The comet swings through the inner Solar System every 33 years, laying down a trail of dust as it goes. It was last here in 1988. The Leonids are caused when the Earth passes through the dusty debris that follows the comet's orbit. Most particles are no bigger than a grain of sand. These hit the Earth's atmosphere at up to 70km per second.
More impressive fireballs are meteors that are just a few centimetres in diameter.