15 Mar 2009, 0313 hrs IST, AP



CAPE CANAVERAL: NASA still doesn't know what caused a hydrogen gas leak that prevented space shuttle Discovery from flying earlier this week.


But officials are hopeful that repairs have solved the problem, and that Discovery will blast off Sunday evening. Good weather is forecast.

Discovery is more than a month late for its scheduled trip to the international space station. First, hydrogen gas valves inside the shuttle had to be double- and triple-checked. Then on Wednesday, hydrogen gas began leaking during fueling where a vent line hooks up to the external fuel tank.

Workers have replaced that hookup and a pair of seals. Nothing obvious was wrong with the removed parts.

Discovery and seven astronauts are set to carry up one last set of solar wings for the space station.