(i)
Comets
A comet is an icy ball of
rock that grows a tail when it travels close to the Sun. It grows a tail
because as comets heat up, gas and dust are forced from the body and trail
behind it. The Sun illuminates this trail, which causes it to glow. The glow is
visible in the night sky, and what we on Earth see is a streak.
The way a comet orbits - and the length of its orbit - varies greatly
from one comet to another. They can have a short period and orbit every few
years, or a very long period and orbit every 100,000 years.
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Most of
the shorter-period comets come from the Kuiper
belt, a massive region of small objects (including Pluto) out beyond
Neptune.
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The
longer-orbit comets come from the Oort cloud, a massive cloud of icy planetesimals very far out in the outer part of the solar
system.
Every
year, Earth sees a few comets. Some of these are comets we expect because they
regularly orbit the sun. The others are new comets that have never been seen
before.
The
best-known comet is Halley's Comet. It last appeared in 1986 and will appear
again in 2061. Halley's Comet has been seen every 76 years since 240 BC.
Figure 4: A comet
