Broken valve grounds shuttle until at least Friday

CAPE CANAVERAL -- Discovery and seven astronauts will remain grounded until at least Friday so technicians can test a faulty propulsion system valve that failed as the shuttle was being fueled Tuesday for its second launch attempt.

In a bit of an unusual twist, the 18-story shuttle and its crew would have two opportunities to launch Friday — the first at 12:22 a.m. and the next at 11:59 p.m. Eastern.

NASA also would have opportunities at 11:33 p.m. Saturday and then 11:11 p.m. Sunday — the 25th anniversary of Discovery's first flight — to send Discovery up to the International Space Station before a looming deadline that would trigger a delay until mid-October.

NASA in that case would yield to previously planned Japanese and Russian missions to the station. But senior managers say NASA still has a chance to get Discovery's mission underway.

The valve failure cropped up as NASA was just about to finish loading supercold liquid hydrogen and liquid oxygen into Discovery's 15-story external tank in advance of an early Wednesday launch attempt. Thunderstorms forced NASA to scrap an initial attempt early Tuesday.

Engineers pouring over data in NASA's Launch Control Center saw that the liquid hydrogen fill-and-drain valve was not operating as intended.

The eight-inch-diameter valve is one of four that control the flow of liquid hydrogen and liquid oxygen from the shuttle?s external tank through the orbiter's main propulsion system and then into its three main engines.

Engineers could not be certain the valve would open and close on command. They feared the valve might close and remain closed. In that case, it would be very difficult for NASA to drain millions of gallons of croygenic hydrogen from the tank.

NASA safety rules prohibit a launch with the valve stuck closed. The valve is one of two that can dump excess hydrogen after the shuttle reaches orbit.

An accumulation of highly flammable hydrogen in the shuttle's engine compartment would be an explosive hazard.

Engineers think the problem might actually be coming from a sensor that is designed to indicate whether or not the valve is open or closed. So they will run several tests today and Thursday, opening and closing the valve, to see whether the valve or the sensor is the root of the problem.

If the sensor is the culprit, NASA could proceed with launch attempts on Friday. If not, NASA would have to replace the valve, which would force a lengthier delay, likely until October.

NASA must launch Discovery by Sunday for the astronauts to complete their station outfitting mission and depart in advance of the debut launch Sept. 10 of a robotic Japanese cargo carrier.

The supplies and equipment being hauled to the station by Discovery and the Japanese freighter are critical to maintaining a full staff of six on the outpost.

The most prominent payload is NASA's new $5 million treadmill, which is named after Comedy Central's Stephen Colbert. He said in a recorded message that he couldn't be prouder that his treadmill soon will be installed at the space station "to help finally slim down all those chubby astronauts."

"Let's face it, being weightless is mostly just a desperate bid to get away from that bathroom scale every morning," Colbert said. "But you guys and gals are ambassadors to the universe. Don't make us look bad. Put down the astronaut ice cream, tubby. Tubby, tubby, two-by-four, couldn't fit through the air lock door."

Colbert campaigned earlier this year to have a space station room named after him. He won the online vote, but NASA went with Tranquility, the name of the dry lunar sea in which Neil Armstrong and Buzz Aldrin landed 40 years ago this summer. As a consolation prize, Colbert got the treadmill. It's full name is Combined Operational Load Bearing External Resistance Treadmill; it will fly up in more than 100 pieces and won't be put together until sometime next month.

Colbert poked fun at NASA's choice of Tranquility for the chamber, which will be launched early next year. "Yeah, that will scare the aliens," he said. He ended his televised message by shouting, "I am 'go' to launch me. Let's light this candle!"

In all, the space shuttle is scheduled to deliver about 17,000 pounds of gear to the space station. The experiments include six mice that will remain at the orbiting outpost until the following shuttle visit in November. Part of a bone loss study, the mice will be the first mammal creatures — other than humans — to spend a prolonged period at the space station.

The next launch opportunity for Discovery would be Oct. 17.

A shuttle mission to the station now set for launch Nov. 12 would slip to Dec. 11 or Dec. 12. The delay also would make it more difficult for NASA to complete seven remaining shuttle missions before scheduled fleet retirement in late 2010.

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The Associated Press contributed to this report.