What is the definition of a comet?

A comet is defined as a ball of frozen gases, rock, and dust that orbits the sun. This definition encompasses the primary characteristics that distinguish comets from other celestial objects. Comets are composed of volatile materials such as water ice, carbon dioxide, ammonia, and gases, which is why they become active and develop a visible atmosphere, or coma, when they get close to the sun. The heat causes the ice to vaporize and release gas and dust, forming the iconic tails that we see streaming away from the nucleus of the comet.

In contrast, the other options describe different celestial phenomena. A small rocky body found in the asteroid belt refers more to asteroids, which are primarily composed of rock and metal. An object burning up upon entering Earth's atmosphere pertains to meteoroids that produce meteors, not comets. Lastly, a fragment of space debris that lands on Earth generally describes meteoroids or space junk, which is distinct from the icy bodies that classify as comets.