Wayne Hale, the shuttle program director, told reporters he had made the decision about two hours earlier during a teleconference with engineers and key staff members. "We had what you might expect was an interesting discussion," he commented.

Hale said NASA has been investigating a potential fault with the wiring in the engine cutoff, or ECO, sensor aboard the main fuel tank, which was built at Lockheed Martin's facility at Michoud, La., outside New Orleans. The sensor warns of a low level in the spacecraft's liquid hydrogen fuel, ensuring the main engines shut down before the tank reaches empty. Running out of fuel, he explained, could damage the engines.

Though the possibility is small that the sensor would give a false reading, Hale said he decided to err on the side of safety and replace it with a newer model. Engineers will do so by entering the huge external tank via an access door on its bottom.

The operation should take about three weeks, but related testing and refitting the tank will push back Discovery's readiness beyond the May launch window. During that time, Hale said, NASA also will conduct aerodynamics tests in wind tunnels and with computer simulations of changes made to the configuration of the tank's foam insulation.

Potential problems from foam breaking off during launch remain a concern.

The next available opportunity to launch in daylight hours from the Kennedy facility, so Discovery can rendezvous easily with the International Space Station, begins July 1 and runs through July 19. "We're all very optimistic that we can get all of our other work done," Hale added.

He said NASA remains confident the shuttle fleet can launch three times during 2006. The first two of those flights constitute the final two or three parts of the space agency's Return-to-Flight program – a critical effort designed to restore the shuttle to routine and regular operations until 2010, with the goal of completing construction of the space station, and possibly mounting one service mission to the Hubble Space Telescope.

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