Comets grow visible tails only when they approach the Sun and begin to warm.

The longer version

A comet's tail appears only near the Sun because a comet is essentially a dirty snowball of ice and dust. Far out in the cold, the ices stay frozen and the body looks like a dark, inactive lump. Warmth vaporizes those ices as the orbit carries it inward. The released gas forms a glowing coma around the nucleus.

Escaping gas drags dust along, forming two distinct tails that point away from the Sun regardless of the direction of travel, pushed by sunlight pressure and the solar wind. Each close pass consumes material, so comets gradually fade, and some eventually break apart or go dormant.

Where this entry sits

The night sky looks calm, yet almost everything up there is moving, burning, or slowly changing. These facts cover planets, stars, the Moon, gravity, black holes, comets, and the story of how people began exploring beyond Earth. The full list sits on the Space and Sky page, where every entry is listed in order.