Due to comets being comprised mostly of various frozen gases and dust, many fans wondered how it was possible for the Comet to provide power to Firebenders. However in the vision Avatar Roku's dragon Fang gave Aang
[6] the comet appeared to be on fire and during the events of Sozin's Comet it was revealed that the Comet passed extremely close to the planet. When it arrived, it brushed against the upper atmosphere, possibly even entering it, causing nearby clouds to break apart and float away (though if it did enter the atmosphere it would have crashed into the planet; it likely skimmed the upper atmosphere, but the heat and force of the comet could be felt in the lower atmosphere causing the clouds to break apart from its force and heat), and so the comet and its tail do indeed catch fire providing a powerful heat source.
The real-world phenomenon that best fits this description is the Earth-grazing fireball, a small space body that enters the upper atmosphere and leaves. However, as an Earth grazer passes through the atmosphere, its mass and velocity are changed, so that when it re-enters space it can be on a new orbit then before it passed and therefore cannot be periodical.
