Page 101 - BBC Sky at Night Beginners Guide to Astronomy - 2017 UK

WHAT TO SEE

Although some comets have
lost much of their lustre, others,
like Comet McNaught, provide
spectacular light shows

a comet to get here from the Oort Cloud, passing those with longer orbits are known as long-period
by Earth for just a day or two and producing comets. It has to be said that most of the short- DIRTY
a tremendous show before fl ying away again. period comets have been around the Sun so many SNOWBALL
However, some comets have their paths changed times that they’ve lost much of their lustre: through
into smaller orbits around the Sun by the gravity of a telescope they appear as little more than a fuzzy In March 1986, the
the gas giant planets, Jupiter in particular. ball. However, even these can sometimes surprise Giotto probe imaged
the nucleus of Halley’s
Those that travel with orbits lasting less than us with an outburst, so do keep an eye out for
Comet. Giotto saw an
200 years are known as short-period comets, while these cheeky cosmic vagabonds.
odd, dark, potato-shaped
object with outgassing
caused by the warming
Sun. It’s funny to think
CHASING THE TAILE TAIL that an icy ball a few
Curved kilometres across can
Comets generally have two dust tail create a roundish coma
visible tails: one composed Tails lengthen as up to about 100,000km
comet nears the Sun
of dust and one made of gas. wide and, even more
It’s the dust one that’s the extraordinarily, a tail
brighter of the two. Both tails that can stretch from the
appear when a comet is Sun to the orbit of Mars.
around 747,989,350km away
from the Sun. Sun
The dust tail is formed by
solar radiation gently pushing Straight
on the particles released from gas tail
the nucleus, while the gas tail Naked
comes from the interaction of nucleus
released gas with the Sun’s
magnetic fi eld.
Because a comet’s speed is
nowhere near that of the solar
radiation, its tails always point Tail points
away from the Sun, regardless away from Sun
of where the comet is in its orbit.

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