Shuttle to try for Tuesday launch

NASA plans to launch the space shuttle Discovery early next week despite lingering uncertainty about why one of the winged ship's eight fuel gauges isn't working properly.

Liftoff is set for 10:39 a.m. EDT on July 26, pending the results of tests that continued through Thursday morning and repairs to be made on Friday. If all goes well, NASA says, countdown clocks at the launch site, Florida's Kennedy Space Center, will start ticking again on Saturday.

"We have a great amount of work in front of us to get ready, but we've all agreed that this work is doable, and that it takes us to a launch on the 26th," shuttle program chief Bill Parsons said in a news conference in Florida late Wednesday. The shuttle's first launch attempt was scrubbed less than three hours ahead of liftoff on July 13 after a crucial sensor registered that Discovery's external fuel tank was empty. At the time, the tank was brimming with 528,000 gallons of liquid hydrogen and oxygen propellants. The small component helps ensure a safe shutdown of the shuttle's three main engines, so the winged orbiter was grounded for repairs.

The postponement has dealt a blow to NASA's comeback effort 2 1/2 years after the re-entry disaster that destroyed a space shuttle and killed seven astronauts. Discovery's 12-day visit to the International Space Station will be the first mission since the February 2003 Columbia disaster.

Mission managers said engineers suspect electromagnetic interference is causing a grounding problem in circuits connected to an electronics box that houses the fuel sensor. But they can't be sure, because testing has been carried out under ambient temperatures, without super-cold propellants in the tank.

Managers ruled out fueling the shuttle merely to test the theory, deciding instead to press ahead with an augmented countdown. Plans are to fuel the shuttle Tuesday morning and let its crew of seven astronauts climb aboard as usual about three hours before the scheduled liftoff. Extra checks of the faulty sensor will be made during the final hours.

"If the sensors work exactly like we think they will, then we'll launch," Parsons said. "If something doesn't go right, then we'll have a scrub and go talk about it."

The sensor is in question is one of four that measure the level of liquid hydrogen propellants in the shuttle's rust-colored tank. Another set of four measures the level of liquid oxygen.

A safety rule that NASA adopted after the 1986 shuttle Challenger disaster prohibits launching if any one of the sensors in either set is malfunctioning. Subsequent shuttle safety improvements have made the rule unnecessary. Managers debated amending the rule since last week's scrub but decided against making such a move in the heat of a launch campaign.