"Space shuttle Discovery's launch to the International Space Station now is targeted for no earlier than March 15. NASA managers postponed Wednesday's planned liftoff due to a leak associated with the gaseous hydrogen venting system outside the external fuel tank. The system is used to carry excess hydrogen safely away from the launch pad. Liftoff on March 15 would be at 7:43 p.m. EDT. The exact launch date is dependent on the work necessary to repair the problem. Managers will meet Thursday at 4 p.m. to further assess the troubleshooting plan. Discovery's STS-119 flight is delivering the space station's fourth and final set of solar array wings, completing the station's truss, or backbone."
Gas Leak Postpones Space Shuttle Launch, AP via NPR
"NASA has until Monday to send Discovery to the space station, otherwise the flight will have to be put off until April. That's because a Russian Soyuz rocket is slated to blast off in two weeks, on a higher priority mission, with a fresh space station crew. Discovery's liftoff originally was targeted for mid-February, but concern about the shuttle's three hydrogen gas valves resulted in four delays."