The longest flight in space shuttle history just got longer. With Florida socked in by stormy weather, Endeavor was unable to touch down at the Kennedy Space Center today. NASA is mulling a Saturday landing either at Kennedy or at Edwards Air Force Base in California. As of Saturday, Endeavor will have been in orbit for 16 1/2 days, surpassing the 14-day, 18-hour flight by Columbia last year. While weather at Kennedy is expected to improve only slightly by tomorrow, nearly perfect conditions are forecast for the Mojave Desert landing strip. NASA prefers Kennedy because it costs $1 million to ferry shuttles from California to Florida on the roof of a jumbo jet.