The shuttle Discovery's crew strapped in for a dress-rehearsal countdown Friday, a milestone on the road to launch around Aug. 24 on a space station resupply mission. Engineers, meanwhile, are continuing a down-to-the-wire assessment of critical insulation on Discovery's external tank in the wake of foam losses during the shuttle Endeavour's launch last month.

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Space shuttle Discovery's seven astronauts climbed aboard their spacecraft at pad 39A for a simulated countdown this morning, completing a full dress rehearsal to practice the launch day coming up sometime in late August.

The Astrovan delivered the astronauts to the seaside pad around 8 a.m. EDT to begin entering the orbiter and taking their assigned seats just like the actual countdown.

Clocks ticked down to the final seconds before ending with a mock abort just after 11 a.m. EDT.

The Terminal Countdown Demonstration Test wrapped up with the suited astronauts exiting the shuttle to rehearse emergency evacuation procedures.

"It was a great day, great training for us, just all good," commander Rick Sturckow said.

Sturckow's crew, which includes pilot Kevin Ford, mission specialists Jose Hernandez, Danny Olivas, Christer Fuglesang, Pat Forrester and station-bound astronaut Nicole Stott, will fly back to Houston later today to finish final training there.

Repairs to the left-hand solid rocket booster hydraulics are underway at the pad, and the mission payloads were loaded into Discovery's cargo bay on Thursday.

The pre-flight schedules continue to be refined and a firm launch date remains up in the air.

A chart of launch windows can be viewed here.

Space shuttle Discovery's seven astronauts were welcomed at the Kennedy Space Center today for upcoming emergency training exercises and a countdown dress rehearsal with the launch team.

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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.

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A meteor streaking over shuttle Discovery! See that and other spectacular images from today's rollout here.

The space shuttle Discovery was hauled from the Vehicle Assembly Building to the launch pad today, slogging through the muddy pathway but eventually reaching its destination to ready the ship for blastoff by month's end.

Bolted to a giant external fuel tank and twin solid rocket boosters atop a mobile launching platform, Discovery embarked on the three-and-a-half-mile trip at 2:06 a.m. EDT.

The typical six-hour trip turned into an epic voyage stalled by a few technical problems and the unusually soggy crawlerway that gave workers a muddy mess.

For a time, it appeared that Discovery might reverse course and return to the assembly building because the conditions were just too much. But the ground crew steered the transporter to the edge of the roadway, where the wetness was less severe, and pushed onward.

An Apollo-era transporter carried the shuttle stack along Kennedy Space Center's rock-covered crawlerway leading to the oceanfront launch complex at far less than a walking pace. Finally getting through the pad gate around 11 a.m., powerful hydraulics jacked up the platform, keeping it level as the crawler ascended the pad's concrete incline after a marathon journey.

A laser alignment system helped technicians precisely position the platform, then the crawler lowered it onto the pad's pedestals to complete Discovery's rollout at 1:50 p.m. EDT.

In the next few hours and days, the methodical process of hooking up the crew module assess and hydrogen vent arms extending from the launch tower, as well as electrical, propellant, communications and other lines between the ground systems and mobile launch platform will begin.

The mission's payload -- the Italian-made Leonardo reusable cargo module -- has been waiting inside the gantry's cleanroom for the past few days for installation into Discovery's bay.

Liftoff of Discovery is targeted for August 25 at 1:36 a.m. EDT (0536 GMT). But there's little margin in the schedule, and a valve failure in the hydraulic steering system for the left-hand solid rocket booster that will require removal and replacement of hardware at the pad will add to the tight processing timelines.

The upcoming mission to the International Space Station follows just after Endeavour's recent flight that featured extensive use of spacewalkers and robotic arms to attach an outdoor platform for Japan and large external spare parts. Discovery's trip will focus on the station's interior by installing more science research equipment and crew accommodations.

"It doesn't have all of the robotic activities like the mission we just accomplished. It has more of a utilization kind of feel where we're now really setting up the internal research capabilities," said Bill Gerstenmaier, NASA's associate administrator for Space Operations.

"The laboratory shells are up there but now you could think of it as research racks being delivered, as well as critical supplies that are needed to maintain the space station as a research facility."

Leonardo is packed with about 15,000 pounds of cargo, including fluid and materials science experiment racks, a laboratory freezer to store samples, another sleep compartment for the station's larger resident crews, a new air purification system and the treadmill named for comedian Stephen Colbert.

It will be the 30th space shuttle flight to the International Space Station dating back to December 1998, the 128th mission overall for the shuttle program and Discovery's 37th spaceflight.

The seven astronauts will fly from Houston to the Cape on Wednesday morning for the start of their Terminal Countdown Demonstration Test, which serves as a launch dress rehearsal for the crew and ground teams.

The mobile launch platform was lowered onto the pad pedestals at 1:50 p.m. EDT, marking the official time for Discovery's arrival at pad 39A.

Space shuttle Discovery has arrived atop the pad. The rollout's official conclusion time will be marked when the launch platform is secured to the pad pedestals.

The crawler is getting the mobile launch platform positioned over the pad pedestals.

It is now ten hours since Discovery departed the Vehicle Assembly Building on what would normally be a six-hour trek to the launch pad.

Discovery atop the crawler transporter is inching its way towards its perch on launch pad 39A.

Discovery has climbed up the concrete ramp of launch pad 39A, its crawler-transporter having used massive hydraulic pistons to keep the mobile platform level.

Now atop the pad surface, a precision laser guidance system will help align the platform over the pad pedestals. The crawler will lower the platform onto the pedestals to complete the rollout.

The crawler transporter hauling space shuttle Discovery is now climbing the concrete ramp to the launch pad.

After a difficult morning, the crawler is nearly at the entrance to launch pad 39A now.

Now passing the eight-hour mark into today's rollout.

The rollout team reports resumption of crawler toward the pad.

The newly arrived external fuel tank has entered into the Vehicle Assembly Building's center transfer aisle. It'll be hoisted into a storage and checkout cell to be prepared for flight.

It's a busy day at America's spaceport. In addition to space shuttle Discovery's trek to the launch pad, other workers are preparing to receive the external fuel tank destined for use by the Atlantis mission in November.

Our live webcast on the right-hand side of this page is streaming video of the tank being unloaded from the barge named Pegasus.

The tank was delivered to the Kennedy Space Center yesterday aboard the covered barge following a seagoing voyage from Lockheed Martin's manufacturing plant near New Orleans. Once a special transport trailer drives off the shipment barge onto KSC soil, it will take the tank into the Vehicle Assembly Building.

The crawler has stopped to reload lubrication. This should take about 20 minutes, the rollout team says.

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 arrive. They ultimately decided to proceed, driving near the slightly higher edge of the soaked gravel roadway instead of down the middle.

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The crawler is passing the fork in the road -- left to pad 39B or straight ahead to pad 39A. The rollout has been underway for more than six hours, but the trip seems to be moving along with a steady pace now.

Space shuttle Discovery has been slogging through a muddy mess on its way to launch pad 39A. Pictures illustrating the conditions on the crawlerway this morning are posted here.

Hauling the shuttle stack out to launch pad 39A on this 3.5-mile trek is one of NASA's two Apollo-era crawler-transporters. The combined weight of the transporter, mobile launch platform and shuttle Discovery is 17.5 million pounds.

The stone-covered pathway connecting the VAB to the launch pad is 130 feet wide -- almost as broad as an eight-lane highway. Two 40-foot-wide lanes are separated by a 50-foot-wide median strip. The average depth is seven feet.

About 30 people are aboard the transporter to operate it during the rollout, including three drivers -- a prime and backup in the front cabin and one in the rear -- a jacking and leveling operator, a control room operator to run crawler systems and talk with the Launch Control Center, two electricians, two electronic technicians and four diesel mechanics for starting, monitoring and shutting down the transporter's engines. The other team members are mechanics watching over the roll and helping with the platform's docking to the launch pad.

The transporter consumes 126 gallons of diesel fuel in each mile it travels from the VAB to pad. The vehicle has a fuel capacity of 5,000 gallons.

By the crawler driver keeping the transporter off center of the roadway and continuously washing away the mud, the team has gotten back to the normal 0.8 mph cruising speed. The official decision has been made to proceed with the rollout to the launch pad.

The team estimates Discovery is still three hours away from the entrance to the pad and a couple additional hours from being secured on the pad pedestals.

There are concerns that Discovery might not make it to the pad before afternoon storms hit. The crawler has moved toward the edge of the crawlerway in the hope of finding drier ground.

The crawler has resumed its trek but its progress will be slowed to 0.25-0.3 mph due to the soggy conditions on the crawlerway. The shuttle has not yet reached the halfway point to the pad.

The crawler has stopped once more for the crew to make some adjustments.

The crawler is moving again after a long pause to debate the muddy mess the rollout team has found on the way to launch pad 39A.

The rollout is going to press ahead at a reduced speed and see how it goes. Returning to the Vehicle Assembly Building today without any obvious ways to improve the crawlerway would likely prevent Discovery from launching this month, officials say.

The question is how to improve the crawlerway conditions given the summertime afternoon thunderstorms and more rain that would be likely.

A gallery of pictures showing Discovery's departure from the Vehicle Assembly Building can be seen here.

The concern with continuing toward the pad is mud or dirt getting into the moving parts of the crawler. The conditions onward to the pad don't look much better. One team member reported their shoes completely sinking into the mud.

The rollout team is recommending the space shuttle be moved back to the Vehicle Assembly Building because of the muddy crawlerway. The slower travel speed and washing the transporter hasn't been helping much. Managers are assessing and no decision has been made yet.

The muddy roadway to the launch pad is making for slow going this morning. The rollout team is using water trucks to clean out the sand and dirt that's collecting in the tracked machine.

Space shuttle Discovery has moved away from the 52-story landmark Vehicle Assembly Building where it spent the past week being attached to the external fuel tank and twin solid rockets atop a mobile platform.

After another pause, the middle-of-the-night move has resumed inching toward pad 39A in the predawn darkness.

It was a brake problem that temporarily halted Discovery's trip to the launch pad, officials say. The crawler was stationary a short distance from the VAB while technicians analyzed the situation. But the problem has been resolved and the crawler is moving again.

The crawler has stopped outside of the Vehicle Assembly Building while workers assess a problem.

ROLLOUT BEGINS. Inside the cavernous Vehicle Assembly Building, the transporter has started driving space shuttle Discovery toward Kennedy Space Center's launch pad 39A.

The overnight journey just got underway, about two hours later than planned because of lightning concerns delayed rollout preparations Monday night.

The crawler-transporter is hydraulically lifting the mobile launching platform off the pedestals in the Vehicle Assembly Building. Shuttle Discovery should be setting sail for the launch pad very shortly.

The rain finally dissipated and the moon is shining through the thin cloud cover.

It's looking like 1:45 or 2 a.m. EDT start time now.

The latest estimate for the start of today's rollout is approximately 1:30 a.m. EDT, according to team members.

Ground technicians are getting the crawler-transporter positioned underneath Discovery's mobile launch platform.

It's a soggy night at the Kennedy Space Center. Steady rains have fallen all evening and it continues to sprinkle at this hour.

NASA now projects Discovery's rollout to pad 39A will begin at 1 a.m. EDT due to the earlier lightning warnings that held up work.

The giant retractable doors on the Vehicle Assembly Building have been opened and activities are proceeding toward the start of the shuttle's move.

The Phase 2 lightning advisory for workers around the Kennedy Space Center has ended. Opening of the doors to the Vehicle Assembly Building's high bay No. 1 will begin shortly in preparation for Discovery's departure tonight.

The weather outlook for Discovery's overnight move to the launch pad looks pretty good. Meteorologists are predicting a minimal chance of lightning and just a slight possibility of rain. Light southerly winds are expected.

Storms have been moving through the Space Coast area this evening. But the weather should diminish over the next couple of hours.

Engineers are readying the shuttle Discovery for rollout to pad 39A early Tuesday, one day later than planned because of work to assess the impact of a check valve failure in the steering system of the ship's left-side solid-fuel booster. The valve problem apparently caused a hydraulic system pump to spin backwards during a high-pressure leak check, possibly damaging the pump and one of the booster's hydraulic power units, officials said.

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NASA is ready to roll the fully assembled space shuttle Discovery to Kennedy Space Center's launch pad 39A beginning at midnight EDT.

The ground team responsible for moving the spacecraft along the 3.5-mile stone-covered roadway from the Vehicle Assembly Building to the seaside pad is scheduled to report for duty at 8 p.m. EDT.

The crawler-transporter will hydraulically lift the mobile launching platform and carry the shuttle on the six-hour trip. If all goes according to plan, Discovery should be secured atop the pad around sunrise.

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An internal valve problem with the hydraulic steering system on the left-hand solid rocket booster of space shuttle Discovery is being analyzed in the Vehicle Assembly Building, NASA said this evening.

Any work to replace hardware associated with the hydraulic power unit would take place after Discovery is rolled out to the launch pad, an agency spokesman said. What, if any, impact this issue could have on the targeted August 25 launch date is not yet known.

"During pre-rollout preparations and testing Saturday morning in Kennedy's Vehicle Assembly Building, a valve failed inside Discovery's left-hand solid rocket booster hydraulic power unit tilt system, which helps steer the SRB during launch," the spokesman said in a statement.

The spokesman indicated that the delay in Discovery's move to the pad from Monday to Tuesday was caused by the ongoing evaluations of the valve problem.

The seven astronauts remain scheduled to arrive at Kennedy Space Center on Wednesday for emergency training exercises and a practice countdown that culminates with a launch morning rehearsal on Friday.

Rollout of space shuttle Discovery to launch pad 39A is now targeted to begin Tuesday morning at 12:01 a.m. EDT (0401 GMT) for its late August flight to the International Space Station.

Originally planned for Wednesday, managers had moved up to Monday the target date for Discovery's 3.5-mile move from the Vehicle Assembly Building to the seaside pad. But a decision to perform another round of foam adhesion tests on the external tank will result in the rollout slipping to Tuesday morning.

This latest testing -- which is in addition to the 150 or so "plug-pulls" already done -- will examine an area of blocked by scaffolding during the earlier checks.

The shuttle was mated up with its fuel tank and solid rocket boosters atop a mobile launching platform inside the Vehicle Assembly Building this week. A combined systems test between all of the shuttle elements is underway.

Meanwhile, the mission payloads were transported to the pad Thursday night and hoisted into the cleanroom of the rotating gantry on Friday. The main cargo is the Italian-made Leonardo logistics carrier packed with new equipment and supplies for the space station. The payloads will be inserted into the Discovery's bay upon the shuttle's arrival at the pad.