The shuttle Discovery's slow trip to launch pad 39A turned positively glacial early today while engineers assessed the condition of a soggy crawlerway and discussed whether to proceed or return to the protection of the Vehicle Assembly Building before expected afternoon thunderstorms. They ultimately decided to proceed, driving on the slightly drier edge of the crawlerway and taking nearly 12 hours to cover 3.4 miles.

Credit: Stephen Clark/Spaceflight Now Galleries: Overnight | Mud | Meteor streak | At pad

Discovery is tentatively scheduled for launch Aug. 25 on a space station resupply and crew rotation mission. Rollout was delayed one day by a discussion of what to do about a suspect check valve in the shuttle's left-side solid-fuel booster that failed during a high pressure leak test.

The valve problem caused a hydraulic pump and one of two auxiliary power units to spin backward, possibly damaging the hardware used to move the rocket's nozzle for steering during the first two minutes of flight. NASA managers decided to proceed with rollout and to replace the hardware at the launch pad.

The 3.4-mile trip to the pad began at 2:06 a.m. EDT. But the roadway that supports NASA's Apollo-era 5.5-million-pound crawler-transporter, the 3-million-pound space shuttle "stack" and its 8.2-million-pound mobile launch platform was saturated with water from recent storms. The massive crawler could do no better than one-quarter to one-third of a mile per hour and frequent stops were required to hose off its huge cleated trucks.

The crawler crew at one point recommended a return to the VAB, concerned that mud, gravel and sand could damage bearings, rollers or other components, leaving the shuttle stranded if repairs were required.

But moving back to the VAB could have triggered a major launch delay - Discovery has to get by the end of the month to avoid significant conflicts with other station missions - and after additional discussion, the crew was able to proceed, driving close to the slightly drier edge of the eight-lane-wide crawlerway.

Five hours into the normally six-hour move, the shuttle was just halfway to the launch pad. But as the morning wore on, the crawler could be seen on NASA television making slow but steady progress, the wet 2,000-pound cleats of its trucks glistening in the sunlight.

A final delay was in store when a pin broke going up the incline to the pad surface. After a stop for repairs, the slow trip continued and the shuttle's mobile launch platform was lowered onto massive support pedestals at 1:50 p.m. to complete the journey.

Discovery commander Frederick Sturckow, pilot Kevin Ford, Jose Hernandez, John "Danny" Olivas, Patrick Forrester, European Space Agency astronaut Christer Fuglesang and space station flight engineer Nicole Stott plan to fly to the Kennedy Space Center early Wednesday to review emergency procedures and participate in a practice countdown Friday.

Assuming the booster hydraulic system repairs can be completed in time, no other major problems develop and NASA managers make the current launch target official, Discovery's countdown will begin Aug. 22, setting up a launch attempt at 1:36 a.m. on Aug. 25.

The primary goals of Discovery's three-spacewalk mission are to deliver science racks and other hardware, including a new air revitalization system, to transfer crew supplies, swap out ammonia tanks for the station's cooling system and retrieve a European experiment package.

Stott, making her first flight, will replace flight engineer Timothy Kopra, launched to the station aboard the shuttle Endeavour July 15, as a member of the Expedition 20 crew. Assuming an on-time launch, Discovery's crew, with Kopra riding home in place of Stott, will land back at the Kennedy Space Center Sept. 6.

Additional coverage for subscribers:
VIDEO: ORBITER HOISTED FOR MATING TO TANK PLAY | HI-DEF
VIDEO: DISCOVERY MOVED TO ASSEMBLY BUILDING PLAY | HI-DEF
VIDEO: TIME-LAPSE OF DISCOVERY ARRIVING IN VAB PLAY
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