NASA: Discovery is on track for Sunday launch

NASA's going for it. It aims to launch Discovery on Sunday at 7:43 p.m. Eastern Daylight Time.

The orbiter originally was set to go up March 11. But mission managers scrubber the launch after controllers couldn't stop hydrogen gas from leaking into the air just outside the external fuel tank. The problem appears to be centered around a seal at the fuel-tank end of a line that draws excess hydrogen from the tank as it's filled.

Engineers still haven't figured out what cause the problem.  And launch director Mike Leinbach acknowledges that it is unusual to launch without know what caused a launch-canceling problem.

But mission managers say they feel comfortable with the decision to try again, for two reasons: They had technicians  replace the hardware that failed last Thursday; and the problem has nothing to do with systems on the orbiter itself. It's what managers call a ground problem, not a flight problem.

Even though engineers have not found the smoking gun, "I'm going to sleep just as good tonight" as if they had, said Mike Moses, who heads the pre-launch mission management team.  He made the comment during a briefing this afternoon. If all goes well, the shuttle and its crew will still be able to complete three of the four spacewalks initially planned for the mission, including the all-important installation of the station's final set of solar panels.

And if the new hardware displays the same problem? It's another scrub, and likely a launch delay until April.

If you'd like to watch the launch, NASA's TV coverage begins at 10:15 a.m. You can watch on-line here. Play close attention to the fueling process!