Earth is entering a stream of debris from giant Comet Swift-Tuttle, source of the annual Perseid meteor shower. Forecasters expect the shower to peak on Aug. 11-12 with many fireballs streaking across the night sky. NASA's network of all-sky cameras is already detecting a few Perseid fireballs every night over the USA. Comet Swift-Tuttle has a huge nucleus--about 26 km in diameter. Most other comets are much smaller, with nuclei only a few kilometers across. As a result, Comet Swift-Tuttle produces a large number of meteoroids, many of which are hefty enough to produce fireballs. Indeed, a 6-year study by NASA's Meteoroid Environment Office found that the Perseids produce more fireballs than any other annual shower. During the Perseid meteor shower, which lasts for weeks, comet debris particles a few cm wide hit the atmosphere with an average speed of 59.6 km/s (133,350 mph). The average magnitude of the resulting fireballs is -3.7, about as bright as Venus. More fireballs are in the offing as Earth moves deeper into the comet's debris stream. Generally speaking, the best time to look is during the dark hours before sunrise when the shower's radiant is high in the sky. Stay Tuned!