NASA called off Thursday's launch of space shuttle Atlantis after detecting problems with fuel gauges in the shuttle's external tank. Officials will try again Friday to launch the shuttle, NASA said. Two of the four sensors in the shuttle's liquid hydrogen tank indicated that the tank was dry, even though there was fuel inside. Three sensors must be working properly to fly, NASA spokesman Paul Foerman said. The sensors are critical to ensure that the shuttle's three main engines do not shut down too soon or too late during liftoff. Problems with the sensors have delayed shuttle launches before, most recently in September 2006.