NASA's mission managers met on Wednesday morning to discuss space shuttle Endeavour's readiness to launch on Friday. They concluded their L-2 meeting with a unanimous "go" for space shuttle Endeavour's launch attempt on Friday, April 29 at 3:47 p.m. EDT.

"Everything is in great shape," said Mike Moses, the chair of the mission management team. "The team is all polled, ready to go and proceed with the countdown."

The official countdown to Friday's launch began at 2:00 p.m. on Tuesday at the T-43 hour mark.

Technicians working at Kennedy's Launch Pad 39A will fill Endeavour's three power-producing fuel cells with cryogenic liquid oxygen and liquid hydrogen today.

Endeavour's six-person STS-134 crew arrived in Florida on Tuesday for their pre-launch preparations. The astronauts flew in on T-38 jets, landing at 12:52 p.m. EDT.

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

Shuttle Weather Officer Kathy Winters reported weather is looking good for launch day and for the loading of propellants into Endeavour's external fuel tank although there may be possibility for a storm during the rollback of the rotating service structure, or RSS, on Thursday evening.

At this time, the forecast calls for only a 20 percent chance of weather hindering a launch on Friday, due to possible windy conditions, reported Winters.

Endeavour's STS-134 crew pose for a photo after arriving in Florida. Credit: collectSPACE

Preparations for Endeavour's launch are continuing as planned at Launch Pad 39A. Liftoff for the STS-134 mission is scheduled for 3:47 p.m. EDT Friday.

"I'm pleased to report that everything is going well at Pad A," said Jeff Spaulding, NASA test director, on Thursday morning. We don't have any issues right now that we're tracking, we're right back on our timelines."

Spalding said that technicians were finishing preparing Endeavour's three main engines for launch and were working on inspections of the orbiter, its external fuel tank and two solid rocket boosters.

"All those are in work and going well," he said.

"Later this afternoon, we do intend to power up our orbiter- and ground-side communications networks, and we will be doing a full end-to-end checkout on all of those systems," Spalding added. "Once we're done with that, we will go into the crew module and start doing our flight crew equipment late stow operations, putting all of our final middeck [items] and some our last provisions into the vehicle for the crew."

Pad 39A's Rotating Service Structure (RSS), which envelops Endeavour, is scheduled to be retracted at 7 p.m. today but may be delayed because of storms in the vicinity. Teams will have about a four-hour leeway for the weather to clear before the delay would affect Friday morning's external tank fueling, now planned to begin at 6:22 a.m.

The weather forecast for launch has slightly worsened, with a 30 percent chance of conditions preventing an on-time liftoff. In addition to concerns over crosswinds at Kennedy Space Center's Shuttle Landing Facility, the chance of a low cloud ceiling on Friday will increase as a front moves into central Florida tonight.

Space shuttle Endeavour was revealed on its launch pad early on Friday morning after a five hour delay as NASA waited for severe thunderstorms to move off-shore.

Lightning in the vicinity of Kennedy Space Center's Launch Pad 39A left Endeavour unscathed but prevented technicians from retracting the pad's rotating service structure at 7:00 p.m. EDT Thursday as planned. NASA safety rules required that its workers wait for the storms to pass, pushing the start of the retraction to just before midnight Friday.

Teams began moving the structure away from Endeavour at 11:58 p.m. It was fully retracted at at 12:32 a.m.

The rotating service structure enveloped the shuttle as it was readied for launch, protecting the orbiter and providing access to its payload bay.

The retraction was required before Endeavour's external tank could begin being fueled for flight, scheduled to begin at 6:22 a.m.

Space shuttle Endeavour is bathed in Xenon light on Launch Pad 39A. Credit: collectSPACE

Fueling of space shuttle Endeavour's external tank with 535,000 gallons of liquid oxygen and hydrogen is now underway.

Space shuttle mission managers met at 5:45 a.m. EDT and gave a "go" to begin loading Endeavour's external tank with its super-cold cryogenic propellants. The operation began on time at 6:22 a.m. EDT and will take approximately three hours.

The first phase of the fueling is called "slow fill" to allow the pumps, lines and tank to condition themselves to the super cold chemicals. The liquid oxygen is at minus-297 degrees Fahrenheit and the liquid hydrogen is at minus 423 degrees F.

The weather forecast calls for a 70 percent chance of favorable weather for launch. The primary concern is for low cloud ceilings and crosswinds at the Kennedy Space Center's Shuttle Landing Facility. A front will pass through Kennedy between 7 and 8 a.m., bringing a chance for showers. After the front passes, high pressure will build into the area and winds will shift to the northeast and be near the peak limit for crosswinds.

Teams are not working any issues that would delay today's liftoff, which is targeted for 3:47:55 p.m. EDT (1947 GMT).

The STS-134 astronauts woke at 6 a.m. and, after having breakfast, are soon to undergo pre-launch medical checks.

NASA's launch team is evaluating an issue with Endeavour's right Orbital Maneuvering System (OMS) fuel tank pressure. It is currently reading at 290 pounds per square inch (psi) and the Launch Commit Criteria limit is 288 psi.

There is also a difference in pressure between the fuel and oxidizer tanks of 18 psi, and the limit is 15. The oxidizer pressure is 272 psi.

All the pressures are stable, and engineers believe the issue may have been caused by a regulator failure locking up and a helium isolation valve leaking.

Launch controllers are working to open the crossfeed valve between the left and right OMS pods to help alleviate the pressure, which is expected to clear the issue.

The two OMS engines, located in "pods" on each side of the orbiter's aft fuselage, provide propulsion for the shuttle while it is in orbit. The OMS is used for orbit insertion, circularization, transfer, rendezvous and deorbit.

Endeavour's external fuel tank has now been filled with 535,000 gallons of liquid hydrogen and liquid oxygen.

"Fast fill" of the tank began at 7:10 a.m. EDT with liquid hydrogen and one minute later for the liquid oxygen. Fueling reached "stable replenish" of both cryogenic propellants at 9:24 a.m. EDT. Liquid hydrogen (LH2) "topping" began at 8:21 a.m.

The three-hour tanking process, which began at 6:22 a.m. EDT, provides the fuel and oxidizer that Endeavour's three main engines need for the eight and a half minute climb into orbit, scheduled to begin with a liftoff at 3:57 p.m. EST.

The earlier issue involving Endeavour's right orbital maneuvering system (OMS) engine has been cleared. A crossfeed valve procedure worked to alleviate the right OMS pressure. Both left and right tanks are balanced and are below limits.

There are no additional technical issues being worked for launch.

A storm cell is rapidly moving through the Kennedy Space Center. Once it passes, forecasters predict a 70 percent chance of favorable weather for launch. The only concerns are low cloud ceilings and high crosswinds at the Shuttle Landing Facility.

Space shuttle Endeavour will wait at least three days before launching on its final mission due to a suspected electrical short in one of the three units that powers its hydraulic systems.

"It's unfortunate for the [Endeavour] team and Mark Kelly and his crew, but today the orbiter is not ready to fly," said shuttle launch director Mike Leinbach, after he scrubbed the planned 3:47 p.m. EDT liftoff attempt at 12:16 p.m. "And as we always say in this business, we do not fly before we are ready."

Leinbach said the specific problem was with a fuel line thermostat inside Endeavour's Auxiliary Power Unit (APU) No. 1.

"We need to keep those lines warm to prevent them from freezing on orbit," explained Leinbach. "So we have two thermostats — two heaters — for each line for each APU."

"On Auxiliary Power Unit No. 1, one of those two heater units failed and the troubleshooting we did on it proved that it was a hard failure. We were not able to get it to come to life no matter what we did."

"We tried to let the line cool down just by normal means and to see if the thermostat on that heater would kick in, that did not happen. We tried to command the heater from the cockpit of the orbiter, that did not happen either. So we know we have a hard failure in that heater stringer for that one auxiliary power unit," said Leinbach.

Another heater upstream of the one that failed was also exhibiting some "funny behavior," said Leinbach, so what they believe they have now is a problem with one of the switch box load control assemblies (LCA).

"That says we probably have a short actually in that box or in one of the electrical lines to that box or from that box, we're not quite sure yet. We won't know until we get into the aft of the orbiter and do some detailed troubleshooting," Leinbach said.

Before technicians can begin inspecting Endeavour, the shuttle's external fuel tank needed to be drained of its propellants, which is now underway.

"We'll set up to gain entry into the aft compartment overnight tonight and probably [by] tomorrow afternoon we'll get into the aft. [We'll] start putting in our platform set to get to this load control assembly, which is down inside the aft quite a ways, and so it is going to take us a bit of time to get in and do that," said Leinbach.

Leinbach said he would meet with his launch team at 2:30 p.m. EDT to discuss the forward plan. If Endeavour is ready to launch on Monday, the new targeted liftoff time will be 2:33 p.m. EDT (1833 GMT).

Engineers will begin troubleshooting the issue that scrubbed Endeavour's Friday launch attempt after they gain access to the rear of the orbiter on Saturday afternoon. Once inside, they'll learn if a Monday launch is still a possibility.

At issue are a series of fuel line thermostats that failed to turn on inside Endeavour's Auxiliary Power Unit (APU) No. 1, which powers the orbiter's hydraulic systems. The problem could be a failed thermostat or it could be a short in a switch box.

"The troubleshooting involves taking in a can of mist and spraying it on the thermostat to see if we can force a lower temperature... and have it kick on," explained shuttle launch director Mike Leinbach. "If that works, we might have just a thermostat that's out of kilter and we could replace that thermostat and be in good shape."

"If it's more than that, then it could be in a box called the LCA, the Load Control Assembly, which is essentially a box full of switches," Leinbach said. "If there is something wrong in that box, if we have to change out that box, that is a much more extensive job and it would take us quite some time to do that."

Leinbach said there is still a chance for launching Endeavour on Monday at 2:33 p.m. EDT.

"If we can go down the easy path, we are still on a track for Monday," he explained. "This initial round of troubleshooting still supports a launch on Monday."

If a Monday liftoff is not an option, then NASA will have until Wednesday to attempt launching Endeavour before having to stand down for an Atlas rocket launch of a military satellite.

"There's an Atlas that is scheduled for launch right now on the 6th, so our last launch attempt would be on Wednesday the 4th, in front of that Atlas launch," said mission management chair Mike Moses.

"If we had to go on the other side of [the Atlas], we'd come back probably around the 8th or 9th," said Moses. "Those aren't really good launch days from the [space] station's perspective."

A May 8 liftoff would couple Endeavour's departure from the International Space Station with Russia's Soyuz TMA-20 undocking and NASA would rather that those two events not occur on the same day.

"We are going to have to go do some homework and talk to the Russians about exactly the right way to deconflict that. But we are not going to do too much about that just yet cause we think we still have a shot in front of Atlas," said Moses.

Technicians began testing various systems in space shuttle Endeavour's aft section Saturday to determine what caused the issue with an auxiliary power unit (APU) heater that scrubbed Friday's STS-134 launch attempt.

The tests verified a fuse panel on Endeavour's flight deck was working, suggesting the likely problem was either with a thermostat or an avionics box called a load control assembly.

Initial reports indicated that the thermostat side that failed during Friday's launch countdown is still not working. More tests were needed to learn if the thermostat is receiving a current or whether the issue is being caused by another part of the system.

Mission managers are not expected to decide until they meet on Sunday morning, at the earliest, whether a launch attempt on Monday is possible.

The flight dynamics office at NASA's Johnson Space Center in Houston refined Saturday the target launch time for Monday to 2:44:00 p.m. EDT.

Space shuttle Endeavour will not launch on Monday, which had been its earliest possible liftoff date after Friday's scrub.

Technicians and engineers are narrowing in on the likely source of what caused heaters on a fuel line for Endeavour's Auxiliary Power Unit, No. 1 (APU-1) to fail Friday, scrubbing the first launch attempt for the STS-134 mission. But because of work associated with fixing the problem, launch teams will not be able to make a launch attempt tomorrow.

Teams will meet Monday and are expected to determine a new "no earlier than" next launch attempt for Endeavour at that time.

Testing of the APU systems associated with the heaters began Saturday afternoon and continued overnight. The results indicated the issue is with the Aft Load Control Assembly-2, a power control box, in Endeavour's aft compartment. Teams are working on plans to replace the box or any faulty associated hardware.

The launch team is currently backing out of countdown operations.

There still are numerous factors to be worked out, but just based on the amount of time needed to complete the repair, a next launch attempt will not happen before the end of the week, at the soonest.

Endeavour's six STS-134 astronauts are heading back to Johnson Space Center in Houston for a few days of additional training before they return for the next launch attempt.

"Things happen fast," wrote Endeavour's STS-134 pilot Greg H. Johnson on Twitter. "Be back in a few days."

Shuttle techs will spend the next several days removing and replacing a faulty fuse box in Endeavour's aft section and then will perform a lengthy retest of its replacement in their effort to bring back online the failed fuel line heaters that scrubbed Friday's launch attempt.

"The bottom line is the failure we have appears to be a power problem, which is in the LCA, the load control assembly, box which is basically a box of switches we use to control power feeds. That basically means the power isn't getting out to the heaters that weren't working on launch day," said mission management team chair Mike Moses on Sunday. "We need to go in and change out that box."


Credit: NASA/Kim Shiflett
Above: A technician crouches in Endeavour's aft section, where teams are preparing to remove and replace the aft load control assembly-2.

The LCA (or specifically, Aft Load Control Assembly-2, ALCA-2) controls more than just the heaters for Endeavour's auxiliary power units but also a long list of other orbiter systems, including the shuttle's main engines, its orbital maneuvering system and life support system. As a result, once replaced, the box and all its connections needs to be retested, a process that will tentatively take an additional two full days.

"Any time you break a connection to a box like this, you essentially invalidate all the testing we've done up to that point," explained shuttle launch director Mike Leinbach. "You could take the tact that saying 'Well, all you are doing is replacing the box, everything downstream of that box should be fine.' Well that's true, but our requirements and the way we do business is that whenever we break a connection, we go back and retest it. That's just the prudent thing to do."

Additionally, they won't know what exactly was wrong with the faulty box until it is removed and inspected, adding to the present uncertainty when Endeavour can launch.

"I am not going to tell you what the new launch date is because I have no idea," said Moses. "We have a lot more to evaluate."

"We can tell you pretty much that it's not going to be any earlier than [May 8]. That doesn't mean we're going to go launch on the 8th, that just means we know right now the 8th is our next available opening, so we'll start looking at that but we're really not even setting schedules today," he said.

"[So] don't take that as a launch date, just take that as a target for when we're ready to start talking again about a launch date," said Moses.

Space shuttle Endeavour will not be ready again to attempt launching the STS-134 mission, its final flight, until Tuesday, May 10, at the soonest.

NASA's shuttle and International Space Station (ISS) managers met on Monday and determined that May 10th is the earliest Endeavour could be ready based on preliminary schedules to replace a faulty Aft Load Control Assembly (ALCA) box in the orbiter's aft compartment.

The managers plan to reconvene on Friday, May 6, to determine a more definite launch date after the box is removed and replaced and the retest of its reconnected systems has been completed.

Shuttle program managers adjusted the date after further evaluating the schedules to change out the box and retest the nine systems associated with the controller. That work would be followed by the standard closeout of the aft compartment before proceeding into the launch countdown.

If a May 10th launch is approved, then the countdown would begin at the T-43 hour mark on Saturday, May 7, at about 9:30 a.m. EDT leading to a liftoff at 11:21 a.m. the following Tuesday.

Sunday night and on Monday, technicians on Launch Pad 39A conducted additional testing of systems associated with ALCA-2, including testing the box itself, which is expected to be removed late on Monday or early Tuesday morning and replaced with an existing spare.

Managers will continue to evaluate the repair process, and may make additional adjustments before scheduling Endeavour for its next attempt at launching the STS-134 mission to the International Space Station.

Technicians at the NASA Shuttle Logistic Depot in Cape Canaveral, Fla., continued testing Thursday a faulty power distribution box, called an Aft Load Control Assembly 2 (ALCA-2). It was removed on Tuesday morning from the aft compartment of space shuttle Endeavour on Pad 39A.

The tests are to try to determine what caused the power box to fail. Initial testing from Wednesday shows a driver (circuit) inside the ALCA shorted out. The ALCA-2 feeds power to heaters on the fuel line for Endeavour's Auxiliary Power Unit, No. 1 (APU-1). Initially, when the shorted out driver was replaced, the ALCA-2 appeared to work as designed.


The faulty Aft Load Control Assembly-2, uncovered for testing. Credit: NASA
Technicians are performing additional failure analysis on the driver in an attempt to confirm whether the short experienced by the system came from inside or outside the ALCA-2. Depending on the results, additional testing of the box may be required.

On Pad 39A, a new power box was installed into Endeavour Wednesday morning, and re-testing of the system is proceeding before Endeavour's next launch attempt is scheduled. Currently, the "no earlier than" launch date remains May 10.

The APU-1 fuel line heaters did not work April 29, prompting the scrub of Endeavour's first launch attempt for the STS-134 mission.

The faulty Aft Load Control Assembly-2, after being removed from Endeavour. Credit: NASA

NASA managers have retargeted space shuttle Endeavour's launch to no earlier than Monday, May 16, 19 years to the day after the orbiter's first mission, STS-49, came to an end.

Meeting Friday, the mission management team also extended the length of Endeavour's final mission, STS-134, from 14 to 16 days. If Endeavour launches on May 16, liftoff would be at 8:56 a.m. EDT.

Shuttle technicians are continuing work to resolve a problem in a heater circuit associated with Endeavour's hydraulic system that resulted in its first launch attempt being scrubbed. Technicians determined the failure was inside Aft Load Control Assembly-2 (ALCA-2), which is a switchbox in the orbiter's aft section, and possibly its associated electrical wiring.

Although the root cause of the failed fuel line heater in ALCA-2 has yet to be found, technicians are replacing hardware that could have caused the issue. The faulty box was changed out on Wednesday, and a test of nine shuttle systems powered by the new box is under way.

This weekend, technicians will install and test new wiring that bypasses the suspect switchbox's connection to the heaters. They will also run the heaters for up to 30 minutes to verify that they are working properly and will complete retesting of the other systems powered by ALCA-2.

The shuttle has three Auxiliary Power Units (APUs) that provide hydraulic power to steer the vehicle during ascent and entry. The hydrazine fuel lines on each APU have two heater circuits that prevent the fuel from freezing while the shuttle is in space.

NASA launch commit criteria and flight rules require all APUs and heater circuits to be operational for launch. On Endeavour's first launch attempt on April 29, one of the two heaters for APU-1 did not turn on. Engineers confirmed that the circuit in the original switchbox that directed power to the heaters was shorted out.

Launch attempts are available through May 26. May 21 is the only day a launch is not an option because it would lead to a May 23 docking with the International Space Station, the same day when three of the space station's Expedition 27 crew members undock and return home in their Soyuz spacecraft.

Managers reviewed the STS-134 timeline and determined the Endeavour crew can still complete all the mission objectives even with the departure of the three station crew members.

Space shuttle Endeavour on Launch Pad 39A awaits its final launch. Credit: NASA

NASA managers on Monday set the liftoff of space shuttle Endeavour for 8:56 a.m. EDT on Monday, May 16.

"We have extremely high-confidence that we have nothing in front of us to prevent us from launching on the 16th," said Mike Moses, chair of the mission management team, during a press conference on Monday.

Shuttle engineers have been unable as of yet to identify the root cause of the issue that scrubbed Endeavour's first launch attempt on April 29, but are confident that the workarounds they have put in place since then will correct for the problem.

"The failure on launch day was a failure of having power supplied to the heaters. The cause of that failure to supply power we're still not exactly sure of, and so we went through what we call our 'unexplained anomaly troubleshooting,' which basically [asks] when you do not have an exact root cause, what are you going to do next?" explained Moses.

A short in a heater circuit associated with Endeavour's hydraulic system resulted in the launch being postponed. Technicians determined the most likely failure was inside a switchbox in the shuttle's aft compartment and associated electrical wiring connecting the switchbox to the heaters. The heater circuits prevent freezing of the fuel lines providing hydraulic power to steer the vehicle during ascent and entry.

The faulty switchbox was replaced on May 4. Since Friday, technicians have installed and tested new wiring bypassing suspect electrical wiring and have confirmed that the heater system is working properly.

"We did various checks along the point but it culminated last night in our full up retest of this [hydraulic] system, where we basically operated the heaters on both an external power supply and then the new LCA-2 power supply, and we saw full functionality out of these five heaters as they were supposed to work. So we kind of end-to-end checked and wrung out the whole system," said Moses.

"Of all the components — the box, the power supply, the wiring to get you out to the heater, and then the heater itself — we have replaced everything except the heaters and we've wrung those out with at least five separate checks and full function afterward and now have extremely high confidence that the problem is no longer on the ship or in any of the electronics."

"We don't still exactly have root cause but we basically have really good comprehensive proof to show that we've managed to remove that failure," said Moses.

With the issue addressed, managers turned their attention to any impacts that the new mission schedule would have to the crew's timeline. While a May 16 launch will allow for the astronauts to complete the tasks set out for them, it will require they follow a slightly offset sleep schedule to their space station counterparts.

On May 23, a week into the STS-134 mission, Expedition 27 crewmates Dmitry Kondratiev, Paolo Nespoli, and Cady Coleman will climb onboard Soyuz TMA-20, undock from the station, and return to Earth.

"To get that lined up over Russian ground sites, if you look at the time that that undock has to occur, based on the time the shuttle crew needs to be on their sleep shift to set up for their undocking, that Soyuz undock will occur about an hour or so after the shuttle crew goes to sleep," said Moses.

"So we wanted to make sure we knew how to deconflict the two crews from each other," explained Moses. "We are effectively kind of setting up a second shift. We'll leave the station crew on the timeline they need to be on, the shuttle crew will be on the timeline that they need to be on and there will be a little disconnect between the two."

At the most, the shuttle crew will wake up four hours before their station crewmates.

To support a May 16 liftoff, NASA's launch team will start the countdown Friday, May 13 at 7 a.m. EDT.

Before then, STS-134 commander Mark Kelly and his five crewmates will return to Kennedy Space Center on Thursday at approximately 11 a.m.

The six astronauts who will fly STS-134, space shuttle Endeavour's final mission, returned to Florida on Thursday, hoping next Monday's (May 16) scheduled second-try at their launch is the trick.

"We are really excited to be here, excited to launch, hopefully on Monday if the weather holds," STS-134 mission commander Mark Kelly said after arriving at the Kennedy Space Center.

For Endeavour's pilot Greg "Box" Johnson, the arrival was also a birthday trip.

"I cannot think of a more perfect way to spend my birthday than to come here and fly over here with my crew and get ready to fly Endeavour next week." said Johnson.

Photo Gallery: Astronauts arrive for Endeavour's final flight


Kelly and Johnson, together with mission specialists Mike Fincke, Drew Feustel, Greg Chamitoff and European Space Agency astronaut Roberto Vittori, arrived at the Florida spaceport at 9:00 a.m. EDT, flying aboard a NASA Gulfstream jet.

They touched down on Kennedy's Shuttle Landing Facility, where Kelly is planning to return Endeavour for a last time from space.

The crew's family members plan to arrive later this week.

"Our families will be down later in the week. Normally we would have them come down typically on the same day we come down but all of our kids, all of them are in school," said Kelly. "So they will be down over the weekend, even though I'm sure they would rather be here in Florida now."

Kelly's wife, Congresswoman Gabrielle Giffords (D-Az) is expected to fly in separately. Giffords is recovering from a gunshot wound to her head after a Jan. 8 assassination attempt in Tucson, Ariz.

This marked the second arrival for the astronauts, who originally came to Florida on April 26. They returned home to Houston and NASA's Johnson Space Center on May 1 after Endeavour's first launch attempt on April 29 was scrubbed.

A short in a heater circuit associated with Endeavour's hydraulic system resulted in the launch being postponed. Technicians determined the most likely failure was inside a switchbox in the shuttle's aft compartment and associated electrical wiring connecting the switchbox to the heaters. The heater circuits prevent freezing of the fuel lines providing hydraulic power to steer the vehicle during ascent and entry.

The faulty switchbox was replaced on May 4. Technicians then installed and tested new wiring bypassing suspect electrical wiring and confirmed that the heater system was working properly.

"We really appreciate all the hard work by the team that has worked over the last couple of weeks to get shuttle Endeavour ready," said Kelly.

The doors to Endeavour's rear compartment were closed on Wednesday in preparation for the launch team picking up the countdown on Friday at 7 a.m. EDT (1100 GMT). Endeavour is scheduled to liftoff on Monday, May 16 at 8:56 a.m. EDT (1256 GMT) to the International Space Station.

The 16-day mission will deliver the Alpha Magnetic Spectrometer (AMS) — a state-of-the-art cosmic ray detector to examine fundamental issues about matter and the origin and the structure of the universe — and the ExPRESS Logistics Carrier 3 (ELC3) carrying spare station components.

"We feel that the AMS, along with the International Space Station, are great symbols of what we human beings can do when we work together constructively. Our crew is very proud to be part of this great endeavor," said Fincke.

Endeavour's final crew arrives in Florida for their May 16, 2011 scheduled launch. [Photo Gallery]

The countdown to Endeavour's final launch began at 7 a.m. EDT, picking up at the T-43 hour mark.

Launch teams are currently not working any issues that would delay the scheduled liftoff on Monday, May 16 at 8:56 a.m.

"We are back officially into the launch countdown," NASA Test Director Jeff Spaulding said. "We did just pick up the clock a few hours ago and we are counting and working on some of our avionics checkouts right at the beginning of the count as we normally do."

"Real pleased to be back here again to ready the vehicle for Monday's launch," said Spaulding.

Technicians at Pad 39A are preparing to load the power reactant storage device (PRSD) early morning on Saturday and Endeavour's external fuel tank with propellants beginning Sunday night.

The current weather forecast calls for a 70 percent chance of acceptable conditions at launch time. There may be some storms around the time that the launch pad's rotating service structure is to be rolled back, now scheduled to begin at noon on Sunday, but launch day is looking good for Kennedy and also for booster recovery and abort landing sites.

"By Monday, [the] front moves off to the east and we're actually going to clear out by launch time, the weather should all be off to the east and southeast, so we expect the conditions to improve," said shuttle weather officer Kathy Winters. "We are concerned a little bit about crosswinds at the Shuttle Landing Facility... but they look right now like they are going to be within limits."

The STS-134 crew woke at 1 a.m. to continue their launch preparations. Commander Mark Kelly and pilot Greg "Box" Johnson performed practice shuttle landings this morning piloting NASA's Shuttle Training Aircraft, a modified Gulfstream jet.

Two weeks after scrubbing space shuttle Endeavour's launch as a result of a failed hydraulic system heater, NASA's mission managers gave their "go" on Saturday for the orbiter's next liftoff attempt on Monday, May 16 at 8:56 a.m. EDT.

"We had a really good meeting today," said mission management team chair Michael Moses. "Unanimous consent... to press on with the launch countdown. Everything is in really great shape."

During the meeting, managers reviewed the heater problem, the work that was done to correct for it and the subsequent discovery of its root cause.

"We took our time to walk through the issue that caused the launch scrub last time, the APU-1 heater problem, to make sure we truly understood our resolution and our fix," said Moses.

Technicians determined the most likely failure was inside a switchbox in the shuttle's aft compartment and associated electrical wiring connecting the switchbox to the heaters. The heater circuits prevent freezing of the fuel lines providing hydraulic power to steer the shuttle during ascent and entry.

The faulty box was replaced and new wiring was installed bypassing the suspect electrical wiring. Testing confirmed that the heater system was working properly.

By testing the faulty box, engineers were able to locate the failure.

"The cause of that short actually occurred back in June, when we were in the orbiter processing facility doing some testing on the circuit. We either had come in contact with a connector on the thermostat that already had bare metal on it or the testing itself caused that inadvertent contact and created the bare metal short," explained Moses.

With that problem resolved, no other issues are being worked that would delay Endeavour's launch.

"The countdown for Endeavour's final mission is going extremely well," said launch director Mike Leinbach. "We got the cryogenic fuels on board for the fuel cell system today."

"Next major milestone is [the rotating service structure] retract tomorrow afternoon," said Leinbach, referring to Launch Pad 39A's movable gantry, which has protected and provided access to Endeavour. "And we'll take a good hard look at the weather before we do that."

There may be some storms around the time that the service structure is to be rolled back, scheduled to begin at noon on Sunday, but the forecast is getting better.

"The good news is by tomorrow the weather should all be pushing out [off the coast] gradually so by RSS retract time the weather is going to be improving," said shuttle weather officer Kathy Winters.

The forecast for Monday remains at 70 percent for acceptable conditions at launch time.

Space shuttle Endeavour stands poised for a Monday morning liftoff, its launch pad's shielding gantry having been retracted on Sunday.

Technicians began rolling the rotating service structure (RSS) away from Endeavour at 11:44 a.m. EDT. It was fully retracted 40 minutes later.

Photo Gallery: Endeavour revealed on launch pad for final flight
The rotating service structure enveloped the shuttle as it was readied for launch, protecting the orbiter and providing access to its payload bay.

The retraction cleared the way for Endeavour's external tank to be fueled for flight, scheduled to begin at 11:36 p.m.

Before then, technicians will configure the switches on Endeavour's flight deck this afternoon, while STS-134 commander Mark Kelly and his crew will begin their launch sleep schedule at 4 p.m.

The shuttle's electricity-producing fuel cells will be activated at 5:46 p.m. and mission managers will meet at 11 p.m. to give a "Go-No Go" to load the external tank with liquid hydrogen and liquid oxygen.

Shuttle Endeavour stands poised on its launch pad, its flag waving in the wind. [Photo Gallery]

Fueling of space shuttle Endeavour's external tank with 535,000 gallons of liquid oxygen and hydrogen is now underway.

Space shuttle mission managers met at 11:00 p.m. EDT on Sunday and gave a "go" to begin loading Endeavour's external tank with its super-cold cryogenic propellants. The operation began on time 36 minutes later and will take approximately three hours.

The first phase of the fueling is called "slow fill" to allow the pumps, lines and tank to condition themselves to the super cold chemicals. The liquid oxygen is at minus-297 degrees Fahrenheit and the liquid hydrogen is at minus 423 degrees F.

The weather forecast calls for a 70 percent chance of favorable weather for launch. The primary concern is for low cloud ceilings and crosswinds at the Kennedy Space Center's Shuttle Landing Facility.

Teams are not working any issues that would delay the launch, which is targeted for 8:56:28 a.m. EDT on Monday.

The STS-134 crew will wake up at midnight and, after sharing breakfast, will undergo pre-launch medical checks.

Endeavour's external fuel tank has now been filled with 535,000 gallons of liquid hydrogen and liquid oxygen.

"Fast fill" of the tank began at 12:25 a.m. EDT with liquid hydrogen and two minutes later for the liquid oxygen. Fueling reached "stable replenish" of both cryogenic propellants at 2:36 a.m. EDT. Liquid hydrogen (LH2) "topping" began at 1:39 a.m.

The three-hour tanking process, which began at 11:36 p.m. on Sunday, provides the fuel and oxidizer that Endeavour's three main engines need for the eight and a half minute climb into orbit, scheduled to begin with a liftoff at 8:56 a.m. EDT.

There are no technical issues being worked for launch.

Forecasters predict a 70 percent chance of favorable weather for launch. The only concerns are for low cloud ceilings and high crosswinds at the Kennedy Space Center's Shuttle Landing Facility.

The STS-134 astronauts left their crew quarters to depart for Launch Pad 39A at 5:11 a.m. EDT, riding the "AstroVan," a modified Airstream trailer.


Commander Mark Kelly, pilot Greg H. Johnson, and mission specialists Mike Fincke and European Space Agency astronaut Roberto Vittori are boarding Endeavour to take their seats on the flight deck.

Mission specialists Greg Chamitoff and Andrew Feustel will ride to space on the shuttle's middeck.

With all the astronauts onboard, the closeout crew will close Endeavour's hatch, pressurize the crew cabin, check for leaks, and then finish up their work inside the White Room prior to leaving the pad.

In the process of closing Endeavour's crew hatch for launch, the closeout crew reported seeing minor damage to one of the thermal protection tiles surrounding the orbiter's entranceway.


Members of the shuttle's close out crew retrieved a repair kit and patched the damaged tile by filling it with a slurry.

NASA stated the damage and subsequent repair is not an issue for flight.

Launch director Mike Leinbach has given Endeavour's crew a "go" after a final "go-no go" poll of the launch team to begin the STS-134 mission at 8:56:28 a.m. EDT.

During the last few minutes remaining in the countdown, the access arm on the rotating service structure will swing away from the shuttle and the vent hood, called the beanie cap, covering the point of the external tank will be lifted up and away.

A computer, called the ground launch sequencer, has taken over all the operations on Pad 39A and will be giving the commands that will launch Endeavour.