Shuttle Stays on the Pad

With thunderheads pressing in on the launch site in central Florida, NASA managers canceled Saturday’s liftoff of the space shuttle Discovery. The launch has been rescheduled for today at 12:26 p.m. PDT.

“This is a dynamic day. I think we’re playing it too close here,” said Steve Stich, the flight director at Johnson Space Center in Houston.

Launch director Mike Leinbach at Kennedy Space Center told shuttle Cmdr. Steve Lindsey, “Well, Steve, sorry to break your string.” Lindsey’s previous three shuttle launches had all gone off without a hitch.

Officials made the call only minutes before the scheduled launch at 12:49 p.m. PDT.

Forecasters say rain and thunderstorms will move from west to east across the central Florida coast for the next few days, a typical summer weather pattern for the hot and humid Southeast.

NASA regulations prohibit a launch in heavy clouds or when thunderstorms are within 20 nautical miles of the launch site. The fear is that the vehicle could trigger outbreaks of lightning as it streaks through the clouds, which could in turn knock out the shuttle’s electronics.

To find a proper vector for its rendezvous with the orbiting International Space Station, the shuttle can only launch within a 10-minute period each day.

This is the shuttle’s 115th mission and the second test flight since the loss of Columbia in 2003.

During the 12-day mission, the seven-member crew is scheduled to deliver two tons of cargo, including a second oxygen generator, to the space station. One of the crew, European Space Agency astronaut Thomas Reiter, will stay aboard the space station when Discovery departs, the first time since the Columbia disaster that the station will have a three-person crew.

NASA worked on another late problem Saturday, the failure of a thermostat on one of the small thrusters near the tail. Since there are 44 thrusters, launch officials decided the crew could work around the problem.

The weather, however, would not yield. The specific problem was “anvils,” the tops of thunderheads that can carry a charge. The anvils were parked off to the west much of the morning and afternoon, but one moved within the 20-mile limit just minutes before launch time.

“We can’t control the weather,” Leinbach said after the launch was scrubbed. “We’re not going to launch a vehicle unless it’s safe to do so.”

The problem with canceling launches is not just the inconvenience. Each launch day, the fuel tank must be filled with 535,000 gallons of liquid oxygen and hydrogen.

If the craft does not launch, the tank is emptied. This repeated fueling process could let cracks develop on the insulating foam that covers the tank. But each tank is allowed 13 fueling cycles, so it should not be a problem, NASA said.

If the launch today is scrubbed, NASA managers are considering taking a day off Monday.