PERSEID FIREBALLS: Earth is entering a stream of debris from Comet Swift-Tuttle, source of the annual Perseid meteor shower. Peak rates of 100+ meteors per hour are expected next week when Earth approaches the heart of the debris stream. The display, however, is already underway. "Our network of meteor cameras has picked up more than 78 Perseid fireballs since July 27th," reports Bill Cooke of NASA's Meteoroid Environment Office. He prepared this solar system diagram showing the orbits of all the fireballs so far.