* The astronomer excitedly pointed out the comet streaking across the night sky.
* The comet's tail glowed brightly, casting a faint ethereal light on the desert landscape.
* The ancient myths tell stories of comets as omens of good or bad fortune.
* The spacecraft successfully flew past the comet, collecting valuable data.
* The comet was named after the scientist who first observed it.
You can also use "comet" in a metaphorical sense:
* His success was a comet - sudden, brilliant, and fleeting.