NASA on Thursday added nearly 24 hours to the delay in the space shuttle Discovery's scheduled launch to the international space station, in order to give engineers more time to study a problem involving a bad fuel valve sensor.
The launch time was moved from just after midnight on Friday (12:22 a.m. ET) to just before midnight the next night (11:59 p.m.). Forecasters predicted a 60 percent chance of acceptable weather for liftoff.
The mission management team ordered a halt to Thursday's countdown just hours after it restarted because engineers said they needed more time to analyze the results of tests conducted on the suspect fill-and-drain valve on Wednesday.
It marked the third time the countdown had to be stopped: Early Tuesday, liftoff was called off nine minutes before launch due to stormy weather around the launch pad at NASA's Kennedy Space Center in Florida. Later that same day, a second count had be stopped when indicators showed that the crucial valve did not close as required during the fueling process.
The 8-inch-wide (20-centimeter-wide) valve controls the flow of liquid hydrogen between Discovery and its external fuel tank. It must be closed during fueling, and opened again either after launch or after a scrub when the tank is drained.
The valve passed five open-and-close tests on the launch pad during Wednesday's troubleshooting, supporting the engineers' suspicions that the problem involved Discovery's indicator system rather than the valve itself. The next step is to find a way to launch the shuttle safely even if the indicators report the problem again, or go ahead with repairs to the indicator or the valve.
Mike Moses, the management team's chief, said planners were "80 to 90 percent" of the way through the process of developing methods to determine whether the valve was closed even if the indicator showed it was open. He said that could involve checking how long the actuator controlling the valve was in operation, as well as looking for characteristic emissions of helium gas from the valve's pneumatic system.
Engineers asked for extra time to make sure the plan was reliable. "We really just needed to polish the plan," Moses said. The revised routine would require NASA to issue a waiver from usual shuttle launch procedures, he said.
NASA has said that if Discovery is not launched by Sunday, the mission would have to be delayed until mid-October to provide launch opportunities for other spacecraft, including a Russian crew capsule and an unmanned Japanese cargo ship destined to visit the space station.
If NASA decides workers have to make repairs, that would guarantee a weeks-long delay, at least under the scenario currently sketched out by the space agency.
Discovery's 13-day mission is to bring the station thousands of pounds of supplies, experiments and equipment, including a treadmill named in honor of TV comedian Stephen Colbert. Among the seven astronauts assigned to the flight is NASA astronaut Nicole Stott, who is due to replace NASA's Tim Kopra on the station's six-member long-duration crew.