Engineers replaced rocket nozzle rain covers Tuesday and prepared the shuttle Endeavour for a sixth launch attempt Wednesday, weather permitting, to get a complex space station construction mission underway. A backup attempt on Thursday, although possible, would have to eliminate the mission's fifth spacewalk.

See our full story.

The latest forecast from the launch weather team continues to show a 60 percent chance of acceptable conditions for Endeavour's 6:03 p.m. EDT liftoff on Wednesday.

The outlook includes showers and thunderstorms inland, a few clouds at 3,000 feet, scattered clouds at 8,000 feet and 25,000 feet, visibility of 7 miles, launch pad winds from the southeast at 8 peaking to 12 knots and a temperature of 83 degrees F.

"A weak surface boundary is situated in Northern Florida. This will aid in keeping the Bermuda High ridge to the south, and southwest flow will be present over Central Florida. The result will be similar conditions to the last few days with late afternoon and evening thunderstorms near the Space Coast," the forecasters said today.

"On Wednesday, the boundary to the north will begin to erode allowing the Bermuda High ridge to migrate towards the north and position itself over central Florida. Surface and mid-level winds will become weaker allowing the east coast sea breeze to push further inland by early afternoon, and keeping a larger majority of the thunderstorm activity to remain further inland as well."

The primary concern for Wednesday will be thunderstorms within 20 miles of the emergency shuttle runway.

That will be the worry again on Thursday along with violations of the lightning and cumulus cloud rules. The odds of allowable weather for launch on Thursday are 40 percent.

The updated NASA Television schedule for Wednesday can be downloaded here.

After returning from the launch pad, commander Mark Polansky tweeted on his Twitter page: "Well, this isn't the tweet I hoped to send, but it's just not easy to launch a shuttle. We're fine in spite of this, and we'll hope for Wed."

Our scrub story has been updated.

The official weather forecast for Wednesday's 6:03 p.m EDT launch time calls for a 60 percent chance of acceptable conditions.

The outlook for Thursday's 5:37 p.m. EDT launch time has a 40 percent chance of good weather.

Previously, NASA had said Tuesday would be the last chance to launch Endeavour and perform its mission at the space station before a Russian cargo freighter would need to dock with the outpost. The shuttle has to depart from the station before the supply ship gets there.

But in light of today's scrub, NASA has decided to reschedule launch for Wednesday. Mission Management Team chairman Mike Moses says Wednesday is now a good launch opportunity because some activities originally planned during the docked phase of the mission -- such as off duty time and packing -- can be deferred until after the shuttle undocks without shortening the flight or impacting the objectives.

An attempt on Thursday likely would result in the deletion of some mission content. However, all remains under review.

What's known for sure is the next launch attempt will be Wednesday at 6:03 p.m. EDT.

And now commander Mark Polansky has climbed out of the spacecraft. The astronauts are heading back to crew quarters to wait out this 48-hour launch postponement.

Julie Payette, Chris Cassidy and Doug Hurley have climbed down from the flight deck.

Tim Kopra and Tom Marshburn have egressed the middeck.

Dave Wolf is the first crewmember to exit the shuttle.

The SpaceX Falcon 1 rocket launch is targeted for 10:30 p.m. EDT (0230 GMT) tonight. Live updates can be read here.

Endeavour's crew module hatch just swung open.

The Orbiter Closeout Crew has arrived at launch pad 39A to reopen Endeavour's hatch and assist the astronauts out of the shuttle.

The post-scrub news conference is planned to start at 8 p.m. EDT.

For the second day in a row, approaching thunderstorms near the Kennedy Space Center Monday forced NASA managers to order another launch delay for the shuttle Endeavour's frustrated crew. It was the fifth delay for the space station assembly mission since a hydrogen leak scuttled the crew's first launch try in June.

See our scrub story.

NASA is planning a post-scrub news conference this evening.

During this extended period between launch attempts, ground teams will place the rotating service gantry back around Endeavour and reattach a partially debonded cover on one of Endeavour's nose thrusters.

In addition, the weather outlook for Wednesday is better than Tuesday evening's forecast.

NEW DATE SET. Liftoff has been rescheduled for Wednesday at 6:03 p.m. EDT.

Mission officials plan to meet and decide within 30 minutes whether the next launch attempt will be made Tuesday or not until Wednesday.

SCRUB! For the second straight day, launch of space shuttle Endeavour has been postponed due to summertime thunderstorms around the Kennedy Space Center.

Tuesday's target launch time to rendezvous with the International Space Station is 6:25:41 p.m. EDT. The time on Wednesday would be 6:03:10 p.m. EDT.

This is the fifth scrub for the mission after two gaseous hydrogen leaks in June and a delay Saturday to check shuttle systems for any harm caused by nearby lightning strikes.

NASA test director Steve Payne is performing his standard pre-launch poll. However, the Range is "no go" for launch weather conditions and the Mission Control Center is "no go" for the emergency landing site weather.

A lightning advisory for the launch pad area has been issued.

The stormy weather remains around the Kennedy Space Center is now violating the lightning, anvil clouds and field mill rules. The field mills measure the electrical potential in the air.

SpaceX is preparing to launch it Falcon 1 rocket tonight from Kwajalein. The actual liftoff time has not been announced by the company. Live updates can be read here.

The storm cell to the south that was violating the lightning rule has moved out. However, there is another cell to the north that's being watched.

At launch, the space station will be flying 220 miles above the mid Pacific Ocean between New Zealand and Chile. Liftoff at 6:51:24 p.m. EDT is timed to place Endeavour on course to dock with the station on Wednesday.

T-minus 9 minutes and holding. Countdown clocks have gone into the planned 45-minute, 24-second built-in hold. Launch is targeted for 6:51:24 p.m. EDT, if the thunderstorms and lightning around the area clear in time. Today's available window to get the shuttle off the ground extends just five minutes to 6:56:25 p.m. EDT.

Now one hour away from launch.

Powering space shuttle Endeavour throughout its eight-and-a-half minute climb to orbit will be the three main engines built by Pratt & Whitney Rocketdyne. The cryogenic powerplants are fed with liquid hydrogen and liquid oxygen stored in the external fuel tank.

In the engine No. 1 position today is the Block 2-2045 engine now making its 10th launch. It has accumulated 5,485 seconds of total firing time on the previous missions, plus ground testing. STS-89 was its debut flight.

Making its first launch is the new Block 2-2060 in the engine No. 2 position. This powerplant has 1,420 seconds of ground test time.

And Block 2-2054 is engine No. 3 on Endeavour. It has eight previous flights, starting with STS-101, and some 5,123 seconds of firing time.

The Main Propulsion System helium system is being reconfigured by pilot Doug Hurley. Soon the gaseous nitrogen purge to the aft skirts of the solid rocket boosters will be started.

Mission Control in Houston is loading Endeavour's onboard computers with the proper guidance parameters based on the projected launch time.

Pilot Doug Hurley is configuring the displays inside Endeavour's cockpit for launch while commander Mark Polansky enables the abort steering instrumentation.

T-minus 20 minutes and counting. The countdown has resumed after a 10-minute hold. Clocks will tick down for the next 11 minutes to T-minus 9 minutes where the final planned hold is scheduled to occur. The hold length will be adjusted to synch up with today's preferred launch time of 6:51:24 p.m.

Endeavour's onboard computers are now transitioning to the Major Mode-101 program, the primary ascent software. Also, engineers are dumping the Primary Avionics Software System (PASS) onboard computers. The data that is dumped from each of PASS computers is compared to verify that the proper software is loaded aboard for launch.

Weather situation is "dynamic" and continues to be monitored closely by meteorologists at the Cape and Houston. Lightning within 10 miles of the launch pad and thunderstorms within 20 miles of the Kennedy Space Center runway are the current issues.

T-minus 20 minutes and holding. The countdown has paused for a 10-minute built-in hold. Launch is scheduled for 6:51:24 p.m. EDT, weather permitting.

During this built-in hold, all computer programs in Firing Room 4 of the Complex 39 Launch Control Center will be verified to ensure that the proper programs are available for the countdown; the landing convoy status will be verified and the landing sites will be checked to support an abort landing during launch today; the Inertial Measurement Unit preflight alignment will be verified completed; and preparations are made to transition the orbiter onboard computers to Major Mode 101 upon coming out of the hold. This configures the computer memory to a terminal countdown configuration.

The launch weather rule for lightning is now being violated due to a storm cell to the south. So weather is "no go" for launch at this time.

There's actually two sets of weather constraints in play for a space shuttle launch. A 10-mile keepout zone for thunderstorms is based on the launch commit criteria. And there's also a broader 20-mile limit as part of the Return to Launch Site abort weather rules that ensure conditions would be acceptable if Endeavour had to make an emergency landing shortly after liftoff.

Concern about storms within the RTLS restricted area remains the chief worry at this point.

The local 45th Weather Squadron is responsible for launch forecasting, while the Spaceflight Meteorology Group based in Houston handles RTLS weather.

The Orbiter Closeout Crew reports Endeavour's side hatch is latched for flight. The team's work on the hatch is complete and now they will prepare to leave the pad.

The ground pyro initiator controllers (PICs) are scheduled to be powered up around this time in the countdown. They are used to fire the solid rocket hold-down posts, liquid oxygen and liquid hydrogen tail service mast and external tank vent arm system pyros at liftoff and the space shuttle main engine hydrogen gas burn system prior to engine ignition.

The shuttle's two Master Events Controllers are being tested. They relay the commands from the shuttle's computers to ignite, and then separate the boosters and external tank during launch.

The targeted liftoff time is 6:51:24 p.m. EDT. That's the moment when Earth's rotation carries the launch pad into the plane of the station's orbit.

The official window for extends from 6:46:24 p.m. to 6:56:25 p.m. EDT. Launching within that 10 minute period will enable Endeavour to dock with the International Space Station on Wednesday.

For the moment, the launch weather rules are "go." The official forecast still predicts a 60 percent chance of conditions violating the rules at launch times.

The odds are 60 percent "no go" on Tuesday and 40 percent on Wednesday.

Just two hours remain until the planned liftoff time.

The reusable solid rocket boosters, built by ATK, provide the primary thrust to propel the space shuttle away from Earth during the initial two minutes of flight. The 11 sections on each booster flying on Endeavour are a mixture of brand new and used hardware. The upper dome on the right-hand booster, for example, flew on STS-2 in 1981. In all, the twin boosters flying this morning have refurbished segments and pieces that trace back to 65 previous shuttle launches.

The boosters will parachute into the Atlantic Ocean where a pair of retrieval ships are standing by to recover the rockets and tow them back to shore, beginning again the process to disassemble, refurbish and reuse the hardware in the future.

The orbiter closeout team at the launch pad is closing Endeavour's crew module hatch for flight.

The astronauts have completed a series of radio communication checks with ground controllers.

T-minus 90 minutes and counting. Countdown clocks continue to tick down to T-minus 20 minutes where the next hold is planned. Activities remain on track for liftoff at 6:51 p.m.

There are no significant technical issues being worked and the countdown is proceeding smoothly. But the weather situation remains the wildcard due to thunderstorms around Central Florida.

Initialization of the Ground Launch Sequencer has been completed. This is the master computer program that will run the final nine minutes of the countdown.

The final Endeavour astronaut has boarded the shuttle today. Julie Payette, mission specialist No. 2 and flight engineer, has now entered the hatch. She will sit in the flight deck center seat, giving an extra set of eyes to aid the commander and pilot watch displays and switches during launch and landing.

The 45-year-old from Montreal, Quebec is a Canadian Space Agency astronaut. She visited the International Space Station once before in 1999.

Read her biography here.

A reminder that if you will be away from your computer but would like to receive countdown updates, sign up for our Twitter feed to get text messages on your cellphone. U.S. readers can also sign up from their phone by texting "follow spaceflightnow" to 40404. (Standard text messaging charges apply.)

Astronaut Dave Wolf, a veteran of three previous spaceflights ranging from a Spacelab mission, a four-month stint working aboard the Russian space station Mir and an earlier assembly flight to the International Space Station is the lead spacewalker on Endeavour. The 52-year-old medical doctor from Indianapolis is mission specialist No. 4.

Wolf will take the center seat on the middeck for ascent.

Read his biography here.

A Navy SEAL is climbing into space shuttle Endeavour's flight deck aft-right seat. Chris Cassidy, 39-year-old from Maine and spaceflight rookie is mission specialist No. 1. He will be one of the spacewalkers on the flight.

Read his biography here.

Three hours and counting until liftoff time.

Tom Marshburn, a 48-year-old former emergency doctor and NASA flight surgeon born in North Carolina, is mission specialist No. 3 and another of the spacewalkers during Endeavour's trip to the station.

He will ride into space for the first time of his career in the middeck's left seat.

Read his biography here.

Doug Hurley, a 42-year-old lieutenant colonel in the Marine Corps born in New York, will be Endeavour's pilot. He has over 3,200 hours in more than 20 different kinds of aircraft, including being the first ever Marine pilot to fly the F/A-18 E/F Super Hornet.

This spaceflight rookie is making his way to the flight deck's front-right seat right now.

Read Hurley's biography here.

Tim Kopra, the station-bound astronaut set to join the Expedition 20 crew living aboard the outpost, serves as Endeavour's mission specialist No. 5 for the launch. The 45-year-old from Austin, Texas is a colonel in the U.S. Army. Once at the space station, he will exchange places with Japanese astronaut Koichi Wakata, who then will return to Earth on the shuttle to conclude three-and-a-half months in space.

Kopra just crawled through Endeavour's hatch to take the right-hand seat on the middeck.

Read his biography here.

Shuttle commander Mark Polansky, a 53-year-old aviator born in New Jersey, leads this voyage of Endeavour. It will be his third spaceflight, having been a pilot and a commander on two previous construction missions to the space station in 2001 and 2006.

He is the first astronaut to board the shuttle this afternoon, taking the forward-left seat on the flight deck.

Read Polansky's biography here.

None of the launch weather rules are being violated at this point in the afternoon. However, weather officer Kathy Winters indicates storms closer to the Cape could fire up in the next 60-90 minutes.

The astronauts are taking a few moments to gaze up at the space shuttle.

At 3:17 p.m. EDT, Endeavour's crew arrived at launch pad 39A. The AstroVan came to a stop on the pad surface near the Fixed Service Structure tower elevator that will take the seven-man crew to the 195-foot level to begin boarding the shuttle.

The AstroVan is passing the 52-story Vehicle Assembly Building where Endeavour was attached to its external tank and solid rocket boosters and the adjacent Launch Control Center.

The Press Site is located across the street, and reporters went outside to watch at the passing convoy. This is a launch day tradition to say farewell and good luck to the astronaut crews.

At 3:01 p.m. EDT, the astronauts emerged from crew quarters. Commander Mark Polansky, pilot Doug Hurley, flight engineer Julie Payette, spacewalkers Chris Cassidy, Tom Marshburn and Dave Wolf, and station-bound astronaut Tim Kopra have boarded the AstroVan for the ride from the Industrial Area to launch pad 39A on the edge of the Atlantic Ocean.

T-minus 3 hours and counting. The countdown clocks are ticking again after the planned two-and-a-half hour built-in hold. Clocks will proceed to T-minus 20 minutes when the next hold is scheduled. A final hold occurs at the T-minus 9 minute mark to synch up with the 6:51:24 p.m. EDT launch time.

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The astronauts just finished getting suited up again in their day-glow orange launch and entry partial pressure spacesuits. After final adjustments and pressure checks, the astronauts will depart the suitup room and take the elevator down to the ground level of the Operations and Checkout Building to board the AstroVan for the trip to launch pad 39A around 3 p.m.

See our latest story on the completion of tanking, the thruster cover issue and launch options beyond today.

The Final Inspection Team is responsible for checking the shuttle and launch pad one last time prior to liftoff. The team is comprised of engineers and safety officials from NASA, United Space Alliance and tank-builder Lockheed Martin. At the conclusion of their two-hour tour-of-duty, the team will have walked up and down the entire fixed service structure and mobile launcher platform.

The team is on the lookout for any abnormal ice or frost build-up on the vehicle and integrity of the external tank foam insulation.

The team uses a portable infrared scanner that gathers temperature measurements on the surface area of the shuttle and can spot leaks. The scanner will be used to obtain temperature data on the external tank, solid rocket boosters, space shuttle orbiter, main engines and launch pad structures. The scanner can also spot leaks of the cryogenic propellants, and due to its ability to detect distinct temperature differences, can spot any dangerous hydrogen fuel that is burning. The team member also is responsible for photo documentation.

The team wears the highly visible day-glow orange coveralls that are anti-static and flame resistant. Each member also has a self-contained emergency breathing unit that holds about 10 minutes of air.

The Final Inspection Team is out at the launch pad doing its thing again today to ensure there's no ice or debris concerns with the shuttle or launch pad.

The countdown entered the T-minus 3 hour hold at 12:26 p.m. EDT. This planned built-in hold will last two-and-a-half hours. Clocks should resume ticking at 2:56 p.m., followed five minutes later by the astronauts departing crew quarters.

Space shuttle Endeavour is fueled up and ready for launch at 6:51 p.m. EDT (2251 GMT) today. That's if the weather cooperates.

The three-hour process to refill the external fuel tank with liquid hydrogen and liquid oxygen went smoothly and was called complete at 12:37 p.m. EDT. No leakage or problems were encountered.

The astronauts are sitting down for lunch now. They'll get suited up at 2:30 p.m., then depart crew quarters for the launch pad at 3 p.m. EDT.

Here's the launch weather outlook from the 45th Weather Squadron:

"The Bermuda high pressure ridge is located to the south of Kennedy Space Center (KSC) creating southwest low-level flow. This causes the sea breeze to develop later in the day and linger closer to the East Coast, increasing the chance for stormy weather near KSC around launch time. As was the case yesterday, the West Coast sea breeze and its associated outflow boundaries will migrate across the state and likely interact with and strengthen the East Coast storms.

"Due to these conditions, our primary concerns for launch are showers and thunderstorms within 20 nautical miles of the Shuttle Landing Facility (SLF), as well as lightning and cumulus cloud Launch Commit Criteria (LCC) violations.

"This weather pattern will persist through Tuesday. Wednesday the high pressure ridge may migrate back over the KSC area, allowing the East Coast sea breeze to progress further inland, providing slightly more favorable weather for launch."

Fueling is nearing completion. Liquid hydrogen went into topping mode and no gaseous hydrogen leakage was detected.

Running a month behind scheduled because of hydrogen leaks, launch pad lightning strikes and stormy weather, NASA managers have cleared the shuttle Endeavour for a fifth launch attempt today to kick off a 16-day space station construction mission. Forecasters are predicting a 60 percent chance of another delay due to expected afternoon storms.

See our morning story.

The space shuttle launch team, working by remote control from Firing Room 4 about three-and-a-half miles away, has begun reloading Endeavour's external fuel tank with a half-million gallons of supercold propellants.

The filling of liquid hydrogen and liquid oxgen into the shuttle at launch pad 39A officially began with the chilldown thermal conditioning process at 9:33 a.m. EDT. The tank had been emptied after last night's scrub.

The Mission Management Team just finished its morning meeting and approved the game plan to proceed with another launch attempt today. Fueling of space shuttle Endeavour should begin at any moment.

The launch team personnel are reporting "ready" status to begin fueling as soon as the Mission Management Team gives the OK.

The management team continues to meet. The topic under discussion at this point in the meeting is the partially debonded cover on one of Endeavour's thrusters. Approval to begin fueling has not been given yet, so loading of the external tank won't begin at 9:26 a.m. as planned, NASA says.

There's a 60 percent chance that weather will again scrub the launch of Endeavour today.

The forecast for the 6:51 p.m. EDT liftoff time includes showers and thunderstorms in the vicinity, scattered clouds at 3,000, 8,000 feet and 25,000 feet, visibility of 7 miles, launch pad winds from the southeast at 10 peaking to 15 knots and a temperature of 83 degrees F.

The outlook for Tuesday is much the same with another 60 percent chance of weather rule violation.

Preparations are underway at Kennedy Space Center for launch attempt No. 5. The management team meeting is set to begin shortly to give approval for re-fueling Endeavour starting around 9:30 a.m. EDT.

The wakeup call to the seven astronauts occurred at 8:15 a.m. EDT. Commander Mark Polansky tweeted on his Twitter page: "Just woke up. While we all hope that today's the day, we have a saying that you never know for sure until the solid rockets light off."

We have updated our countdown, ascent and flight plan charts to reflect Monday's planned launch.

Stormy weather approaching the Kennedy Space Center forced NASA launch managers to order another 24-hour delay for the hard-luck crew of the shuttle Endeavour Sunday, their fourth slip since a hydrogen leak derailed an initial launch try in June.

See our scrub story.

Stormy weather approaching the Kennedy Space Center forced NASA launch managers to order another 24-hour delay for the hard-luck crew of the shuttle Endeavour Sunday, their fourth slip since a hydrogen leak derailed an initial launch try in June.

See our scrub story.

The Mission Management Team will gather at 8:45 a.m. EDT for the pre-fueling meeting to review shuttle status and the latest weather forecast. Fueling is scheduled to start at 9:26 a.m. EDT.

The AstroVan is driving out of the pad now.

And now commander Mark Polansky has climbed out of the spacecraft. The astronauts are heading back to crew quarters for the night.

Chris Cassidy and Doug Hurley have egressed.

Dave Wolf, Tom Marshburn, Tim Kopra and Julie Payette have exited the shuttle.

Tomorrow's live launch webcast with Miles O'Brien and David Waters will begin at 3:30 p.m. EDT.

Endeavour's crew module hatch just swung open.

The Orbiter Closeout Crew has arrived at launch pad 39A to reopen Endeavour's hatch and assist the astronauts out of the shuttle.

Draining of propellant from the external tank is getting underway as the launch team works through post-scrub activities.

SCRUB! Thunderstorms west of the launch site will prevent space shuttle Endeavour from blasting off today during its brief launch window to rendezvous with the International Space Station.

NASA will try again Monday. The target liftoff time will be 6:51:24 p.m. EDT.

Lightning and cumulus cloud rules are "no go" for launch.

NASA test director Steve Payne is performing his standard pre-launch poll. However, the Range is "no go" for launch weather conditions and the Mission Control Center is "no go" for the emergency landing site weather.

"Weather is not encouraging," NASA commentator Mike Curie says from the Firing Room.

Today's launch opportunity beyond the 7:13 p.m. EDT target liftoff time extends only five additional minutes to 7:18 p.m. EDT. The window is driven by the orbital mechanics for Endeavour to dock with the space station on Flight Day 3.

Now 30 minutes away from the scheduled launch time. However, the stormy weather is now approaching the 10 mile circle around the launch pad.

There's actually two constraints in play. The 10-mile keepout zone for thunderstorms is based on the launch commit criteria. The broader 20-mile limit is part of the Return to Launch Site abort weather rules that ensure conditions would be acceptable if Endeavour had to make an emergency landing shortly after liftoff.

Thunderstorms continue to approach from the west. At this point, conditions remain unacceptable for launch.

At launch, the space station will be flying 220 miles above Turkey. Liftoff at 7:13:54 p.m. EDT is timed to place Endeavour on course to dock with the station on Tuesday.

T-minus 9 minutes and holding. Countdown clocks have gone into the planned 45-minute, 54-second built-in hold. Today's launch is targeted for 7:13:54 p.m. EDT, if the thunderstorms to the west clear out of the area in time.

Mission Control in Houston has loaded Endeavour's onboard computers with the proper guidance parameters based on the projected launch time.

The Main Propulsion System helium system is being reconfigured by pilot Doug Hurley. Soon the gaseous nitrogen purge to the aft skirts of the solid rocket boosters will be started.

Now one hour away from launch.

Pilot Doug Hurley is configuring the displays inside Endeavour's cockpit for launch while commander Mark Polansky enables the abort steering instrumentation.

T-minus 20 minutes and counting. The countdown has resumed after a 10-minute hold. Clocks will tick down for the next 11 minutes to T-minus 9 minutes where the final planned hold is scheduled to occur. The hold length will be adjusted to synch up with today's preferred launch time of 7:13:54 p.m.

Endeavour's onboard computers are now transitioning to the Major Mode-101 program, the primary ascent software. Also, engineers are dumping the Primary Avionics Software System (PASS) onboard computers. The data that is dumped from each of PASS computers is compared to verify that the proper software is loaded aboard for launch.

The work to seal the shuttle's crew compartment hatch for flight is complete. And the closeout team that assisted the astronauts into Endeavour this afternoon is preparing to leave the launch pad now.

T-minus 20 minutes and holding. The countdown has paused for a 10-minute built-in hold. Launch is scheduled for 7:13 p.m. EDT, weather permitting. Storms to the northwest of the Kennedy Space Center have moved within the restricted 20 mile zone around the shuttle runway.

During this built-in hold, all computer programs in Firing Room 4 of the Complex 39 Launch Control Center will be verified to ensure that the proper programs are available for the countdown; the landing convoy status will be verified and the landing sites will be checked to support an abort landing during launch today; the Inertial Measurement Unit preflight alignment will be verified completed; and preparations are made to transition the orbiter onboard computers to Major Mode 101 upon coming out of the hold. This configures the computer memory to a terminal countdown configuration.

The Closeout Crew is finishing up its work to button up the side hatch on Endeavour.

The primary weather concern will be showers or storms within 20 miles of the Kennedy Space Center runway where Endeavour would make an emergency landing in the event of a problem during launch. At the present time, the current conditions are observed "no go" due to a storm cell northwest of the spaceport.

The ground pyro initiator controllers (PICs) are scheduled to be powered up around this time in the countdown. They are used to fire the solid rocket hold-down posts, liquid oxygen and liquid hydrogen tail service mast and external tank vent arm system pyros at liftoff and the space shuttle main engine hydrogen gas burn system prior to engine ignition.

The shuttle's two Master Events Controllers are being tested. They relay the commands from the shuttle's computers to ignite, and then separate the boosters and external tank during launch.

This evening's launch time, based on the International Space Station's orbit and Endeavour's trajectory, has been tweaked by one second. Liftoff is now targeted for 7:13:54 p.m. EDT.

The orbiter closeout team at the launch pad is closing Endeavour's crew module hatch for flight.

Just two hours remain until the planned liftoff time.

The reusable solid rocket boosters, built by ATK, provide the primary thrust to propel the space shuttle away from Earth during the initial two minutes of flight. The 11 sections on each booster flying on Endeavour are a mixture of brand new and used hardware. The upper dome on the right-hand booster, for example, flew on STS-2 in 1981. In all, the twin boosters flying this morning have refurbished segments and pieces that trace back to 65 previous shuttle launches.

The boosters will parachute into the Atlantic Ocean where a pair of retrieval ships are standing by to recover the rockets and tow them back to shore, beginning again the process to disassemble, refurbish and reuse the hardware in the future.

The final report from the inspection team's walkdown of the launch pad this afternoon had only one item of interest, NASA says. On the liquid hydrogen-side umbilical that connects to the side of Endeavour's aft compartment, there's what is being termed by the engineers as slushy ice on the aft seal edge. Officials are discussing the observation.

Also, one of the orbiter's thruster covers is partially debonded. But that is not a constraint to launch.

A reminder that if you will be away from your computer but would like to receive countdown updates, sign up for our Twitter feed to get text messages on your cellphone. U.S. readers can also sign up from their phone by texting "follow spaceflightnow" to 40404. (Standard text messaging charges apply.)

T-minus 90 minutes and counting. Countdown clocks continue to tick down to T-minus 20 minutes where the next hold is planned. Activities remain on track for liftoff at 7:13 p.m.

The countdown is proceeding smoothly with no significant technical issues. The weather predictions still call for a 70 percent chance of acceptable conditions. The only concern could be thunderstorms within 20 miles that would violate the weather rules.

The final Endeavour astronaut has boarded the shuttle today. Julie Payette, mission specialist No. 2 and flight engineer, has now entered the hatch. She will sit in the flight deck center seat, giving an extra set of eyes to aid the commander and pilot watch displays and switches during launch and landing.

The 45-year-old from Montreal, Quebec is a Canadian Space Agency astronaut. She visited the International Space Station once before in 1999.

Read her biography here.

Initialization of the Ground Launch Sequencer has been completed. This is the master computer program that will run the final nine minutes of the countdown.

Astronaut Dave Wolf, a veteran of three previous spaceflights ranging from a Spacelab mission, a four-month stint working aboard the Russian space station Mir and an earlier assembly flight to the International Space Station is the lead spacewalker on Endeavour. The 52-year-old medical doctor from Indianapolis is mission specialist No. 4.

Wolf will take the center seat on the middeck for ascent.

Read his biography here.

A Navy SEAL is climbing into space shuttle Endeavour's flight deck aft-right seat. Chris Cassidy, 39-year-old from Maine and spaceflight rookie is mission specialist No. 1. He will be one of the spacewalkers on the flight.

Read his biography here.

Three hours and counting until liftoff time.

Tom Marshburn, a 48-year-old former emergency doctor and NASA flight surgeon born in North Carolina, is mission specialist No. 3 and another of the spacewalkers during Endeavour's trip to the station.

He will ride into space for the first time of his career in the middeck's left seat.

Read his biography here.

Doug Hurley, a 42-year-old lieutenant colonel in the Marine Corps born in New York, will be Endeavour's pilot. He has over 3,200 hours in more than 20 different kinds of aircraft, including being the first ever Marine pilot to fly the F/A-18 E/F Super Hornet.

This spaceflight rookie is making his way to the flight deck's front-right seat right now.

Read Hurley's biography here.

Tim Kopra, the station-bound astronaut set to join the Expedition 20 crew living aboard the outpost, serves as Endeavour's mission specialist No. 5 for the launch. The 45-year-old from Austin, Texas is a colonel in the U.S. Army. Once at the space station, he will exchange places with Japanese astronaut Koichi Wakata, who then will return to Earth on the shuttle to conclude three-and-a-half months in space.

Kopra just crawled through Endeavour's hatch to take the right-hand seat on the middeck.

Read his biography here.

Shuttle commander Mark Polansky, a 53-year-old aviator born in New Jersey, leads this voyage of Endeavour. It will be his third spaceflight, having been a pilot and a commander on two previous construction missions to the space station in 2001 and 2006.

He is the first astronaut to board the shuttle this afternoon, taking the forward-left seat on the flight deck.

Read Polansky's biography here.

Formal approval from the Firing Room has been given for the astronauts to start ingressing the orbiter.

Endeavour's crew has arrived at launch pad 39A. The AstroVan came to a stop on the pad surface near the Fixed Service Structure tower elevator that will take the seven-man crew to the 195-foot level to begin boarding the shuttle.

The AstroVan is passing the 52-story Vehicle Assembly Building where Endeavour was attached to its external tank and solid rocket boosters and the adjacent Launch Control Center.

The Press Site is located across the street, and reporters went outside to watch at the passing convoy. This is a launch day tradition to say farewell and good luck to the astronaut crews.

Commander Mark Polansky, pilot Doug Hurley, flight engineer Julie Payette, spacewalkers Chris Cassidy, Tom Marshburn and Dave Wolf, and station-bound astronaut Tim Kopra just departed the Kennedy Space Center crew quarters to board the AstroVan for the ride from the Industrial Area to launch pad 39A on the edge of the Atlantic Ocean.

The crew is coming down the hallway from the suitup room.

T-minus 3 hours and counting. The countdown clocks are ticking again after the planned two-and-a-half hour built-in hold. Clocks will proceed to T-minus 20 minutes when the next hold is scheduled. A final hold occurs at the T-minus 9 minute mark to synch up with the 7:13:55 p.m. EDT launch time.

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The crew has donned the day-glow orange launch and entry partial pressure spacesuits. After final adjustments and pressure checks, the astronauts will depart the suitup room and take the elevator down to the ground level of the Operations and Checkout Building to board the AstroVan for the trip to launch pad 39A around 3:23 p.m.

A short time ago, commander mark Polansky, pilot Doug Hurley and flight engineer Julie Payette received a weather briefing on conditions at the Kennedy Space Center and the primary abort landing sites. Weather is not expected to be a constraint to launching on time today.

Join anchor Miles O'Brien for our launch webcast live from Kennedy Space Center. The show is underway on the right-hand side of this page.

The inspection team hasn't reported any problems or concerns during its observations today.

The Final Inspection Team is performing its observations of Endeavour this afternoon.

The team is responsible for checking the shuttle and launch pad one last time prior to liftoff. The team is comprised of engineers and safety officials from NASA, United Space Alliance and tank-builder Lockheed Martin. At the conclusion of their two-hour tour-of-duty, the team will have walked up and down the entire fixed service structure and mobile launcher platform.

The team is on the lookout for any abnormal ice or frost build-up on the vehicle and integrity of the external tank foam insulation.

The team uses a portable infrared scanner that gathers temperature measurements on the surface area of the shuttle and can spot leaks. The scanner will be used to obtain temperature data on the external tank, solid rocket boosters, space shuttle orbiter, main engines and launch pad structures. The scanner can also spot leaks of the cryogenic propellants, and due to its ability to detect distinct temperature differences, can spot any dangerous hydrogen fuel that is burning. The team member also is responsible for photo documentation.

The team wears the highly visible day-glow orange coveralls that are anti-static and flame resistant. Each member also has a self-contained emergency breathing unit that holds about 10 minutes of air.

The crew had lunch at 12:55 p.m. EDT today. The menu included steak and lobster tails for commander Mark Polansky, steak and mashed potatoes for Chris Cassidy and Tim Kopra, lobster and salad for Tom Marshburn, and cheeseburgers for Doug Hurley, Julie Payette and Dave Wolf.

Out at the launch pad's White Room next to Endeavour's hatch, the Orbiter Closeout Crew is preparing for arrival of the astronauts shortly before 4 p.m. EDT. The team will help the seven astronauts get strapped into the spacecraft.

The updated flight plan for Endeavour's mission can be read here.

For those keeping score at home, the fueling of space shuttle Endeavour officially began at 9:48 a.m. and concluded at 12:48 p.m. EDT, precisely on-time, NASA says.

Progress has completed its engine burns to fly away from the International Space Station following today's rendezvous test. The small cargo-delivery spacecraft, which delivered equipment, fuel and food to the station over the past couple of months, will be deorbited on Monday.

The freighter moved within approximately 33 feet of the station and has begun the backaway sequence.

The Progress is performing its approach toward the space station now.

Tank now full! The external fuel tank has been fully loaded with a half-million gallons of supercold liquid oxygen and liquid hydrogen.

But given the cryogenic nature of the oxidizer and propellant, the supplies naturally boil away. So the tanks are continuously topped off until the final minutes of the countdown in a procedure called "stable replenishment."

With the hazardous tanking operation completed, the Orbiter Closeout Crew and Final Inspection Team will head out to the pad to perform their jobs. The closeout crew will ready Endeavour's crew module for the astronauts' ingress in a couple of hours; and the inspection team will give the entire vehicle a check for any ice formation following fueling.

T-minus 3 hours and holding. Clocks have entered a planned two-hour, 30 minute built-in hold.

Up at the International Space Station, a Russian Progress cargo supply ship is performing a close approach test today to test rendezvous antennas and equipment that spacewalkers recently installed on the zenith docking port of the Zvezda service module. That port will become the home of a new Russian module later this year.

This Progress freighter was launched to the station on May 7. It undocked June 30 and flew away from the station. Today's rendezvous test will bring the craft within 10 to 15 meters of the station before being commanded to retreat.

Liquid hydrogen entered stable replenish mode at 12:23 p.m. EDT.

No leakage is being observed in the gaseous hydrogen venting system, NASA says.

Technicians fixed the leak in the ground umbilical carrier plate following scrubs last month and then performed a special fueling test July 1 to ensure the system was tight. The repair efforts overcame a slight misalignment between the external tank and venting system by using a new seal and washer-like shims.

The weather forecast is looking very promising at this point in the countdown.

"The long complicated version can be boiled down to saying that we got about the best chance we can today to keep those afternoon storms off of us," Mission Management Team chairman Mike Moses says.

The liquid hydrogen tank reached the 98 percent level at 11:47 a.m. EDT, NASA says. The topping mode will be used to finish filling the tank. It is during the topping sequence that the leaks developed in the gaseous hydrogen venting system during previous launch attempts.

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.

Mission Management Team chairman Mike Moses just stopped by the public affairs console in the Launch Control Center to give an update on the lightning analysis work performed.

"We gave them an extra day to do that work and it paid off big time. We came in last night around 8 or 9 o'clock local time. I had gotten enough feedback from the teams to know that we were going to be in really good shape and proceed with a launch countdown today. They had some open testing they did over night."

Teams completed dedicated tests on 38 systems to make sure Endeavour is undamaged from the lightning strike Friday, Moses said.

Engineers could not directly test the pyrotechnic initiator controller (PIC) circuitry that fires ordnance on the solid rocket boosters. But Moses said the team compared data from similar systems and officials are comfortable the PIC is ready for flight.

The thrust vector control system, a hydraulic drive that steers the solid rocket boosters, was not checked. That system will be powered on late in the countdown and tested then, Moses said.

After filling a small portion of both the liquid oxygen and liquid hydrogen tanks in slow-fill mode, both loading operations have transitioned to fast-fill. NASA says no problems are being addressed and launch remains scheduled for 7:13 p.m. EDT.

The slow-fill of liquid oxygen began at 10:28 a.m. following completion of the chilldown conditioning.

The liquid hydrogen loading has transitioned from the chilldown thermal conditioning sequence to the slow-fill mode at 9:58 a.m.

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 9:48 a.m. EDT as scheduled.

The Mission Management Team met earlier this morning to review the lightning strike situation and determined Endeavour had not been harmed. Officials then gave the "go" to begin fueling for launch at 7:13 p.m. EDT today.

The launch director in Firing Room 4 has been given the OK to begin the fueling operation.

T-minus 6 hours and counting! The countdown has resumed for today's launch. 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 7:13:55 p.m. EDT.

NASA says no technical issues are being worked in the countdown and fueling should begin shortly.

Commander Mark Polansky tweeted on his Twitter page: "Just got the word that we are GO to fuel the external tank. Hope that today is the day! Watching the Tour de France at the moment."

Go for launch! The formal decision was just made by the Mission Management Team to press ahead with today's launch of Endeavour.

Presentations to the Mission Management Team of the engineering reviews have been completed and NASA says no problems with shuttle systems have been found in the post-lightning checks. Managers just received a weather briefing and will soon decide whether to give final approval to proceed with the launch.

At launch pad 39A, workers are clearing the area in preparation for fueling this morning.

The weather team reports there's no worries about conditions in Central Florida this morning for fueling space shuttle Endeavour's external tank. And the outlook for the 7:13 p.m. EDT launch time continues to be favorable with a 70 percent chance of good weather.

The forecast for this evening includes showers and thunderstorms inland, just a few clouds at 3,000 feet, scattered cirrus at 25,000 feet, visibility of 7 miles, launch pad winds from the southeast at 8 peaking to 12 knots and a temperature of 82 degrees F.

An updated countdown timeline for today, plus the ascent chart have been posted.

NASA leaders are meeting at this hour to determine whether space shuttle Endeavour is clear for launch today. Engineers have been working diligently to sort through data and perform selective retesting on systems to verify no damage was caused by lightning strikes during a severe thunderstorm Friday.

Fueling of Endeavour is scheduled to start at 9:48 a.m. EDT, if the Mission Management Team grants approval.

Today's target launch time is 7:13:55 p.m. EDT.