SLEEP WELL, THE DINOSAUR: It has long been assumed that there are between 1,000 and 2,000 Near Earth Asteroids large enough to cause a major global catastrophe if they hit the planet. A new study, however, has put the number at between 470 and 930, and probably near 700. Military missiles with nuclear weapons are generally thought to be the only current defense against such a threat unless it is detected years in advance. Chances for such an impact, which would probably end civilization but not mammalian life, are rated as 1% per million years. Smaller impacts that could cause regional problems and 100,000 deaths are rated at 1% per century.--Stephen V Cole