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.
-
Step 1 of 5
A comet is mostly ice.
A dirty snowball, and the description is meant literally. 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.
-
Step 2 of 5
Most of the time it is far away and dead.
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.
-
Step 3 of 5
Sunlight turns the ice 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.
-
Step 4 of 5
The solar wind blows that gas into a tail.
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.
-
Step 5 of 5
It comes back, again and again.
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 a dirty snowball a few kilometres wide, dead for centuries until sunlight turns its ice straight to gas. The solar wind blows that gas outward, so the tail points away from the Sun, never backwards.
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.