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A dirty snowball

What Is a Comet?

A lump of ice and dust that grows a glowing tail when it comes close to the Sun.

1 Frozen lump farout2 Comes near theSun3 Grows a tail
The tail is not fire. It is the comet evaporating.

  1. Step 1 of 5

    A comet is mostly ice.

    A comet Ice, dust and frozen gas

    Under a dark crust it is frozen water, frozen gas and dust, packed into a lump a few kilometres across. Astronomers call it a dirty snowball.

  2. Step 2 of 5

    Most of the time it is far away and dead.

    Comet's far pointfar beyondNeptuneNeptune30x Earth'sdistanceEarth1

    Comets spend centuries in the freezing outer solar system where nothing happens. No tail, no glow — just a dark lump on a very long loop.

  3. Step 3 of 5

    Sunlight turns the ice straight to gas.

    1 Solid ice2 Sunlight heatsit3 Straight to gas

    As the comet swings near the Sun, the surface heats fast. The ice does not melt into water — it goes directly from solid to gas and blasts off the surface.

  4. Step 4 of 5

    The solar wind blows that gas into a tail.

    Sunlight and solarwind push Tail blownbackwardsThe tail leads the way on the trip home

    The escaping gas and dust get pushed away by sunlight and the solar wind. That is why a comet's tail always points away from the Sun — even when the comet is flying away.

  5. Step 5 of 5

    It comes back, again and again.

    Halley's Cometevery 76 yearsA human lifeabout 80 yearsEncke's Cometevery 3.3 years

    A comet is in orbit, so it returns. Halley's Comet swings past every 76 years — most people see it once, if they are lucky.

    Each pass boils away a little more ice. Comets have a limited number of visits left.

The short version

A comet is an icy lump that grows a glowing tail because the Sun is boiling it away.

Try it yourself

Take an ice cube outside on a hot dry day. The wisp coming off it is the same process that makes a comet's tail.

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