Hit by more valve trouble, NASA postponed the launch of space shuttle Discovery just hours before it was to head to the international space station Wednesday because of a hydrogen gas leak. The potentially catastrophic leak was in a different part of the system that already had caused a vexing one-month delay. Shuttle managers put off the launch until at least Sunday and indicated that Monday might be more likely. The latest delay means Discovery’s two-week flight must be shortened and some spacewalks cut out of the mission.
