NASA will run tests later this week to determine whether its safe to fly shuttle Discovery despite valve trouble that cropped up over the weekend during a critical propellant-loading operation at Kennedy Space Center.

The tests, if successful, could provide managers with the data required to prove Discovery could launch as scheduled on April 5 and still fly its International Space Station outfitting mission safely.

Discovery and seven astronauts are scheduled to blast off from launch pad 39A at 6:21 a.m. Eastern April 5, hauling up an Italian cargo carrier filled with science racks and tons of supplies and equipment to the International Space Station. The mission is one of four station outfitting missions remaining before shuttle fleet retirement.

The Discovery launch plans hit a snag late Friday when a helium leak was detected in one of the shuttle's two hump-like orbital maneuvering system pods, which house engines that are used to make large course corrections in orbit. The twin maneuvering engines are used to drop the shuttle out of orbit for atmospheric reentry and landing. Small steering thrusters used for precision piloting also are housed in the pods.

Helium plays a critical role in pushing propellant into the engines and thrusters while maintaining proper pressures within the system. Engineers detected a leak in an isolation valve that separates a helium supply tank from a hydrazine fuel tank in Discovery's right-hand pod. The helium line in question is used to pressurize thrusters in the pod. It appears the valve might have been stuck in the open position, or that it might have a significant leak.

The propellant-loading operation involved pumping monomethyl hydrazine and nitrogen textroxide into separate tanks and filling up the helium supply tank. The operation was completed over the weekend and engineers were able to close the faulty valve after pressurizing the helium tank.

The tests late this week will determine whether two helium regulator valves downstream from the faulty valve are working properly. If so, then the regulator valves would ensure proper pressures would be maintained in the system even if the faulty valve leaked or failed to close during flight.

Knowing whether the regulators are operating properly will be a key factor for managers who must decided whether to fly the shuttle as-is or move Discovery back to its processing hangar for repairs. A decision to roll the shuttle off the pad and remove Discovery from its external tank would cause a significant launch delay. It also would scramble the schedule for the last three shuttle missions, which now are scheduled to launch May 14, July 29 and Sept. 16.

NASA and shuttle fleet operator United Space Alliance will calibrate a test panel over the next several days. The helium system will be brought up to full pressure and engineers will monitor gauges on the panel to determine whether the regulators are operating properly.

By Todd Halvorson, FLORIDA TODAY