Mission Status Center

Our post-scrub story has been updated after today's news conference.

The shuttle briefing begins now.

Still waiting for the news conference to begin. We'll stream the briefing when it occurs.

Problems with the shuttle Endeavour's hydraulic power system forced NASA managers to scrub Friday's planned launch on a space station assembly mission, disappointing thousands of spectators and spoiling a visit by President Obama and his family.

Read our updated story.

A post-scrub press conference is planned for approximately 4 p.m. EDT.

NASA now estimates Endeavour won't launch before Monday at 2:33 p.m. EDT.

Launch director Mike Leinbach says the team believes there's an electrical short in one of the shuttle's load control assemblies (LCA) or one of the lines going to that box. The LCA routes power to the two heaters that exhibited problems on Auxiliary Power Unit No. 1, forcing today's scrub.

Requirements for the mission payloads to be readied for the next launch attempt could push liftoff until Monday. But no firm decisions have been made yet.

The launch team plans to meet at 2:30 p.m. to review the game plan going forward and determine when the next countdown is possible.

Problems with the shuttle Endeavour's hydraulic power system forced launch controllers to scrub Friday's planned launch on a space station assembly mission, disappointing thousands of spectators and spoiling a visit by President Obama and his family.

Read our full story.

NASA spokesman George Diller says more than 48 hours may be required before another launch attempt is possible.

Right after the scrub was called, the AstroVan turned around at the Launch Control Center and headed back to the crew quarters building.

Technicians need to gain access into the aft compartment of Endeavour for further troubleshooting. Obviously, that cannot occur while the space shuttle is fueled in the countdown. So the time necessary has delayed the mission at least 48 hours.

NASA estimates the next launch attempt won't come before Sunday at 2:59 p.m. EDT, if the problem can be understood and resolved by then.

SCRUB! Today's launch of Endeavour has been postponed by the APU heater problem.

The AstroVan has pulled into the parking lot at the Launch Control Center.

Troubleshooting on the APU is checking the thermostats to see if they are causing the heater problem.

Two of the heaters appear to have failed on Endeavour's Auxiliary Power Unit No. 1 and technicians are assessing whether that is going to be a problem for launch. The shuttle has three APUs that provide the hydraulic power to drive the ship's aerosurfaces, deploy the landing gear and gimbal the main engine nozzles.

HERE COMES THE CREW. Space shuttle Endeavour's six veteran astronauts have departed Kennedy Space Center's crew quarters building for today's launch into Earth orbit.

Commander Mark Kelly, pilot Greg Johnson, flight engineer Roberto Vittori and spacewalkers Drew Feustel, Mike Fincke and Greg Chamitoff will ride the AstroVan to pad 39A on the edge of the Atlantic Ocean where they will climb aboard the orbiter this afternoon.

The astronauts are leaving the suitup room and heading down the hallway to board the elevator that will take them down to the AstroVan parked outside the Operations & Checkout Building in KSC's Industrial Area.

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 3:47:55 p.m. EDT launch time.

Mission specialist Mike Fincke just tweeted: "Headed for the pad soon. Last tweet for a while. Gonna see some sights and get the job done."

President Barack Obama, the first family and up to 750,000 onlookers are flocking to Florida's Space Coast for Friday's launch of the shuttle Endeavour, the second-to-last flight of NASA's fleet of winged spaceships.

Read our full story.

Scarred by the ravages of Hurricane Katrina and subsequent repairs, Endeavour's external fuel tank went through years of stress before ever encountering the trials of a space shuttle launch.

Read our full story.

Mission specialist Mike Fincke just tweeted a before photo and an after photo for suitup.

Space shuttle Endeavour's astronauts -- all experienced veterans -- are donning 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 at 11:57 a.m.

For here at the Kennedy Space Center, the primary weather concern will be gusty crosswinds at the spaceport's runway for the shuttle's Return to Launch Site abort capability.

The latest weather forecast for the three Transoceanic Abort Landing sites in France and Spain is a little iffy. At least one of the sites must have acceptable weather for the shuttle in case Endeavour must make an emergency landing during launch today. The outlook is a chance of showers at the Zaragoza and Moron runways in Spain. The Istres landing site France had been promising, but the most recent update from meteorologists calls for a "slight chance" of showers there today.

Our launch webcast anchored by Miles O'Brien, with journalist David Waters and for space station commander Leroy Chiao is streaming live on this page.

The Final Inspection Team is out at the launch pad to scan the vehicle for any ice or debris concerns following fueling operations. 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 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 countdown remains on schedule and free of any problems for space shuttle Endeavour this morning. And the weather continues to improve.

Recent activities in the countdown have included calibrations of the orbiter's inertial measurement units, powering up Endeavour's navigation systems, the pre-flight alignment of ground station antennas with the launch pad and communications checks with the Eastern Range.

"Dear God -- thank you for my beautiful wife and awesome kids. Thanks for my family and friends," Mission specialist Mike Fincke just tweeted. "Thank you, dear Lord for the chance to fly in space again. Please don't let me mess it up!"

CONTACT AND CAPTURE! The latest Russian-made resupply ship has arrived at the International Space Station with a smooth docking at 10:28 a.m. EDT while two spacecraft flew 220 miles above western Mongolia.

Inside 30 feet.

Now just 55 feet left to go.

The craft is bringing two-and-a-half tons of supplies to the station, continuing the constant stream of provisions sent up the orbiting complex. The "dry" cargo tucked aboard this Progress amounts to 2,976 pounds in the form of food, spare parts, life support gear and experiment hardware.

The refueling module carries 1,940 pounds of propellant for transfer into the Russian segment of the complex to feed the station's maneuvering thrusters. The vessel also has 926 pounds of water and 110 pounds of oxygen.

Progress is 165 feet away, now closing at 0.4 mph.

The 24-foot long ship will attach itself to the open port on the Pirs compartment, which was vacated last Friday when a previous Progress was jettisoned to burn up in the atmosphere.

The ship is closing at 1.16 miles per hour.

Progress is 500 feet from docking.

Approval has been given to commence the final approach. Progress 42P is closing toward its automated linkup with the International Space Station.

The spacecraft is holding 635 feet away from Pirs.

Progress has reached the stationkeeping hold point while ground controllers verify all is in readiness to begin final approach to docking.

About 800 feet now separate the freighter from the space station.

The space station is staffed by the Expedition 27 crew of commander Dmitry Kondratyev, Cady Coleman, Paolo Nespoli, Ron Garan, Andrey Borisenko and Alexander Samokutyaev.

A flyaround maneuver is underway by the robotic resupply ship to align with the docking port.

Shuttle Endeavour's original April 19 launch date was postponed by 10 days to deconflict with this vessel's arrival. Flight controllers opted against having the freighter rendezvous with the station while the shuttle was there, forcing Endeavour to wait on the ground until after the Progress was launched and docked.

Our stream is showing the Russian Progress 42P cargo freighter docking to the International Space Station right now. Today's space shuttle clearance to launch today is depending upon on good rendezvous by the resupply ship. The linkup is expected at 10:29 a.m. EDT.

Only the ceiling/visibility weather rule is "red" now as the weather gradually improves this morning.

Here is a timeline of major events through the remainder of today's countdown:

HH...MM...SS...EDT...........EVENT

05...35...00...10:07 AM......Pre-ingress switch reconfig
05...12...00...10:30 AM......NASA TV launch coverage begins
04...25...00...11:17 AM......Final crew weather briefing
04...15...00...11:27 AM......Crew suit up begins
03...50...00...11:52 AM......Resume countdown (T-minus 3 hrs)

03...45...00...11:57 AM......Crew departs O&C building
03...15...00...12:27 PM......Crew ingress
02...25...00...01:17 PM......Astronaut comm checks
02...00...00...01:42 PM......Hatch closure
01...30...00...02:12 PM......White room closeout

01...10...00...02:32 PM......Begin 10-minute hold (T-minus 20)
01...00...00...02:42 PM......NASA test director briefing
01...00...00...02:42 PM......Resume countdown (T-minus 20m)

00...59...00...02:43 PM......Backup flight computer to OPS 1
00...55...00...02:47 PM......KSC area clear to launch

00...49...00...02:53 PM......Begin final hold (T-minus 9m)
00...24...00...03:18 PM......NTD launch status verification
00...09...00...03:38:55 PM...Resume countdown (T-minus 9m)

00...07...30...03:40:25 PM...Orbiter access arm retraction
00...05...00...03:42:55 PM...Launch window opens
00...05...00...03:42:55 PM...Hydraulic power system start
00...04...55...03:43:00 PM...Terminate LO2 replenish
00...04...00...03:43:55 PM...Purge sequence 4 hydraulic test
00...04...00...03:43:55 PM...IMUs to inertial
00...03...55...03:44:00 PM...Aerosurface profile
00...03...30...03:44:25 PM...Main engine steering test
00...02...55...03:45:00 PM...LO2 tank pressurization
00...02...35...03:45:20 PM...Fuel cells to internal reactants
00...02...30...03:45:25 PM...Clear caution-and-warning memory
00...02...00...03:45:55 PM...Crew closes visors
00...01...57...03:45:58 PM...LH2 tank pressurization
00...00...50...03:47:05 PM...SRB joint heater deactivation
00...00...31...03:47:24 PM...Shuttle computers take control
00...00...21...03:47:34 PM...Booster steering test
00...00...07...03:47:48 PM...Main engine start (T-6.6 seconds)
00...00...00...03:47:55 PM...Booster ignition (LAUNCH)

With the hazardous tanking operation now completed, the Orbiter Closeout Crew and Final Inspection Team have been dispatched 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.

A reminder that if you will be away from your computer but would like to receive occasional 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.)

TANK FULL. Liquid oxygen has entered stable replenishment mode, officially completing today's three-hour external tank filling process.

For those of you keeping score at home, fueling began at 6:22 a.m. and concluded at 9:24 a.m. EDT.

A variety of the launch weather rules are "red" at the moment due to the cold front moving through the area. The current violations are the rules for lightning, the electrical charge in the air, cumulus clouds, attached anvil clouds, disturbed weather, flight thru precipitation and ceiling/visibility.

T-minus 3 hours and holding. Clocks have entered a planned two-hour, 30-minute built-in hold. Additional pauses are scheduled at the T-minus 20 and T-minus 9 minute marks, setting up the countdown for launch at 3:47 p.m. EDT.

Mission specialist Mike Fincke just tweeted a photo of the crew out for a morning run.

Liquid hydrogen loading has been completed. After reaching the 98 percent level, the topping sequence was performed. And now the stable replenishment mode has been activated to keep the tank full through the rest of the countdown, replacing the supercold propellant that naturally boils away.

NASA assistant launch director Pete Nickolenko says the orbital maneuvering system issue has been corrected and the tank pressure is back within limits. "This problem is behind us."

No leakage is being seen around the ground umbilical carrier plate on the backside of shuttle Endeavour's external tank as the liquid hydrogen system is topped off.

Teams are working an issue with the right-hand orbital maneuvering system engine pod on the tail of Endeavour. But NASA is hopeful this won't be a constraint to launching the space shuttle today as scheduled.

"The launch team is evaluating an issue with the right orbital maneuvering system fuel tank pressure. Currently it's at 290 psi (pounds per square inch) and the launch commit criteria, which are the launch rules we have, the limit is 288," NASA launch commentator Allard Beutel says.

"There's also a difference in pressure between the fuel and oxidizer tanks -- 3 psi.

"All of the pressures are stable and engineers believe that this is an issue may have been caused by a regulator failure. In particular, a regulator locking up and a leaking helium isolation valve within the right OMS pod itself. Launch controllers are in the process now of evaluating a procedure of basically opening up an crossfeed interconnect valve between the left and the right orbital maneuvering systems...to help balance the pressure out."

The cold front is traversing through Central Florida this morning. There is a pretty impressive weather cell moving toward Merritt Island now, but it will stay south of the pad.

In a pre-flight interview, Endeavour pilot Greg Johnson offered this quick overview of the mission:

"STS-134, we are an International Space Station assembly mission, one of the final ones. We're taking up a very important experiment, the Alpha Magnetic Spectrometer, a science experiment that could change the course of physics. And we're also taking some pieces and parts up to the space station in anticipation of the shuttle retiring, because we only have a few more shuttle flights and so we're taking those parts up in an EXPRESS Logistics Carrier and perching that on top of the space station as well."

Mike Fincke tweeted this photo of his launch morning breakfast.

All is going well 80 minutes into the fueling operations for Endeavour. Both liquid oxygen and liquid hydrogen loading have switched to the "fast-fill" mode as fueling of the space shuttle proceeds via remote control at launch pad 39A.

The cryogenics flow 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 fueling sequence started with the chilldown of the liquid oxygen system at 6:22 a.m. EDT. The transfer lines on the liquid oxygen side will be chilled down, then the main propulsion system conditioning is completed.

The liquid hydrogen loading has transitioned from the chilldown thermal conditioning process to the "slow-fill" mode. This fills a small fraction of the tank, then the loading switches to "fast-fill" mode. The propellant started flowing into the tank at 6:32 a.m.

FUELING UNDERWAY. Today's filling of space shuttle Endeavour's external tank has started. It will take three hours to get the half-million gallons of fuel aboard the shuttle for launch.

T-minus 6 hours and counting. The countdown has resumed ticking after a two-hour hold. The next scheduled built-in hold will occur at T-minus 3 hours.

Fueling should be getting underway shortly. No problems are being worked in the countdown this morning.

The launch team conducts a safety check on the pyrotechnic circuits to ensure they are in a good configuration before fueling starts. That's the final step before the tanking preps can commence.

The testing is then re-performed after the shuttle gets into cryogenic conditions and before the ice inspection team and orbiter closeout crew head to the pad.

The Mission Management Team met for the pre-fueling meeting and granted approval to load a half-million gallons of supercold rocket fuel into Endeavour's external tank for launch.

The weather is cloudy but acceptable out at launch pad 39A for fueling Endeavour this morning.

And the latest forecast for today's 3:47 p.m. EDT launch remains at a 70 percent chance of acceptable weather for Endeavour to fly as scheduled.

The latest outlook at launch time is expected to include some scattered clouds at 4,000 feet, good visibility, north-northeasterly winds of 12 knots gusting to 18 knots and a temperature of 77 degrees.

T-minus 6 hours and holding. The countdown has gone into the scheduled two-hour built-in hold prior to the start of fueling. The Mission Management Team is scheduled to convene its critical pre-fueling meeting around 5:45 a.m. EST.

On the night before her last launch, the space shuttle Endeavour has been unveiled from the cocoon-like service gantry at Kennedy Space Center's pad 39A for liftoff.

The stage is now set for tomorrow's big show. Hundreds of thousands are flocking to the Space Coast to witness the launch, including President Obama and the first family.

"When you think about why the people are coming, to experience something that's uniquely American and be able to see one of the last two flights, that gives me a lot of pride," says shuttle launch director Mike Leinbach.

Technicians will spend the next few hours getting ground equipment configured and secured in preparation for tomorrow's fueling with supercold rocket propellant. Inside Endeavour's cockpit, meanwhile, support personnel will verify all of the switches are in the correct positions for ascent.

Clocks resumed counting at 11:22 p.m. EDT after the half-day hold at T-minus 11 hours. The orbiter's fuel cells will be activated about an hour later, and the hazard area around the pad gets is scheduled to be cleared of all workers around 3 a.m.

The next planned hold is T-minus 6 hours beginning at 4:22 a.m. EDT. During this two-hour pause of the clocks, the Mission Management Team convenes its pre-fueling meeting around 5:45 a.m. to review the status of work, the readiness of shuttle systems and the latest weather forecast.

If all goes according to plan, loading of the external tank with propellant will start at 6:22 a.m. EDT. The process should take three hours to complete.

Join us here in the Mission Status Center for live play-by-play updates throughout the day. And don't miss our launch webcast anchored by Miles O'Brien that begins at 11 a.m. EDT (1500 GMT). Miles will be joined by former space station commander Leroy Chiao, journalist David Waters and several special guests.

Liftoff time is 3:47 p.m. EDT (1947 GMT) as Endeavour sets sail on her final space voyage.

This is the next-to-last space shuttle mission. The final one goes up June 28 when Atlantis brings the storied 30-year program to its forced retirement.

"It's like breaking up a family. It's tough to deal with. But we're moving on and we're going to fly these last two missions safely and brings the crews home. And then that will be it," Leinbach says.

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The gantry has arrived in its parking spot for launch.

The structure is nearing its launch position.

After initially moving at a glacial pace, the speed has picked up and the massive gantry is clear of the shuttle now as it continues to back away.

Rollback of the rotating service structure to reveal shuttle Endeavour has started at launch pad 39A. You can watch the tower retraction in our live streaming video.

The observers are on station, inspections are complete and the pathway that the rotating structure will follow has been cleared of personnel. The move is expected to begin shortly.

The walkdown inspections at the pad prior to gantry rollback have been completed with no descrepencies reported.

Pad 39A is busy with activity again as the pad technicians work through the final steps to ready the mobile gantry for retraction away from Endeavour to reveal the spacecraft for launch.

We've posted a YouTube video showing some of tonight's dramatic lightning behind shuttle Endeavour.

After a 3-hour and 40-minute lightning alert tonight, the all-clear has been sounded for technicians to return to the launch pad. The engineering walkdown inspections that were halted around 6:40 p.m. can begin again.

Observers for gantry rollback are scheduled to report on station at 11:15 p.m.

The weather folks expect to drop the lightning warning in the next 5 to 10 minutes, allowing the launch pad crew to resume its work. The current target time for rolling the tower is 11:45 p.m. EDT in support of liftoff tomorrow afternoon.

And check out this image taken from a launch pad camera a short time ago.

Rain is falling at the launch pad again.

Bolts of lightning continue to illuminate the night around space shuttle Endeavour's launch site. The weather has kept launch pad prep work suspended for nearly three hours now, and the schedule was running a bit behind before activities were halted.

A new weather alert has been announced through 11 p.m. for high winds and the chance of one-inch hail at the space center.

Another weather cell is crossing the river toward the Cape.

Check out this stunning lightning photography taken from the Kennedy Space Center press site this evening with the Vehicle Assembly Building and shuttle Endeavour as backdrop.

NASA officials say they can delay rolling the gantry by four or five hours without impacting tomorrow's launch.

The lightning warning that is preventing workers from doing their activities at the launch pad remains posted. There's no estimate when that alert will be relaxed.

The weather advisory for winds gusting to 55 knots and the chance of one-inch hail has been terminated. But a wind alert of 25 knots gusting to 35 knots is in effect through 6 a.m.

The current hope by the team is getting the tower retraction underway between 10 and 10:30 p.m. EDT, if the weather allows. But the good news is there's no indications of any damage to the shuttle from the storm that passed through the area a little while ago.

Now it's a matter of waiting until the lightning threat passes and the launch pad can be reopened for workers to resume their countdown activities tonight. The team has lost a couple of hours already.

The skies are brightening again as the storm is pushing on through the area.

Rain is pouring at the launch pad. But the severe weather is skirting to the north of Endeavour.

Hail is being reported in downtown Titusville.

The latest weather advisory for Kennedy Space Center is predicting winds of 45 knots gusting to 55 knots and one-inch hail possible as the storm moves through between now and 9 p.m. EDT.

A Phase 2 lightning warning has been posted for Kennedy Space Center. That means launch pad activities will have to stop while workers to take precautionary shelter.

Rollback of the rotating service structure to reveal shuttle Endeavour is coming up later this evening. The plan calls for final walkdown inspections of the pad starting at 6:30 p.m., observers at their duty posts around 7:30 p.m. and gantry retraction starting at 8:30 p.m. EDT.

A low-level lightning advisory has been issued for all areas of Kennedy Space Center through 10 p.m. tonight.

NASA now estimates tower rollback will begin no sooner than 8:30 p.m. EDT, based on the weather forecast.

The official launch window for Friday's shot at getting space shuttle Endeavour into orbit for docking with the International Space Station extends for 10 minutes.

Based on the latest radar tracking of the space station's orbit and subsequent revision from Mission Control, the single-pane window leading to rendezvous on Flight Day 3 will open at 3:42:55 and last until 3:52:56 p.m. EDT.

The targeted liftoff time occurs in the middle of the period at 3:47:55 p.m. EDT. That's the moment when Earth's rotation carries the launch pad into the plane of the station's orbit.

We have updated our countdown timeline and launch events pages with this latest tweak to the liftoff time.

If liftoff slips to Saturday for some reason, the backup launch opportunity offers a window of 3:17:13 to 3:27:13 p.m. EDT. The preferred launch time would be 3:22:13 p.m. EDT.

With some stormy weather moving toward Central Florida, the launch pad team expects the planned 7 p.m. EDT retraction of the rotating service structure will be delayed a bit. There's no estimate how long the weather hold could last. NASA can delay rolling the tower by four or five hours without impacting tomorrow's launch.

"We had a 'wave across' today with our families at the base of the launch pad. Due to quarantine requirements, we had to keep our distance," pilot Greg Johnson just tweeted. "Weather is looking good tomorrow after the cold front passes. Sooner is better! Our European abort site weather is iffy, but OK in France."

Endeavour's six astronauts are spending today visiting with their families and touring the shuttle at the pad. They also received briefings on orbiter preparations, the payload status and the weather forecast from the ascent team of flight controllers in Houston.

Check out this great photo of them at the launch pad today.

They will go to sleep at 10 p.m. EDT and will be awakened for launch day at 6 a.m. tomorrow. They'll have breakfast at 6:30 a.m. and then undergo final medical exams at 7 a.m. Suit up begins about 11:27 a.m. and departure from crew quarters is scheduled for 11:57 a.m. in preparation for blastoff at 3:47 p.m. EDT.

A storm front that wreaked havoc across the southeastern United States overnight Wednesday will reach the Florida space coast tonight, but forecasters expect the sky will clear in time for the shuttle Endeavour's launch Friday on a long-awaited space station assembly mission.

Read our full story.

With just two flights remaining and thousands of layoffs looming, NASA is readying the shuttle Endeavour for launch Friday on its 25th and final mission, a four-spacewalk voyage to deliver supplies, spare parts and a $2 billion particle physics detector to the International Space Station.

This is our 6-page, 10,300-word mission preview.

Shuttle Endeavour remains poised for blastoff tomorrow at 3:47 p.m. EDT (1947 GMT) on a two-week flight to the International Space Station.

"I'm pleased to report that everything is going well out at pad A. We don't have any issues right now that we're tracking," says Jeff Spaulding, the NASA test director.

Final tests of the avionics, pneumatics and controllers for the three main engines were conducted overnight. Countdown clocks then entered the lengthy T-minus 11 hour planned hold period at 10 a.m. That built-in hold should last 13 hours and 22 minutes.

Today's chores at launch pad 39A are focused on 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.

The giant gantry-like rotating service structure is scheduled for retraction from around Endeavour at 7 p.m. Fueling of the shuttle's giant external tank begins tomorrow at 6:22 a.m.

"Endeavour, like all of our shuttles, has had a very distinguished history throughout its life. Tomorrow we're going to add to that distinction as she departs for the mission to deliver the Alpha Magnetic Spectrometer to the station," Spaulding said.

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The weather forecast for Friday's launch has dropped the odds of acceptable conditions to 70 percent due to the chance of low clouds and high crosswinds.

The outlook from Air Force meteorologists: "The cold front causing severe weather in the Southeast U.S. yesterday will move into Central Florida this evening. Although the front will lose a lot of its energy, late afternoon to evening thunderstorms are expected at Kennedy Space Center (KSC), and an isolated severe storm is possible causing concern for the rotating service structure retract operation.

"Meteorological models are a few hours slower with the movement of the front; therefore, showers may linger into the morning tanking operation, and a low-cloud ceiling is also a concern for launch. The ceiling will be over the area through the countdown, but is expected to clear before launch as the front continues to move southeast.

"Winds will shift to the north-northeast, and Shuttle Landing Facility (SLF) crosswinds continue to be a concern. Our primary concerns for launch are a low-cloud ceiling and a SLF crosswind violation."

The launch time conditions are expected to include some scattered clouds at 3,000 feet, good visibility, north-northeasterly winds from 020 degrees on the compass at 12 knots gusting to 18 knots and a temperature of 77 degrees.

The forecast for Saturday is 70 percent favorable and Sunday is 80 percent "go" for weather.

The cryogenic reactants for the space shuttle's electricity-generating fuel cells were successfully loaded aboard Endeavour during a multi-hour operation at pad 39A this afternoon.

Work performed tonight adjusted the tank levels, actually removing some of the reactants off the vehicle, to optimize Endeavour's launch weight.

Meanwhile, Endeavour's crew underwent medical exams today, received a briefing on the orbiter ingress plan for launch day, plus got some flying time in T-38 jets and met with NASA Administrator Charlie Bolden.

Space shuttle main engine checkouts will be performed early Thursday while other pad technicians continue work to button up the orbiter and ground support equipment for launch.

Liftoff remains targeted for Friday at 3:47 p.m. EDT (1947 GMT).

Our live launch webcast hosted by Miles O'Brien, David Waters and former astronaut Leroy Chiao begins Friday at 11 a.m. EDT (1500 GMT) from the Kennedy Space Center. Join us for the show!

The shuttle Endeavour's countdown is proceeding smoothly toward launch Friday with no technical problems of any significance and forecasters predicting an 80 percent chance of good weather. Rep. Gabrielle Giffords, the wounded wife of shuttle commander Mark Kelly, flew to Florida from Houston Wednesday to witness the blastoff.

Read our full story.

The Kennedy Space Center fire department reports the brush fire has been contained. Helicopters and fire crews are dealing with a few hotspots following today's brush fire.

Helicopters are now dumping water alongside the crawlerway that stretches from the Vehicle Assembly Building to the launch pads as a preventative measure to stop the fire jumping the canal used by the external tank barges.

Helicopters are fighting the brush fire at the Kennedy Space Center, scooping up water from the turn basin in buckets. The efforts appear to be paying off with the fire appearing to be more under control.

The doors of the Vehicle Assembly Building were closed as thick smoke drifts across the Kennedy Space Center, obscuring views of space shuttle Endeavour on the launch pad.

Kennedy Space Center's fire department has stationed crews along the shuttle crawlerway, where embers from today's brush fire are falling. So far there are no reports of fire breaking out in that area.

Kennedy Space Center's fire department is responding to a brush fire that erupted this afternoon about 3 miles southwest of Endeavour's launch pad. Thick gray and black smoke is streaming north over the crawlerway between the Vehicle Assembly Building and launch pad 39A.

Winds are gusty at KSC this afternoon out out the south and southeast up to 20 knots.

A NASA helicopter and patrol boat are in the area around the KSC press site, where reporters and photographers are gathered to cover Endeavour's countdown.

The Mission Management Team gathered this morning for its pre-launch meeting and verified everything remains on track for shuttle Endeavour's blastoff Friday at 3:47 p.m. EDT.

"We had an easy poll to say we're ready to go. We're looking forward to Friday's launch," says pre-launch MMT chairman Mike Moses.

"The countdown for Endeavour's final mission is going extremely well," shuttle launch director Mike Leinbach added. "We're not working any significant issues at all. The team is upbeat."

Liquid hydrogen and liquid oxygen for Endeavour's electricity-generating fuel cells will be loaded into storage spheres beneath payload bay beginning around 11:30 a.m. EDT today as standard work continues at pad 39A.

The cryogenic reactants are combined by the three onboard fuel cells to produce power and a byproduct of drinking water during the shuttle's mission.

Technicians flow the cryogenics into small tanks aboard the orbiter during a multi-hour operation. Later, they will demate the pad umbilical system used in the loading process and stow that equipment.

Earlier this morning, tests of the shuttle's pyrotechnic initiator controllers and range safety systems were completed while the pad was cleared of personnel.

The countdown had a planned four-hour hold at the T-minus 27 hour mark that began at 6 a.m. and lasted through 10 a.m. EDT ahead of the fuel cell servicing work. Clocks now will continue until the next hold point at T-minus 19 hours starting at 6 p.m., which is when the ground team wrap up the fuel cell operations.

Before the shuttle Endeavour can receive final clearance for takeoff Friday afternoon, a Russian-made cargo freighter must make a smooth flight to the International Space Station. Step number one was accomplished this morning when that unmanned resupply ship successfully launched atop a Soyuz booster from Kazakhstan at 9:05 a.m. EDT (1305 GMT).

Read our full story.

The shuttle Endeavour's crew flew to Kennedy Space Center today for the start of their countdown to blastoff Friday on a long-awaited mission to deliver supplies, spare parts and a $2 billion cosmic ray detector to the International Space Station. Forecasters are predicting an 80 percent chance of good weather.

Read our full story.

A check out this crew arrival photo gallery.

Wow! Look at this photo just tweeted by Mike Fincke that he took while buzzing over launch pad 39A before arriving today.

COUNT BEGINS. Countdown clocks at the Kennedy Space Center just began ticking toward Friday's scheduled launch of the shuttle Endeavour on the veteran ship's final trek to space.

The official countdown sequence started at 2 p.m. EDT inside Firing Room 4 of the Complex 39 Launch Control Center. Launch team members had gathered for the "call-to-stations" at 1:30 p.m.

The count commenced from the T-minus 43 hour mark. But a series of pre-planned holds are timed throughout the next few days, leading to the targeted liftoff time of 3:47 p.m. EDT.

The weather forecast looks promising for the launch opportunity and there are no hardware worries being worked as the count starts.

Activities planned during the early portion of the countdown for shuttle workers include buttoning up launch pad equipment and removing platforms inside the orbiter'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.

If you missed any of our live streaming video coverage of this morning's countdown preview briefing or the astronauts arriving for launch, Spaceflight Now+Plus subscribers can watch both events in standard or high-definition video downloads:


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Six astronauts traveled from their home base in Houston to the Kennedy Space Center launch site in sleek T-38 jets today, trained and ready to take shuttle Endeavour for its final spaceflight.

Commander Mark Kelly, pilot Greg Johnson, flight engineer Roberto Vittori and spacewalkers Drew Feustel, Mike Fincke and Greg Chamitoff arrived at the Florida spaceport to begin counting down to Friday's blastoff.

"We're really happy to be here today," Kelly told reporters at the runway. "It's a beautiful day. We got a chance to take a look of the orbiter as we flew over, first the field and then the pad. It's great to see Endeavour all ready to go again."

The crew will tag up with their support personnel and check on flight data files this afternoon. Kelly and Johnson plan to fly landing approaches into KSC's runway using Shuttle Training Aircraft this evening. The astronauts head to sleep at 10:30 p.m. EDT, then will be awakened at 6 a.m. Wednesday.

On hand to greet the arriving astronauts are KSC center director Bob Cabana, shuttle launch director Mike Leinbach and former space station commander Scott Kelly, Mark's twin brother.

Welcome to the spaceport! All six Endeavour astronauts have arrived for launch on Friday afternoon.

The astronauts are soaring into the Kennedy Space Center's Shuttle Landing Facility, which is the same three-mile-long concrete field where Endeavour will touch down to conclude the upcoming mission.

The tower is giving the T-38s jets clearance for touchdown.

The T-38s are headed for a flyby of shuttle Endeavour at launch pad 39A before landing.

Commander Mark Kelly has made voice contact with the KSC tower. This lead aircraft is about 20 minutes away from the spaceport and will be targeting Runway 15 for landing.

The NASA aircraft carrying the astronauts' families has arrived at Kennedy Space Center.

Reporters and photographers are gathered at the runway to cover the astronauts' arrival. You can watch it live with our streaming video coverage when it happens.

Given the crew's stop at Tyndall Air Force Base on the trip to Kennedy Space Center, NASA now says the astronaut arrival isn't expected until around 12:50 p.m. EDT or so.

An incredible photo just tweeted by Mike Fincke of the T-38s making their way to Florida.

Mission specialist Mike Fincke tweeted a few minutes ago: "Just landed at Tyndall AFB on the way to the cape. Taxied by some F-22 Raptors. Nice!"

At launch pad 39A, the payload bay doors of space shuttle Endeavour have been swung shut and latched for flight. The next time the doors are opened will be about 90 minutes into the mission on Friday.

The astronauts are traveling in four T-38 jets:

• Tail number 955: Commander Mark Kelly and mission specialist Mike Fincke
• Tail number 966: Pilot Greg Johnson and mission specialist Roberto Vittori
• Tail number 961: Mission specialist Drew Feustel (with non-crew pilot)
• Tail number 908: Mission specialist Greg Chamitoff (with non-crew pilot)

NASA says the shuttle Endeavour astronauts have embarked on their trip from Ellington Field near the Johnson Space Center in Houston over to the Kennedy Space Center in Florida. Arrival at the launch site is expected around 12:15 p.m. EDT.

After reaching the spaceport, the crew will pose for photos and make a brief statement to the gathered news media.

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"Teams here at Kennedy Space Center and at NASA centers around the country have been working for over a year to get Endeavour ready for the STS-134 mission," says Jeremy Graeber, NASA test director.

"We are not working any issues at this time."

Clocks will begin ticking at 2 p.m. EDT for the three-day countdown sequence to Friday's planned launch of Endeavour.

"All of our vehicle and ground systems are ready, the STS-134 flight crew, Endeavour and launch team here at Kennedy Space Center are all ready to proceed. We'll looking forward to getting (the) launch countdown underway, leading to a spectacular Friday launch," Graeber said.

Get a live update on shuttle Endeavour preparations and hear from the launch weather officer in the NASA news briefing beginning now in our streaming video.

The weather outlook issued moments ago by the launch meteorology team gives an 80 percent chance for Endeavour to fly on Friday as planned. The only potential for weather prohibiting launch is the concern for crosswinds at the shuttle's Florida runway that must be available in case an emergency landing is declared just after liftoff.

The specifics call for a few clouds at 3,000 feet, good visibility, north-northeasterly winds of 12 peaking to 18 knots and a temperature of 77 degrees.

"High pressure is located well east of Florida and the ridge associated with the high is extended north of Florida. Southeast flow is prevalent over the East Central Florida Coast, and showers are present offshore moving north-northwest parallel to the coast. A wave of lower pressure is migrating northwest from the Bahamas and will cause more widespread offshore showers this afternoon," the weather team says."

"Kennedy Space Center may experience an isolated shower or thunderstorm Tuesday and Wednesday in the early afternoon as the sea breeze causes low level convergence, but the sea breeze will migrate inland quickly, and the majority of afternoon weather will occur inland. On Thursday, a cold front will move into Florida, causing moist, unstable conditions in the afternoon and evening. Also, a late afternoon sea breeze may occur, causing additional low-level convergence. With all of these factors, late afternoon to evening thunderstorms are expected, and some could be severe causing concern for the rotating service structure retract operation.

"The front will move through overnight, and the weather will improve through Friday morning. Tanking weather will be cloudy, but favorable, and by launch time, the only issue is the wind will shift to the north-northeast, causing a concern for a Shuttle Landing Facility (SLF) crosswind violation. Our primary concern for launch is a SLF crosswind violation.

"The concern for a SLF crosswind violation increases the following day as the winds shift more easterly, and by Sunday, the winds become more favorable from the east-southeast," forecasters report.

If Endeavour's launch slips to Saturday for some reason, the odds of acceptable weather dips to 70 percent and then improves to 80 percent on Sunday.

Endeavour's six astronauts will arrive at the Kennedy Space Center today as the launch team begins the countdown for Friday's scheduled blastoff of the next-to-last space shuttle mission.

Meet the astronauts.

The space shuttle team is back from the holiday weekend to begin launch week for Endeavour. Final touches are being put on the orbiter's payload bay today in advance of closing the 60-foot-long doors tomorrow. In firing room 4 of the launch control center, countdown preps are in work for starting the clocks ticking tomorrow afternoon.

Join us here on this page tomorrow for live streaming video coverage of the countdown status and weather briefing at 10 a.m. EDT and the astronauts' arrival at 12:15 p.m. EDT.

Space shuttle Endeavour's three-level aft engine compartment has been buttoned up for launch and the access doors on either side of the orbiter are now installed and sealed as the team heads into some time off for the holiday weekend.

The six-man astronaut crew has completed its year-and-a-half training regimen in Houston and enters preflight medical quarantine today. They are scheduled to arrive at the Kennedy Space Center on Tuesday around 12:15 p.m. EDT in advance of the countdown clocks starting to tick at 2 p.m.

Liftoff remains on schedule for next Friday at 3:47 p.m. EDT.

Flight pressurization of the gaseous helium and nitrogen systems for Endeavour's orbital maneuvering system engine pods and reaction control thrusters, plus the main propulsion system pressure vessels was successfully completed overnight and into this morning.

The astronauts performed their final training inside the motion based simulator today to practice launch and ascent procedures.

Meanwhile, check out our story on the STORRM experiment the astronauts will conduct after undocking from the International Space Station. It's a test pilot's dream.

And here's our story posted yesterday about President Obama's plans to attend Endeavour's launch.

We also have a list of all previous presidential and vice presidential visits made to Kennedy Space Center.

Endeavour and her six-man crew are moving ever closer toward flight next week as work continues smoothly on the space shuttle at launch pad 39A and the astronauts finish their training in Houston.

"The final processing flow for Endeavour is going extremely well out at the pad," launch director Mike Leinbach says.

Work to install the ordnance used to release the shuttle from the launch pad and separate the various parts of the vehicle during ascent has been accomplished. "That's all complete and good," Leinbach said.

Next up is the pre-flight charging of the high-pressure gas bottles for the hypergolic and main propulsion systems beginning tonight.

"The rest of the pad flow is really pretty easy. We have all of Easter weekend off, which is nice for us. We'll come back in next Monday, close the payload bay doors and a week from right now we'll be into launch countdown for STS-134."

Commander Mark Kelly, pilot Greg Johnson and mission specialists Mike Fincke, Roberto Vittori, Drew Feustel and Greg Chamitoff are honing their skills at the Johnson Space Center before heading over to the Cape next Tuesday for the countdown.

"Official launch date of April 29 in hand. On flight deck for final integrated entry sim today in the motion base simulator," Kelly tweeted this morning.

This is the crew's final re-entry simulation with Mission Control to practice responding to problems that could arise during Endeavour's landing.

"2 failures so far for entry. Cabin leak and an OMS engine fail," Fincke tweeted during the training. Then later he wrote: "Just 'landed' in the simulator. Many small malfunctions great job to MCC."

Fincke even posted a photo of the commander at work.

NASA managers attending an executive-level flight readiness review Tuesday formally cleared the shuttle Endeavour for launch April 29 on its 25th and final mission, a four-spacewalk flight to deliver supplies, spare parts and a $2 billion particle physics detector to the International Space Station.

Read our full story.

Today's Flight Readiness Review has concluded with a unanimous "go" for launch of Endeavour next Friday, April 29 at 3:47 p.m. EDT. Senior managers reviewed the readiness of the space shuttle and International Space Station for the mission to deliver the Alpha Magnetic Spectrometer and another spare parts pallet.

A news conference with NASA officials is expected at 4 p.m. EDT today.

The morning session of the Flight Readiness Review received briefings from the mission operations folks who will run STS-134 from Houston and discussed the status of life-support elements aboard the space station.

After a break for lunch, managers have moved into the space shuttle program topics.

The Flight Readiness Review is underway today at the Kennedy Space Center. Senior officials are examining all aspects of mission preparations to ensure space shuttle Endeavour, the astronauts and payloads are ready to go fly the STS-134 mission, as well as the International Space Station is ready to receive this flight.

The meeting will culminate with managers setting the formal launch date. The shuttle is targeting blastoff in 10 days.

NASA plans to hold a post-FRR news conference this afternoon. The exact start time has not been determined. We'll post updates on this page and dispatch Twitter alerts as further details become available.

NASA officials have reversed the decision to lengthen Endeavour's mission to 15 days, opting instead to wait until after the shuttle in orbit to make that call.

"The program had extended the mission one day on Wednesday, but managers determined late yesterday that they will keep it at a 14-day flight for the time being," the space agency announced today. "Once Endeavour is in space, managers will evaluate the shuttle's performance and other mission objectives and then decide whether to add another day or two to the flight to support operations aboard the International Space Station."

Today's work at pad 39A is focusing on closing up Endeavour's aft engine compartment for launch. Ordnance installation is scheduled to begin Monday.

Things have been fairly quiet at launch pad 39A. Tankers have been coming in waves to replenish the liquid oxygen and liquid hydrogen storage spheres. Final pre-flight testing of the Alpha Magnetic Spectrometer was completed over the weekend and the payload has been powered down for launch. And the spacewalking suits were loaded into shuttle Endeavour's airlock Wednesday in preparation for checkout today.

A bonus day has been added to space shuttle Endeavour's flight to the International Space Station, managers decided today. The mission now lasts a planned 15 days with launch on April 29 at 3:47 p.m. and landing back at Kennedy Space Center on May 14 at 9:51 a.m. EDT.

The extra time will be inserted into the flight as the new Flight Day 10 and be spent with internal maintenance work with the station's U.S. carbon dioxide removal system.

There is the potential that even another day could get added to the mission later. As it currently stands, Endeavour should have enough consumables for a further one-day extension, plus a pair of landing waveoff days for weather or system problems.

NASA Administrator Charles Bolden unveiled which museums will house the retired space shuttle vehicles during a ceremony today on the 30th anniversary of the program's first launching.

Read our full story.

Bracing for a potential government shutdown, NASA managers are assessing shuttle launch processing and putting plans in place to continue near-normal operation of the International Space Station if a workforce furlough is ordered.

Read our full story.

Despite the 10-day delay in shuttle Endeavour's launch, work continues at pad 39A to prepare the spacecraft and payload for flight.

Servicing of the hydraulic power units on the solid rocket boosters was successfully completed yesterday by loading hydrazine fuel. The HPUs enable the twin motors' nozzles to swivel during ascent and steer the shuttle.

Meanwhile, preparations are underway to perform a end-to-end systems test on the Alpha Magnetic Spectrometer.

IT'S OFFICIAL: Attempts by NASA to resolve the scheduling conflict between Endeavour's mission to the International Space Station and the exchange of Russian resupply ships at the complex have failed to secure the shuttle's planned April 19 launch date. Instead, this final voyage by Endeavour will be delayed 10 days.

Manifests call for the Russian Progress 41P resupply ship to depart the space station on April 26, the fresh Progress 42P freighter to launch from Baikonur Cosmodrome on April 27 and the automated linkup to happen April 29.

Conducting those undocking and docking events while Endeavour is parked at the space station isn't something NASA wants to do. An April 19 launch by the shuttle would have brought Endeavour to the complex on April 21 for the two-week mission.

Options had included postponing the new Progress cargo ship's launch or launching it on-time and then loitering in orbit until after the shuttle left the station.

But a time-sensitive science experiment being carried aboard the cargo freighter made delays to its launch and docking dates undesirable to the Russians.

Negotiations ultimately led to delaying Endeavour's launch a week-and-a-half to wait for the Russian traffic to clear. Liftoff is retargeted for April 29 at 3:47 p.m. EDT (1947 GMT).

The Russians send up a half-dozen unmanned Progress spacecraft each year to deliver food, oxygen, equipment, experiments and rocket fuel to the space station. Before undocking from the station, the crews fill the ships with trash to burn up in the atmosphere.

Endeavour's mission will put final touches out the space station's structure by installing the $2 billion Alpha Magnetic Spectrometer physics experiment and one last collection of external spare parts.

Given this delay, the flight readiness review moves to April 19, the astronauts' arrival in Florida for launch is rescheduled for April 25 and the countdown begins ticking on April 26.

After docking to the station on May 1, the AMS instrument will be attached on May 2 and the mission's four spacewalks occur on May 3, 5, 7 and 9.


Read our earlier status center coverage.