Smaller chunks would burn up as they came through the atmosphere, but larger pieces could survive and hit Earth . A really big strike would be like the explosion of many H-bombs. If it splashed down in a ocean, tidal waves would swamp coastal areas. Boiling steam would shoot into the air and cover much of the world in dense cloud. This would cut off the sun, perhaps starting a new ice age.
Comets seem to appear suddenly because they brighten when they come close to the sun.
A comet usually follows a very elongated orbit. A bright comet can be a spectacular sight, moving slowly against the background stars, it seems to hang in the sky like a ghostly sword for days or weeks. A head of a comet is called a nucleus. The gas that evaprorates forms a cloud called the coma that is the ball at the front of the comet you see in the night sky when you are looking at a comet. People long ago thought that comets were stars growing hair.