Answer:

The gases liberated in the comet's tail are very diffuse. So it takes many trips of a comet about the sun to bleed off a lot of mass, unless the comet passes sufficiently close to the sun (or collides with it or with something else). Eventually the comet would become a much smaller, rocky body, sort of like an asteroid. All the water vapour, methane, and ammonia will have bled off.

Explanation: