Comets — Astronomy, 4–6 years
A comet is a small icy and dusty object travelling through space. Near the Sun, some of its material makes a bright cloud and a tail.
What a comet is
A comet is a cold mixture of ice, dust and rocky material. Far from the Sun, it may look like a dark, frozen lump. When it travels nearer to the Sun, sunlight warms it and gas and dust spread around it. This can make a bright head and a long tail.
Why comets have tails
People wondered why a bright object sometimes appeared in the sky and then disappeared for years. A comet’s tail is a clue: heat from the Sun changes some ice into gas and pushes dust away. The tail is not a trail left behind like smoke from a train; sunlight and a stream of particles shape it.
Watching Halley’s Comet
Halley’s Comet was seen from Earth in 1986 and is expected again around 2061. Step one: it travels a very long way from the Sun, where it stays cold and has little visible tail. Step two: it comes closer and sunlight warms it. Step three: gas and dust brighten around it, making the comet easier to see.
A reasonable mix-up
Pictures often show a comet with a tail pointing behind it, so it is natural to think the tail shows the comet’s direction of travel. But the tail usually points generally away from the Sun, pushed by sunlight and particles from the Sun. As a comet moves around the Sun, its tail can point in a different direction from its motion.
Comets and what we learn
Scientists study comets because their ice and dust have stayed since the early Solar System formed. Space missions have flown close to comets, photographed them and tested their material. Comets can also leave dust along their path; when Earth crosses that dust, small pieces burn in the atmosphere and make a meteor shower.
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