Our scrub story has been updated with additional details.

Following space shuttle Endeavour's launch postponement this morning, work began immediately to reconfigure the Eastern Range to support the Atlas 5 rocket and its Moon-bound mission. The rocket will be rolled out to the Complex 41 pad at 10 a.m. EDT today in preparation for launch Thursday at 5:12 p.m. EDT.

Read more about that launch here.

"Our next steps are really still to be determined. We'll be meeting successively with the investigation team members and compare the data from the experience we had from tonight's tanking with the experience we had from the tanking for the June 13 attempt as well as from STS-119 really to ascertain what differences there were and what this really means for our troubleshooting solutions," said Launch Director Pete Nickolenko.

"We gotta step back and try to understand this problem. We will do that, I'm confident we will do that. It's going to take us a little time to do that," said Mission Management Team Chairman LeRoy Cain. "As a result of this scrub, we will be targeting our next earliest available launch date which would be after the beta cutout. That will be as early as July 11."

Commander Mark Polansky tweeted on his Twitter page: "I'm sure you all know that we postponed again. It's a reminder that spaceflight is NOT routine. We will fly home to Houston this morning."

Senior Launch Director Mike Leinbach, serving as deputy to Pete Nickolenko for the shuttle Endeavour's countdown, said the gaseous hydrogen leak that grounded the spacecraft Wednesday for the second time in four days was too severe to continue with a launch attempt. Speaking on NASA television, Leinbach explained what happened leading up to the launch scrub.

"We got into tanking and everything looked like it was going fine until we were about an hour away from the end of tanking and we picked up the hydrogen leak at the umbilical again," he said. "This time, it was a little bit different. We were actually picking up the leak before we got into the topping sequence, which is where we've seen this leak before.

"So at that time, we knew we had a little bit different signature. We watched the data very closely, we did our standard troubleshooting techniques by cycling the valve to see if we could clean up that leak again and indeed, it never cleaned up. The signature was a little bit different, but that doesn't surprise me. With cryogenic leaks it would be very unusual to have an identical leak.

"And so the fact that this one was a little bit different and we thought we might be able to work our way through it, that didn't surprise me we were trying to do that.

"The team did an outstanding job over these last four days to get to this point," Leinbach said. "I sure wish we could have rewarded them and the astronauts and everybody with a launch this morning. But the leak was way out of spec again and so we were just not comfortable pressing on. As much as we tried to fix the leak, we just couldn't do it so we had to scrub and secure again.

"We're in the process of draining the external tank now. ... Sometime tomorrow evening, we'll be able to get our hands on this disconnect again and go into taking it apart. I imagine we'll put together a more detailed troubleshooting plan this time and go execute that once we get our hands on the disconnect again."

NASA says the next attempt at launching space shuttle Endeavour will come after the temperature constraint with the space station's orbit ends. That puts launch on July 11 at 7:39 p.m. EDT.

The post-scrub news conference is targeted to begin no sooner than 3:30 a.m. EDT.

"The leak was way out of spec again and so we were just not comfortable pressing on. As much as we tried to fix the leak, we just couldn't do it. So we had to scrub and secure again," launch director Mike Leinbach says.

The launch team is draining the cryogenic propellants from space shuttle Endeavour's external fuel tank after this morning's postponement. The tank will be inerted and workers should be able to get their hands on the ground umbilical carrier plate by tomorrow evening.

After a lengthy fueling delay because of stormy weather, launch of the shuttle Endeavour on a space station assembly mission was scrubbed early Wednesday when a presumably repaired hydrogen vent line umbilical began leaking potentially dangerous vapor.

Read our full story.

NASA plans to hold a post-scrub news conference from the Kennedy Space Center at some point this morning.

"This is Shuttle Launch Control. At 1:55 a.m. EDT, we are scrubbing the launch attempt for today. The troubleshooting efforts have not resulted in a significant decrease in the amount of gaseous hydrogen that's being detected outside of the ground umbilical carrier plate, the same area where we experienced a leak in the last launch attempt," launch commentator Mike Curie said in announcing the scrub.

"STS-127 launch director Pete Nickolencko has just given the team a 'go' to scrub the launch attempt after two more more attempts to open the vent valve again displayed much higher than expected amounts of gaseous hydrogen."

SCRUB! Launch of space shuttle Endeavour has been postponed due to another leak in the gaseous hydrogen venting system between the launch pad and external fuel tank. This is the same problem that cancelled Saturday's initial countdown.

When Endeavour could make another launch attempt is not known. Obviously, the leak will have to be fixed. NASA faces a Saturday deadline the get the shuttle launched or else wait until July 11 due to a period of unfavorable thermal conditions in the orbit with the International Space Station.

At this point, there's been no official decision about calling off the countdown. Testing and analysis of the leak situation continues.

This leak is in the same area that scrubbed Endeavour's first launch attempt on Saturday and a countdown on the earlier STS-119 mission of Discovery back in March.

Flow of liquid hydrogen has been stopped again. NASA says troubleshooting could continue for some time.

The launch team has been given permission for to restart "fast-fill" loading of liquid hydrogen.

Liquid hydrogen loading of the external tank is going to resume in order to get the tank at a higher level for additional vent valve leak testing.

A second cycling of the vent valve resulted in a slightly lower leak rate. Further testing is being performed.

The vent valve has been cycled in the ground umbilical carrier plate in hopes of resolving the gaseous hydrogen leak. However, NASA says the leak rate remains high.

Liquid hydrogen loading into the external tank has been stopped due to the gaseous hydrogen leak in the venting system. The hydrogen fueling of Endeavour was halted at 1:02 a.m.

A leak has reoccurred in the gaseous hydrogen venting system between the launch pad and space shuttle Endeavour, NASA confirms. The leak rate is greater than expected at this point in the countdown. Engineers are watching the data.

"We enjoy challenges and the team is performing in an outstanding manner," launch director Mike Leinbach says.

"We're not going to short cut anything. If we get to the situation where we just can't make it tonight, we'll just call it and say we can't make it. But right now, we think we can get there fine."

NASA launch director Mike Leinbach says officials have added another astronaut support member of the Orbiter Closeout Crew to help quicken the process of getting Endeavour's astronauts strapped inside the shuttle today.

The astronauts' departure from the crew quarters building is going to be delayed an hour -- to 2:50 a.m. EDT -- because of the countdown activities running behind schedule by the weather.

Liquid hydrogen and liquid oxygen loading have switched to the "fast-fill" mode as fueling of space shuttle Endeavour proceeds via remote control at launch pad 39A.

There are two tanks inside the shuttle's external fuel tank. The liquid oxygen tank occupies the top third of the bullet-shaped tank. It will be filled with 143,000 gallons of liquid oxygen chilled to minus 298 degrees Fahrenheit. The liquid hydrogen tank is contained in the bottom two-thirds of the external tank. It holds 385,000 gallons of liquid hydrogen chilled to minus 423 degrees Fahrenheit.

The gaseous hydrogen vent system that had the leak on Wednesday will be checked out once the tank reaches 98 percent full.

Both liquid hydrogen and liquid oxygen are in "slow-fill" mode.

The liquid hydrogen loading has transitioned from chilldown to the "slow-fill" mode. This fills a small fraction of the tank, then the loading switches to "fast-fill" mode.

T-minus 3 hours and holding. Clocks have entered a planned two-hour, 30 minute built-in hold. Launch remains scheduled for 5:40 a.m. EDT.

After stormy weather delayed fueling for almost three hours tonight, filling of space shuttle Endeavour's external fuel tank is finally getting underway with the chilldown thermal conditioning process. This will be followed by the slow-fill mode and then the fast-fill mode to load the tank over the next three hours.

GO FOR FUELING.

Lightning advisories for the launch pad areas have been discontinued.

The astronauts are scheduled to depart their quarters and head for launch pad 39A three hours from right now.

Commander Mark Polansky tweeted on his Twitter page: "Waiting for word on whether we can tank the vehicle or not. Time will tell. Can't worry about things that we can't control."

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Mission managers are watching the radar and waiting for the storms to move away, hopefully before it's too late to begin fueling for launch in the morning.

Thunder and lightning continue to pound Central Florida, now two hours after fueling of shuttle Endeavour was supposed to start.

Countdown clocks continue to tick but fueling still hasn't begun.

This unplanned hold in the start of fueling has lasted 90 minutes now. But NASA has not yet given up hope that the stormy weather will clear out of the area and allow the countdown activities to resume in preparation for liftoff at 5:40 a.m. EDT.

Weather conditions remain the obstacle preventing the start of fueling. The launch team is looking to find efficiencies in the countdown timeline to catch up after this delay, if the weather clears and fueling can get underway.

NASA's weather rules prohibit the start of fueling if there is forecast to be greater than a 20 percent chance of lightning within five nautical miles of the launch pad during the first hour of tanking.

The thunderstorms in the Kennedy Space Center area are not dissipating as quickly as shuttle weather forecasters had hoped. Fueling remains on hold while launch controllers hope for a break in the weather.

T-minus 6 hours and counting. Clocks have just resumed ticking following another of the countdown's planned built-in holds. The next hold occurs at the T-minus 3 hour mark. Launch of Endeavour remains targeted for the precise moment of 5:40:52 a.m. EDT (0940:52 GMT).

Again, fueling remains on hold due to lightning concerns around the launch site. NASA is waiting for the weather picture to improve before starting the fueling process. The countdown has about 90 minutes of slack to accommodate fueling delays such as this.

The Endeavour astronauts were awakened at 6:45 p.m. EDT to begin their launch day activities.

Commander Mark Polansky just tweeted on his Twitter page: "As Yogi said, it's Deja vu all over again! It's launch day, and we're just relaxing before it's time to get ready at about 0100 EDT."

The Mission Management Team completed its pre-fueling meeting and given the launch team a tentative approval to begin filling Endeavour's external tank once the weather clears.

Past experience says NASA could delay fueling as much as 90 minutes and still hit the planned launch time. The tanking procedure was supposed to start at 8:15 p.m. EDT.

Stormy weather around the Kennedy Space Center will delay the start of fueling space shuttle Endeavour with explosive rocket propellant at launch pad 39A tonight.

NASA hopes to get the fueling operation underway perhaps later this hour.

T-minus 6 hours and holding. Countdown clocks have just paused for a planned two-hour hold prior to fueling of space shuttle Endeavour.

During this hold, the Mission Management Team will gather for its readiness meeting at 7:30 p.m. to give approval for fueling while the launch team determines if there are any constraints with loading the shuttle's external tank starting at 8:15 p.m. tonight.

Launch remains scheduled for 5:40 a.m. EDT tomorrow.

Engineers pulled a protective gantry away from the shuttle Endeavour and restarted the orbiter's countdown today, setting the stage for launch Wednesday on a delayed space station assembly mission.

Read our full story.

Space shuttle Endeavour has been unveiled from the cocoon-like rotating service structure at Kennedy Space Center's pad 39A in preparation for tonight's fueling and tomorrow's predawn liftoff on the next construction flight to the International Space Station.

Countdown clocks are scheduled to resume ticking from the T-minus 11 hour mark at 1:15 p.m. EDT. This afternoon's work includes activating the Endeavour's inertial measurement units and power-generating fuel cells, plus checking all of the switches in the cockpit to ensure they are in the correct positions for launch.

The count will proceed down to the T-minus 6 hour point where a two-hour hold begins at 6:15 p.m. During that pause, the launch team will verify all systems are ready to begin loading a half-million gallons of supercold rocket fuel in the shuttle's external tank.

The three-hour fueling process is scheduled to commence around 8:15 p.m. EDT, pending a final "go" from the Mission Management Team's evening meeting to review the status of the count.

Launch remains scheduled for 5:40 a.m. EDT.

Rollback of the launch pad's service gantry has begun as preparations get into full swing for tonight's countdown.

Work on the ground umbilical carrier plate was completed this morning and the access platforms used by the workers to reach the backside of the external tank were stowed for launch.

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Meteorologists continue to predict an 80 percent chance of acceptable conditions for Wednesday morning's launch attempt of space shuttle Endeavour.

At the 5:40 a.m. EDT liftoff time, the outlook calls for a few clouds at 3,000 feet and scattered clouds at 6,000 and 25,000 feet, good visibility, westerly winds from 270 degrees at 6 to 10 knots and a temperature of 77 degrees F.

The only slight concerns will be potential violation of the cumulus cloud rule and showers within 20 miles of the shuttle's emergency runway at Kennedy Space Center.

Updated countdown, ascent and flight plan timelines have been posted to reflect Endeavour's new launch date.

NASA managers today formally cleared the shuttle Endeavour for a delayed launch Wednesday on a space station assembly mission. Launch of the agency's Lunar Reconnaissance Orbiter atop an unmanned Atlas 5 rocket was delayed to June 18 or 19 to make room for the shuttle in an effort to maximize launch opportunities for both missions.

Read our full story.

There's an 80 percent chance of acceptable conditions for Wednesday's predawn launch time. Here's the outlook from the launch weather team:

"An upper-level ridge is located over the Gulf of Mexico. An upper-level disturbance will progress south down the east side of the ridge and move through Central Florida this afternoon; in addition, an afternoon sea breeze will develop. With both of these triggers in place this afternoon, thunderstorms are forecast for Kennedy Space Center (KSC). Tomorrow, another upper-level disturbance may again move down the east side of the ridge, causing a 20 percent chance of thunderstorms within 5 nautical miles during tanking operations. By Wednesday morning, a low pressure area will move off the east coast of the Carolinas, dragging a front into North Florida. Weather conditions are generally favorable with only a 20 percent chance KSC weather will prohibit launch due to showers and cumulus clouds. Our primary concerns for launch are showers within 20 nautical miles of the Shuttle Landing Facility (SLF) and cumulus clouds."

Space shuttle Endeavour will bump its way into the Cape Canaveral launch lineup and target a 5:40 a.m. EDT liftoff Wednesday, NASA officials decided today. That means the Atlas 5 rocket originally scheduled to launch at that day carrying two Moon-bound science satellites will be pushed back.

Both launches face Saturday deadlines due to the lunar launch window closing and thermal constraints on Endeavour's orbit with the space station.

NASA will hold a news conference from the Kennedy Space Center at 3 p.m. EDT today.

Working at the end of an access platform high above the ground, technicians continue their efforts to fix the gaseous hydrogen leak in the venting system between the launch pad and space shuttle Endeavour. The crews are replacing internal seals in that umbilical in hopes of solving the problem.

Mission managers will meet again this afternoon to decide, for sure, whether Endeavour can proceed toward launch on Wednesday and if the Atlas 5 rocket will be delayed to Friday.

NASA will hold a news conference at 3 p.m. EDT today.

Watch this page for live updates as information becomes available.

NASA managers Sunday deferred making a formal decision on whether to reschedule the delayed shuttle Endeavour for launch Wednesday or whether to press ahead instead with launch of the agency's $583 million Lunar Reconnaissance Orbiter mission aboard an unmanned Atlas 5 rocket.

Read our full story.

Senior NASA leaders today deferred their decision about whether shuttle Endeavour or the Moon-bound Atlas 5 rocket will get to launch on Wednesday. For now, both missions will continue preparations and await further word.

The Air Force-run Eastern Range can support only one vehicle at a time and needs about 48 hours between different launches to reset tracking and support systems. Given the early morning launch window for Endeavour and late afternoon for the Atlas, that means the Range could handle the Atlas on Wednesday and the shuttle on Saturday. If Endeavour goes on Wednesday morning, the Atlas could try on Friday afternoon.

Both launches face Saturday deadlines due to the lunar launch window closing and thermal constraints on Endeavour's orbit with the space station.

NASA will hold a news conference from the Kennedy Space Center shortly.

News conference now no sooner than 5:30 p.m. EDT.

NASA now projects a news conference start time no sooner than 5 p.m. EDT.

The news conference start time has moved to 4:30 p.m. EDT.

Images showing the hydrogen swing arm moving away can be seen here and here.

The hydrogen swing arm has been moved away, setting the stage for workers to get into the GUCP.

Pad crews have disconnected the pad hydrogen vent swing arm from the ground umbilical carrier plate (GUCP) that remains attached to the shuttle's external tank. This will allow the technicians to gain access to the GUCP for repairs.

Meanwhile, the management team is underway at Kennedy Space Center. NASA plans a news conference no sooner than 4 p.m. EDT.

At launch pad 39A earlier today, technicians extended an access arm and began work on the leaky ground umbilical carrier plate. They plan to replace internal seals in the system later this afternoon, NASA says.

A picture showing the repairs now underway can be seen here.

An earlier closeup view of the GUCP taken Saturday morning is posted here.

The residual traces of cryogenic propellants inside space shuttle Endeavour's fuel tank should be boiled away by midnight, enabling ground technicians to begin preparations for inspecting and repairing the ground umbilical carrier plate that leaked during Saturday morning's countdown.

The GUCP fits to the backside of the space shuttle to funnel gaseous hydrogen from the external fuel tank. Plumbing then carries the gas across an umilical arm to the pad tower, down to the ground and eventually out to the flare stack for burning. Precisely what is causing the leak where the ground equipment attaches to the shuttle is not yet known. In fact, the root cause of a similar leak back in March was never pinpointed.

A serve thunderstorm rumbled over pad 39A Saturday evening as the shuttle sat exposed to the elements. The rotating service gantry is scheduled to be swung back around Endeavour as part of the post-scrub procedures at 7:30 a.m. EDT Sunday.

Mission managers still plan to meet at 2 p.m. EDT Sunday to examine the repair plans and determine the strategy for getting Endeavour a new launch date.

Preliminary timelines, based on the work performed in March to replace the GUCP hardware and fix that leak on shuttle Discovery, indicate Wednesday is the soonest the shuttle could be ready to fly. But an Atlas 5 rocket launch from the Cape already has that date booked on the Eastern Range. The Air Force-controlled Range needs a couple of days between launches to reset its equipment.

If the Atlas launches on Wednesday afternoon, that would push the shuttle's liftoff to the predawn hours of Saturday. NASA officials say Saturday is the last day the shuttle could launch and fly its full 16-day mission because of thermal concerns between Endeavour and the space station's orbit. The orbital constraint prohibits launch from Sunday through about July 11.

NASA leaders could opt to delay the Atlas mission, which carries a pair of lunar spacecraft for the space agency, and let Endeavour attempt a launch on Wednesday morning. But the Atlas has a tight lunar window that offers launch opportunities on Wednesday, Thursday, Friday and Saturday. Otherwise, it would be delayed until around July 1 and potentially disrupt the already-packed schedules for subsequent Atlas missions from the Cape.

A late afternoon news conference following the management team meeting is planned for Sunday.

Launch of the shuttle Endeavour, grounded by a gaseous hydrogen leak during fueling Saturday, is off until Wednesday at the earliest, NASA officials say. But because of the already planned launch of NASA's Lunar Reconnaissance Orbiter satellite Wednesday, the shuttle team could be delayed to June 20, the last day this month Endeavour can be launched.

Read our full story.

The Mission Management Team will meet at 2 p.m. EDT on Sunday to discuss the repair plan and launch date options, NASA says. A post-meeting news conference will be held.

Shuttle launch director Mike Leinbach says this morning's leak is identical to the one that occurred in the Ground Umbilical Carrier Plate during Discovery's first countdown back in March. It will be Sunday when technicians can get their hands on the GUCP to begin repair efforts. The soonest Endeavour could be ready to launch is Wednesday.

The Mission Management Team will meet on Sunday to assess the go-forward plans, chairman Mike Moses says.

The launch team is entering a four-day delay scenario, meaning the earliest Endeavour could fly is Wednesday at about 5:40 a.m. EDT. However, that would conflict with the Atlas rocket launch carrying the Lunar Reconnaissance Orbiter and the Lunar Crater Observation and Sensing Satellite from Cape Canaveral already planned for that day.

If NASA proceeds with the Atlas launch as scheduled and the rocket flies on time, the soonest the Eastern Range could be reconfigured to support the next shuttle launch attempt would be Saturday, June 20.

A news conference is coming up at 1:45 a.m. EDT.

A chart showing Endeavour's available launch windows for June can be seen here.

A picture of the GUCP attached to the backside of shuttle Endeavour's external tank can be seen here.

Commander Mark Polansky just tweeted on his Twitter page: "Got the word that we're not going today. That's part of r business. Most important thing is to not launch til everything is ready."

A NASA spokesman says draining of the external tank is underway. Preliminary information from the space agency says a leak in the Ground Umbilical Carrier Plate, or GUCP, was detected as the fueling process was nearing conclusion. The GUCP is part of the launch pad equipment that vents gaseous hydrogen away from the space shuttle.

There was a leak with the GUCP during the STS-119 mission back in March. In that case, the system leak was discovered as the liquid hydrogen tank was entering final topping mode. Technicians replaced the ground assembly and the system was tight for the subsequent launch attempt four days later.

When Endeavour could be ready for another launch attempt is pure speculation at this early stage. But there is an Atlas 5 rocket scheduled for liftoff from nearby Cape Canaveral on Wednesday. NASA had three days to get the shuttle off the ground or else wait until after the rocket's flight carrying a pair of lunar exploration spacecraft.

The Air Force-controlled Eastern Range that provides tracking, safety and communications services to all launches from the Cape needs about 48 hours between flights to reconfigure equipment.

NASA will hold a news conference at some point overnight to provide further details. We'll update this page as information becomes available.

SCRUB. NASA has called an official cancellation of today's launch attempt because of a leak that developed late in fueling. A possible source of the leak could be the gaseous hydrogen vent system, which also caused a scrub during the March shuttle mission.

NASA is planning a news conference overnight to brief reporters on the situation.

It is not yet known when the launch will be rescheduled.

A leak has developed in fueling of space shuttle Endeavour. The launch team is troubleshooting to determine the source. It appears that launch will be scrubbed for today. However, NASA has not yet announced that.

A problem has developed in the countdown. Standing by for official word.

The fueling operation is proceeding smoothly tonight, NASA says. Clocks continue to count down toward a liftoff at 7:17 a.m. EDT.

Filling of the space shuttle's external tank takes about three hours to complete, and keeping the tank fully loaded continues until just minutes before liftoff time.

The process starts with the chilldown thermal conditioning of the system, followed by a slow-fill mode and then the fast-fill mode. The tank then enters a stable replenishment mode that ensures the liquid hydrogen and liquid oxygen supplies are topped off through the final minutes of the countdown.

The cryogenics are pumped from storage spheres at the pad, through feed lines to the mobile launcher platform, into Endeavour's aft compartment and finally into the external fuel tank.

There are two tanks inside the shuttle's external fuel tank. The liquid oxygen tank occupies the top third of the bullet-shaped tank. It will be filled with 143,000 gallons of liquid oxygen chilled to minus 298 degrees Fahrenheit. The liquid hydrogen tank is contained in the bottom two-thirds of the external tank. It holds 385,000 gallons of liquid hydrogen chilled to minus 423 degrees Fahrenheit.

The filling of space shuttle Endeavour's external fuel tank with a half-million gallons of supercold propellants is underway at launch pad 39A.

The tanking operation commenced with the chilldown thermal conditioning process at 9:52 p.m. EDT as scheduled.

The Mission Management Team met in the past hour and gave the "go" to begin fueling. There are no significant problems being worked and liftoff remains on schedule for 7:17 a.m. EDT.

T-minus 6 hours and counting! The coundown has resumed ticking after the two-hour planned hold. The next scheduled hold occurs at T-minus 3 hours. Later pauses happen at T-minus 20 minutes and T-minus 9 minutes to give the launch team time to catch up on work running behind schedule and deal with any problems. Liftoff is targeted for precisely 7:17:19 a.m. EDT.

We're now standing by for the start of fueling operations.

The Mission Management Team has given approval to begin fueling space shuttle Endeavour for tomorrow morning's launch.

The astronauts were awakened at 8:15 p.m. They're having breakfast right now.

A short time ago, commander Mark Polansky tweeted on his Twitter page: "Good morning. It's launch day! Want to know what an astronaut does on launch day? I'm watching Game 7 of the Stanley Cup Finals."

There are no worries about weather prohibiting the Endeavour fueling tonight. The official forecast calls for some low- and high-level clouds around the area, southeasterly winds of 6 to 10 knots, a temperature of 80 degrees and relative humidity of 75 percent.

T-minus 6 hours and holding. Countdown clocks have just paused for a planned two-hour hold prior to fueling of space shuttle Endeavour. This is the latest in a series of holds that are built into the three-day countdown sequence leading to launch at 7:17 a.m. EDT tomorrow.

During this hold, the Mission Management Team will gather for its readiness meeting at 9:15 p.m. to give approval for fueling while the launch team determines if there are any constraints with loading the shuttle's external tank starting at 9:52 p.m. tonight.

It has been a clear and dry day at the Kennedy Space Center, albeit incredibly hot. The weather outlook for Saturday's 7:17 a.m. EDT launch time looks to be favorable too, with just a few clouds, no rain, good visibility and very light winds.

Out at the launch pad 39A, Endeavour looks ready to receive a half-million gallons of rocket fuel tonight and blast into orbit in the morning.

The launch team in Firing Room 4 continues to march through pre-flight preparations. The start of fueling operations remains scheduled for just before 10 p.m. EDT.

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A protective gantry was rolled away from the shuttle Endeavour today, exposing the orbiter to view atop launch pad 39A and setting the stage for fueling and liftoff Saturday on a space station assembly mission.

Read our full story.

The gantry-like rotating service structure at Kennedy Space Center's pad 39A has been retracted from Endeavour in preparation for tonight's fueling and tomorrow's liftoff on the next construction flight to the International Space Station.

Currently, countdown clocks are stopped at the T-minus 11 hour mark for a planned built-in hold. The count is scheduled to resume ticking at 2:52 p.m. EDT. Today's work includes activating the Endeavour's inertial measurement units and power-generating fuel cells, plus checking all of the switches in the cockpit to ensure they are in the correct positions for launch.

Final closeouts of the pad and clearing of the hazard area will be finished this afternoon. The count will proceed down to the T-minus 6 hour point where a two-hour hold begins at 7:52 p.m. During that pause, the launch team will verify all systems are ready to begin loading a half-million gallons of supercold rocket fuel in the shuttle's external tank.

The three-hour fueling process is scheduled to commence around 9:52 p.m. EDT, pending a final "go" from the Mission Management Team's evening meeting to review the status of the count.

Launch remains scheduled for 7:17 a.m. EDT Saturday morning.

The gantry is clear of the orbiter and continuing to swing back. Endeavour has been unveiled for its 23rd launch into space.

The tower has been moved back a few feet to the normal stop point while technicians confirm all is ready to drive the gantry all the way to the launch position.

Good morning from launch pad 39A where ground crews are preparing to roll back the rotating service structure away from space shuttle Endeavour.

The gantry has provided the primary access and weather protection for the shuttle during its stay on the seaside complex. The structure's retraction will reveal the orbiter and accomplish a key milestone in the countdown.

The astronauts' last day in the ground before launching on their marathon spaceflight actually began last night with an 8:15 p.m. EDT wakeup call. Later, they conducted a review of the mission timeline and then received status briefings on the shuttle, payloads and weather outlook. They'll begin a final pre-launch sleep period at 12:15 p.m. EDT.

Flight Day 1 for the mission starts with a wakeup call in crew quarters at 8:15 p.m. tonight. Breakfast is scheduled for 8:45 p.m., followed by medical exams. After some time to relax, they'll have lunch at 1:25 a.m.

The seven astronauts will suit up shortly before 3 a.m., followed by departure from crew quarters at 3:27 a.m. and arrival at the launch pad a half-hour later to start strapping aboard the shuttle. Endeavour's crew module hatch is scheduled to be closed for flight around 5:12 a.m. EDT.

NASA's Mission Management Team met this morning and confirmed that all systems remain "go" for Saturday's launch of the space shuttle Endeavour.

"We're in really good shape to fly. As a management team, we had a good unanimous 'go' for launch," chairman Mike Moses said.

Countdown activities at launch pad 39A this morning have focused on loading of the cryogenic reactants for Endeavour's three fuel cells. That work should be completed early this afternoon, allowing the pad to be reopened.

The planned activities for later in the evening and into the overnight hours of Friday morning will include final tests of the three main engines, functional checks of the orbiter's star trackers, activating the inertial measurement units, thoroughly testing the communications network, loading the last items into the crew module, filling of the launch pad's sound suppression system water tank and installing film in pad cameras.

Countdown clocks will enter the lengthy T-minus 11 hour planned hold period at 1 a.m. EDT. The built-in hold will last 13 hours and 52 minutes.

The giant gantry-like rotating service structure is scheduled for retraction from around Endeavour at 10:30 a.m. tomorrow.

"In summary, I would like to report that the countdown is proceeding nominally, we're not tracking any issues and our teams here and across the country and around the world, as a matter of fact, are excited and looking forward a successful launch and mission," says Pete Nickolenko, the shuttle launch director.

Air Force weather forecasters continue to predict a 90 percent chance of favorable conditions for Saturday morning's launch.

"There is no significant change to the launch-day forecast," meteorologists reported this morning. "High pressure is located over the Gulf of Mexico, and a sea breeze will develop again today. Isolated showers and thunderstorms will develop along the sea breeze each afternoon. Steering flow winds are from the north; therefore, thunderstorm activity will remain inland each day and should not interfere with pre-launch preparations. By launch day, the Bermuda High pressure ridge will be located over Central Florida causing favorable weather. There is a slight chance for cumulus cloud development during the launch window. Our primary concern for launch is cumulus cloud development."

The outlook for the 7:17 a.m. EDT launch time includes a few clouds at 3,000 and 25,000 feet, visibility of 7 miles, southwesterly winds of just 3 knots and a temperature of 75 degrees F.

If the launch is delayed for some reason, the odds of good weather are 90 percent for Sunday's 6:51 a.m. EDT launch time and 80 percent for Monday's 6:29 a.m. EDT launch opportunity.

Liquid hydrogen and liquid oxygen for space shuttle Endeavour's electricity-generating fuel cells will be loaded into storage spheres beneath payload bay beginning around 6:30 a.m. EDT today as standard work continues at pad 39A for Saturday's launch.

The cryogenic reactants are combined by the three omboard fuel cells to produce power and a byproduct of drinking water during the shuttle's mission. Technicians pump the cryogenics into small tanks aboard the orbiter during a multi-hour operation, then later demate the pad umbilical system used in the loading process and stow that equipment for launch.

The astronauts, continuing to adjust into the overnight work hours for the mission, were awakened in crew quarters at 7 p.m. EDT and underwent medical exams a short time later. The commander and pilot plan to make further landing approaches at Kennedy Space Center's runway starting around 5 a.m. Other crew members will have the opportunity for some T-38 jet flights after sunrise. Bedtime will be 12:15 p.m.

The Mission Management Team will hold its Launch Minus-2 Day meeting in the morning to review the status of work and grant approval to continue with the countdown toward launch. A post-meeting news conference is planned for 12 noon EDT.

See our full story on the countdown beginning.

"Right now, Endeavour is in great shape, the launch countdown is progressing nominally, the weather looks like it might cooperate and we're ready to fly this mission," says Steve Payne, NASA test director.

The weather forecast for launch time now shows a 90 percent chance that conditions will be within limits for Endeavour to fly on time. A slight possibility of cumulus clouds developing over the launch site is the only concern.

"The teams here at Kennedy Space Center have been working extremely hard to try to get us to this point. But it's paid off. We have a vehicle poised at the pad," said Payne.

"I know the launch team and the flight crew are all very excited to be in (the) launch countdown. We worked hard to get here and we're all eager to get Endeavour and her crew on her way to the International Space Station."

COUNT BEGINS. Countdown clocks at the Kennedy Space Center just began ticking toward Saturday morning's scheduled launch of the space shuttle Endeavour and the marathon 16-day mission to the International Space Station.

The official countdown sequence began at 9 a.m. EDT inside Firing Room 4 of the Complex 39 Launch Control Center. Launch team members had gathered for the "call-to-stations" at 8:30 a.m. EDT.

The early portion of the count involves buttoning up launch pad equipment and removing platforms inside the shuttle's crew module, reviewing flight software stored in Endeavour's mass memory units, loading backup software into the general purpose computers and testing navigation systems.

The count began from the T-minus 43 hour mark. But a series of holds are timed throughout the next few days, leading to Saturday's targeted liftoff time of 7:17 a.m. EDT.

"All of our launch countdown preparations are nearly complete. There's nothing standing in our way as we prepare for the launch countdown call-to-stations at 8:30 tomorrow morning. Our countdown clock will begin counting at 9," said Charlie Blackwell-Thompson, NASA test director.

"I have no issues to report. The STS-127 flight crew, Endeavour and the launch team are all ready to proceed."

The astronauts, currently asleep in the crew quarters building, will be awakened at 5 p.m. EDT today. A fit check of their orange launch and entry spacesuits is planned for tonight, as well as shuttle landing practice for commander Mark Polansky and pilot Doug Hurley in the Shuttle Training Aircraft at Kennedy Space Center's three-mile-long runway.

The early weather forecast for Saturday's planned 7:17 a.m. EDT launch of space shuttle Endeavour predicts an 80 percent chance of acceptable conditions at the Kennedy Space Center, Air Force meteorologists reported today.

"During the past week, thunderstorms have developed along the sea breeze each day and southwesterly winds caused them to migrate toward the east coast of Florida. The weather pattern is changing as an upper level trough moves to the east and high pressure builds in to the west, causing northerly steering flow winds. This should cause afternoon Central Florida thunderstorms to remain inland rather than pushing toward the east coast, providing favorable weather conditions during launch preparations. By launch day, the Bermuda High pressure ridge is located over Central Florida causing variable winds and favorable weather on launch morning. There is a slight chance for a morning coastal shower. Our primary concerns for launch are cumulus clouds and showers within 20 nautical miles of the Shuttle Landing Facility," the forecast says.

The outlook for launch time includes a few cumulus clouds at 3,000 feet, a few cirrus clouds at 25,000 feet, visibility of 7 miles, winds of just 3 knots and a temperature of 74 degrees F.

If the launch is delayed for some reason, the odds of good weather remain at 80 percent for Sunday's 6:51 a.m. EDT launch time and Monday's 6:29 a.m. EDT launch opportunity.

The crew of the shuttle Endeavour flew back to Florida late Monday to prepare for blastoff Saturday on a complex space station assembly mission.

Read our full story.

The Mark Polansky-led crew has arrived at the Kennedy Space Center for Saturday morning's launch of the space shuttle Endeavour on the construction flight that will finish building the Japanese science facilities aboard the International Space Station.

The six-man, one-woman team of astronauts flew from Houston to the Cape aboard a Gulfstream jet, landing just before midnight EDT on the same runway Endeavour will use to conclude the 16-day spaceflight.

"We are absolutely thrilled to be down here in preparation for Saturday's launch of Endeavour," Polansky told reporters just after arriving.

The crew also includes rookies Doug Hurley as pilot, Chris Cassidy, Tom Marshburn and Tim Kopra serving as mission specialists, plus experienced Canadian astronaut Julie Payette flying for the second time and Dave Wolf, a veteran of previous shuttle and space station Mir flights.

"We are so ready to go. We just can't wait," Marshburn said.

"The space station is really one of the great wonders of the human race that's ever been produced," added Wolf, the flight's lead spacewalker. "We have a very challenging mission, we have a very talented team, that's a perfect combination."

Before leaving their home training base at the Johnson Space Center on Monday, the astronauts completed a few more launch simulations. On his Twitter page, Polansky said one ascent practiced a launch abort to the emergency landing site in Zaragoza, Spain. Another mock launch saw the crew reach orbit but they had to deal with various problems with onboard systems.

After making brief statements to reporters at the runway and a subsequent photo opportunity, the crew was bussed away. Upcoming on their schedule over the next few hours are reviews of flight data files and suit up procedures.

Bedtime will be 9 a.m. EDT, as the crew continues shifting its wake/sleep cycle for the mission that will see most of the activities occur during the overnight hours U.S. time.

"This represents, of course, one of those flights where we have to shift into a completely different time zone in order to sync up with the space station. And for that, we become a bit of a stealth flight. But because you are paying interest to the program and what we are doing in space right now and because it is so exciting that I think people just might wake up in the middle of the night to see some of the action on the five EVAs and the robotic operations that will happen in this mission," Payette said.

Touchdown. Shuttle Endeavour's crew has arrived at the launch site a few minutes ahead of schedule.

The crew has flown across the Gulf and into Central Florida on this 900-mile trip. They'll be landing shortly.

The astronauts have left Houston for the two-hour plane ride over to the Cape. Touchdown at the spaceport is expected a little after midnight EDT.

As space shuttle Endeavour's targeted Saturday morning launch looms large on the horizon, the seven astronauts are packing up for tonight's trip from Houston to the Kennedy Space Center where final preps to begin the countdown are underway.

Commander Mark Polansky, pilot Doug Hurley, flight engineer Julie Payette, spacewalkers Chris Cassidy, Tom Marshburn and Dave Wolf, and station-bound astronaut Tim Kopra are scheduled to depart Ellington Field near the Johnson Space Center at 10 p.m. EDT aboard a Shuttle Training Aircraft. Arrival at KSC's runway is expected around 12:15 a.m. EDT.

The astronauts visited the Florida spaceport last week for the countdown dress rehearsal activities. They flew home Thursday night, got Friday off to spend with family, then entered into pre-flight quarantine on Saturday.

"In quarantine," Polansky tweeted on his Twitter page. "I'll slowly go over the flight. Day by day. System by system. Task by task. It'll be a good 'attention to detail' review."

The crew completed a rendezvous and docking simulation early Sunday to practice linking Endeavour with the space station on the mission's third day. They have one final run in the launch simulator this afternoon.

Meanwhile, technicians at launch pad 39A closed up Endeavour's payload bay on Saturday. The shuttle's cargo hold is jam-packed on this mission. From forward to aft: the docking port occupies the front portion of the bay, followed by the Japanese science exposure facility that will be attached to the station during the mission, a smaller platform that ferries up some of the experiments for the station's new external porch and another carrier that houses spare parts to be transferred onto the station. In addition, two tiny deployable satellites are stowed on the starboard wall of the bay for release late in Endeavour's flight.

Final installation and connection of Endeavour's ordnance systems occurred on Sunday, and the ship's aft engine compartment will be buttoned up today.

The three-day launch countdown sequence will begin in Firing Room 4 at 9 a.m. EDT Wednesday.

Liftoff of Endeavour remains targeted for 7:17 a.m. EDT Saturday.

Read our earlier status center coverage.