Fuel valve busted? Shuttle can't leave until NASA knows.

NASA engineers are troubleshooting what appears to be a balky fuel-tank valve that turned a planned liftoff of the space shuttle Discovery early Wednesday into another Mr. Fixit moment.

The glitch appeared the Tuesday evening, as technicians filled the shuttle's external tank with liquid hydrogen and liquid oxygen. The valve in question governs filling and draining of the tank holding liquid hydrogen. It's role is critical enough to require a scrub if it appears to be malfunctioning.

The valve problem cropped up as technicians were moving into the final stages of filling the shuttle's external tank. The valve was supposed to close, but engineers received no direct confirmation that it had shut from sensors designed to alert them to the valve's status.

There are actually two of these valves along the plumbing that lead to the hydrogen tank – one valve backs up the other. If one fails to close at the end of the fill up, the second will keep the tank closed.

Problems arise if one of the valves fails to open again. Flight controllers sometimes need to drain the tank in a hurry – for instance, when they have to abort a launch. With the valves open, the tanks can drain in about two hours. Without them, it can take several days for the liquid hydrogen to boil away naturally.

During Tuesday evening's fill up, the launch team received other, less direct, evidence that the valve had shut, says Mike Moses, who heads the prelaunch mission management team. That suggested that the problem involved the sensors and not the valve itself.

But strict launch requirements on this component and the approaches to troubleshooting it prevent the mission from proceeding unless the valve is confirmed to be in working order. NASA developed the strict guidelines after the agency experienced problems with similar valves on two previous missions.

In order to figure out what went wrong, technicians are required to drain the super-cold fuel and allow the valves to return to warmer temperatures before they can inspect the system.

The delay, currently envisioned to last 48 hours, has sent astronauts back into their prelaunch quarantine quarters and their relatives and guests back to motels.

Depending on the diagnosis engineers develop, Discovery could launch at 12:22 a.m. Eastern time Friday, with a second attempt possible some 23 hours later. Weather forecasters are now watching tropical storm Danny to see if it could lead to weather-related delays. If those two opportunities are scrubbed, NASA can launch Aug. 30. After that, other launch schedules for Cape Canaveral as well as Russian and Japanese cargo traffic around the space station would push a launch to mid-October.

Moreover, if the sensors are the problem, managers could still scratch the launch this weekend if they are too uncomfortable with the prospect of relying on indirect evidence that the valve is behaving properly. The mission management team could opt to stand down for repairs.