of the comet cannot be directly observed as it passes
us in the inner Solar System, because the gases boiling off its surface
form a bright atmosphere, or coma, of gas and dust which
reflects light from the Sun, hiding the much smaller solid nucleus inside.
These same gases and dust are then blown away from the comet
by the bombardment of solar wind particles from the Sun
to form a tail. This bright tail always points away from the Sun and
changes direction as the comet passes by the Sun. A second, dimmer,
tail can often be seen marking the orbit of the comet. This is created
by dust, too big to be affected by the solar wind, coming off the comet
nucleus and being left behind in a second trail marking the comets
orbit.
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