More than a trillion comets surround the planetary
region of the Solar System. They follow long orbits
around the Sun and together make up a vast sphere
called the Oort Cloud. Each comet is a lump of dirt
and snow, called a nucleus, or “dirty snowball”.
Comets are so small that they are only visible
when they travel close to the Sun and grow
large and bright enough to be seen.Comets
Tails The comet’s tails
increase in length as it nears
the Sun. They are pushed
away from the Sun and
always point away from it.142
COMET STRUCTURE
▶city-sized lump, two-thirds snow The nucleus of a comet is a
and one-third rock dust. Halley’s
Comet orbits the Sun every 76
years. When it drew close to it in
1986, the
Giotto
captured images of its nucleus. into the comet’s coma and spacecraft flewPASSING THE SUN
▶These images from the SOHO spacecraft,
track the progress of Comet McNaught as
it rounds the Sun. Like most comets,
it is named after its discoverer,
Robert McNaught, who saw it
first on 7 August 2006. It was
at its biggest and brightest
in January 2007, when
closest to the Sun.CHANGING COMET
▶As a comet approaches the Sun it warms
up. The snow turns to gas, which, along
with loose dust, flows from the nucleus.
When the comet passes closer to the
Sun than the orbit of Mars, this material
forms a head (called a coma) and two
tails, one of gas and one of dust.◀ COMET DISPLAY
More than 2,300 comets have been
identified as they passed through
the Sun’s neighbourhood. About
200 make return visits, but most
pass by just once. Three or four
times a century, a spectacular one,
such as Comet McNaught in January
2007, makes a stunning display.Coma A coma has
formed around the
nucleus of Comet
Hale-Bopp as it travels
towards the Sun.This bright dot is
the planet MercuryThe comet grows as
it approaches the SunComet
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