I said examples don’t necessarily have to be from anime, and here’s my first one that doesn’t refer to a Japanese series. Though the fact that Avatar: The Last Airbender is anime-influenced isn’t really in dispute.
Shortly after Aang was frozen in ice, Fire Lord Sozin took advantage of a special comet that enhanced the power of firebenders and wiped out the Air Nomads in a day, causing the comet to be renamed in his honor. (The fact that the date of the comet’s next return was known with precision suggests that it had been observed in previous visits, and likewise the fact that Sozin was so quick to take advantage of it’s effect on firebenders suggests that was already known beforehand.) In the last few episodes of the series, the comet returns, and Fire Lord Ozai attempts an even more evil (and impractical) plan.
Obviously, the idea of a comet having that kind of effect on benders is just as fantasy-based as bending itself. But let’s talk about the comet’s orbit.
Its period is roughly 100 years, which is reasonable enough; the most famous comet of all, Comet Halley, has a period of 76 years. Short-period comets can have periods of up to about 200 years.
The issue, aside from how bright and not-comet-looking it is, is that in the finale, Sozin’s comet is only close to the planet for what seems to be a few hours, defininitely less than a full day. Comets move faster when they are closer to the sun, but not that fast. A comet that was going to be that bright at any point would certainly be easily visible in the sky for months. Now, maybe it has to be either very close to the planet or in a very specific place relative to the sun for the effect on firebending (for this kind of positioning to repeat on every return, its orbit would have to be an exact multiple of a year), but it should still look almost exactly the same for days before and after that. This isn’t what we see; it speeds into the sky and back out of it within a short time, which is impossible.
In fact, it actually enters the atmosphere. It is possible for a comet or meteoroid to graze the atmosphere and still escape, but such a close approach would drastically change its orbit; nothing could repeat that kind of approach in a regular period.
The short duration of the comet is also what makes Ozai’s plan of using the comet to burn the entire Earth Kingdom not make sense. The exact size of the Earth Kingdom isn’t specified, but it’s based on China, is the largest nation in a world that only has four of them, and takes up almost all of the largest continent in the Avatar world, so it’s probably really big. Even if there had been no one trying to stop them at all, Ozai’s airship fleet, at the rate they were moving, wouldn’t have actually covered very much of it at all in one day.