In a comet’s case, ice turns into gas when heated by the Sun, and that gas escapes into space (a process called outgassing), usually also carrying away some amount of dust along with it.

Comet Tsuchinshan-ATLAS will sweep around the sun on Sept. 27 to make a brief foray into the morning sky. Will it be a bright naked-eye object with a significant tail? Here's where and when you ...