Astronauts Douglas Wheelock and Tracy Caldwell Dyson wrapped up a successful spacewalk Monday, installing a new ammonia pump to help flight controllers recover from a failure that shut down half the International Space Station's cooling systems.

Read our full story.

This was the 150th spacewalk devoted to station assembly and maintenance and the 14th so far this year. Total space station EVA time since construction began in 1998 stands at 944 hours and 24 minutes.

Today marked the sixth EVA in the career of Doug Wheelock and third for Tracy Caldwell Dyson. Wheelock has accumulated 43 hours and 30 minutes of spacewalking time on his previous excursions and 22 hours and 49 minutes for Caldwell Dyson.

EVA ENDS. Repressurization of the Quest airlock module began at 1:40 p.m. EDT, marking the official end of today's spacewalk. The EVA lasted 7 hours and 20 minutes. The three EVAs to remove and replace the failed pump module totaled 22 hours and 49 minutes.

The hatchway leading to space has been closed and locked. Standing by for airlock repressurization.

Doug has entered the airlock and closed the outer thermal cover.

Tracy has climbed back inside the airlock.

The decision has been made to skip the J612 extension cable routing outside the Unity node of the International Space Station for today. There isn't sufficient time left in the spacesuit consumables to go tackle the job in this EVA.

But this get-ahead task needs to be completed before the Permanent Multipurpose Module is installed on the Earth-facing docking port during the next shuttle flight in November.

The PMM will cover up an external power outlet. So this 10-foot extension cord ensures the socket can be utilized in future if needed. The installation will be accomplished during another spacewalk.

Doug has removed the foot restraint work platform from the robotic arm that he'd used on the EVAs. It has been stowed on the ESP 2 spare parts deck outside the airlock.

Flight controllers are assessing the clock and spacesuit consumables to decide whether the cable installation task can be accomplished before the EVA ends. That job is budgeted to last 45 minutes.

Archived video coverage from today's spacewalk for our Spaceflight Now+Plus subscribers:

Additional coverage for subscribers:
VIDEO: AMMONIA HOSES ARE ATTACHED TO THE PUMP PLAY
VIDEO: POWER AND DATA LINES BRING PUMP TO LIFE PLAY
VIDEO: REPLACEMENT PUMP SECURELY BOLTED DOWN PLAY
VIDEO: PUMP INSERTED INTO SPACE STATION SLOT PLAY
VIDEO: SPACEWALKER HAULS PUMP OVER TO TRUSS PLAY
VIDEO: RETRIEVING THE REPLACEMENT PUMP PLAY
VIDEO: SPACEWALK NO. 3 BEGINS PLAY
SUBSCRIBE NOW

The spacewalkers are moved the Crew Equipment Translation Aid (CETA) carts into the desired location along the truss and positioned them with the robot arm's mobile railcar.

Still working through the cleanup tasks out on the Starboard 1 truss as the EVA clock nears the 6-hour point.

If time permits after the lengthy cleanup activities are finished, plus repositioning the CETA crew carts on the station rails and removing the work platform from the robot arm, the spacewalking duo could install an external umbilical line that needs routing prior to the next space shuttle mission. The Discovery flight will be installing the new Permanent Multipurpose Module onto the Unity node.

Read our update story on the spacewalk's progress.

The spacewalkers are marching into the Starboard 1 truss cleanup chores following the ammonia pump removal and replacement over the past three EVAs, stowing tools and equipment that had been used in the work.

After tidying up some wire ties, Doug will drape a thermal blanket around the pump module connectors.

All of the ammonia valves are opened. No leakage seen.

Passing the 4-hour, 30-minute mark of the spacewalk.

The next step underway is opening the quick-disconnect valves on the M1, M2 and M3 lines.

And now the M1 hose has been mated to the new pump. All of the fluid lines are attached, joining the five electrical plugs.

Doug just attached the M2 connector, the second of three 1.5-inch ammonia umbilicals.

Ammonia has begun flowing to the new pump.

The M3 ammonia line has been fitted to the pump. This particular connector, as you may recall, was the troublesome hose that leaked during the first spacewalk when the astronauts were trying to unplug it from the failed pump.

The nitrogen has been vented and Mission Control says the system appears to be tight with no leaks. The "go" has been given to install the M3 ammonia hose now.

Doug has opened the valve on the M4 line to vent nitrogen from the new pump.

The first connection to the new pump was the quarter-inch diameter M4 line. The three 1.5-inch ammonia umbilicals will be hooked up shortly.

The M2 ammonia umbilical and the smaller M4 line have been disconnected from the temporary jumper box that was used to maintain system pressure during the pump replacement. They will be mated to the new pump shortly.

Read our update story on the new pump's installation.

Good power, pressure readings and driveshaft motion from the initial testing of the new pump!

Continuing to move through their checklist, the spacewalkers are getting started on the ammonia fluid lines.

This pump was delivered to the International Space Station by the shuttle Discovery's STS-121 mission four years ago. Spacewalkers Piers Sellers and Mike Fossum assisted in moving the device from the shuttle's payload bay to the station's external stowage deck. The EVA occurred on July 10, 2006.

The pump was prepositioned on the station as a spare, ready to replace an existing one whenever the time came. It was two weeks ago, on July 31, when the station suffered its first pump failure.

NASA has three additional spares at the station that were launched on STS-127 and STS-129.

The five power and data lines have been plugged to the module by Tracy.

Flight controllers in Houston will be testing the module's electrical aliveness and command a brief spinup of the pump's driveshaft.

The four bolts have been driven home, firmly securing the pump into its new location.

Tracy has three of connectors hooked up already.

First bolt has been torqued.

For the next phase of installation tasks, Doug will engage the four structural bolts and Tracy will attach the five electrical connectors.

The replacement pump module has arrived inside the International Space Station's Starboard 1 truss, ready to become an integral part of the outpost's thermal cooling system.

The spacewalkers are delicately sliding the pump into the truss.

Now Tracy is lending an extra set of eyes to help Doug get the new pump aligned into the Starboard 1 truss slot and insertion guide rails.

Tracy has made her way over to the Starboard 1 truss to inspect the worksite and ensure there are no wires or obstructions in the way.

These 780-pound pumps measure 69 inches by 50 inches by 36 inches.

With the pump module in his hands, Doug is being maneuvered by the space station's robot arm to the Starboard 1 truss for installation.

An hour and 40 minutes into the EVA, the new pump has been released from the space station's spare parts depot.

Doug has started the final unbolting to retrieve the replacement pump module.

Three of the four structural bolts have been disengaged from the pump by Doug. The last bolt will be removed when the spacewalkers are ready to lift the device off the deck.

Using his cordless power tool, Doug is now releasing the bolts that he broke torque on.

All the caps are off and the torque has been released on the four bolts a special tool.

As Tracy removes the various dust caps have been removed, enabling the station's umbilicals and wires to be attached to the pump later today, Doug is starting to break torque on the bolts that hold the pump module on the ESP 2 platform.

Doug has jumped aboard the space station's robotic arm where he will work to release and then transfer the new pump over to the Starboard 1 truss today. Tracy is removing dust caps from electrical and fluid umbilical ports on the new pump.

Both spacewalkers are headed to the External Stowage Platform No. 2, which is mounted right next to the airlock module. The spare ammonia pump to be installed into the space station has been resting in storage on that platform for four years. They'll spend the initial portion of the EVA getting the pump ready.

Both spacewalkers have emerged from the airlock. This is Doug's sixth EVA. In addition to the pair of Expedition 24 spacewalks, he did three excursions to install the Harmony module and relocate the P6 solar array power tower on the STS-120 shuttle flight to the station in 2007. And this is Tracy's third spacewalk, all on this current Expedition mission.

Read our full story on the spacewalk's start.

This is the 150th spacewalk devoted to station assembly and maintenance and the 14th so far this year. Total space station EVA time since construction began in 1998 stood at 937 hours and 4 minutes going into today's spacewalk.

EVA BEGINS. The spacewalkers switched their suits to internal battery power at 6:20 a.m. EDT, marking the official start time for today's EVA by Doug Wheelock and Tracy Caldwell Dyson. It's the third spacewalk to replace a failed cooling pump outside the International Space Station.

The depressurization just finished and the Quest airlock's outer hatch leading to space is now open.

Running well ahead of schedule this morning, the airlock depressurization is underway in advance of beginning the spacewalk a short time from now.

Astronauts Douglas Wheelock and Tracy Caldwell Dyson are gearing up for a third spacewalk Monday to install a replacement ammonia pump that should allow flight controllers to reactivate a critical space station coolant loop that broke down two weeks ago.

Read our full story.

Spacewalker Douglas Wheelock, given permission to "shake violently" a stuck ammonia connector, used a healthy dose of elbow grease to finally get a troublesome coolant system quick-disconnect fitting to release Wednesday. With the fourth and final ammonia line finally free, Wheelock and crewmate Tracy Caldwell Dyson removed a faulty coolant pump, clearing the way for installation of a replacement during a third spacewalk Monday.

A fourth spacewalk likely will be required later to wrap up loose ends and move the old pump to a permanent stowage location.

Read our full story.

Officials have decided to delay the next spacewalk until Monday, giving the astronauts and ground teams a bit of rest. That EVA, originally targeted for Sunday, will install the new pump.

Archived video coverage from today's spacewalk for our Spaceflight Now+Plus subscribers:

Additional coverage for subscribers:
VIDEO: FAILED COOLANT PUMP REMOVED FROM THE STATION PLAY
VIDEO: ROBOT ARM MANEUVERS SPACEWALKER AROUND PLAY
VIDEO: POWER AND DATA LINES UNPLUGGED PLAY
VIDEO: TROUBLESOME UMBILICAL DETACHED PLAY
VIDEO: NO REPEAT OF AMMONIA LEAK PLAY
SUBSCRIBE NOW

Continuing with the ammonia checks, Doug used an instrument to measure the atmosphere in the airlock. No problems were detected and repressurization is proceeding.

This was the 149th spacewalk devoted to station assembly and maintenance and the 13th so far this year. Total space station EVA time since construction began in 1998 stands at 937 hours and 4 minutes.

Today marked the fifth EVA in the career of Doug Wheelock and second for Tracy Caldwell Dyson. Wheelock has accumulated 36 hours and 10 minutes of spacewalking time on his previous excursions and 15 hours and 29 minutes for Caldwell Dyson.

EVA ENDS. Repressurization of the Quest airlock module began at 3:53 p.m. EDT, marking the official end of today's spacewalk. The EVA lasted 7 hours and 26 minutes.

Standing by for airlock repressurization.

The airlock hatch has been closed and locked. Doug is digging out the ammonia detection kit purely as a precaution to make sure the spacewalkers don't track in any contamination.

Doug and Tracy are back in the airlock. But they'll need to complete some precautionary ammonia decontamination steps before returning inside the cabin.

Now passing seven hours into this EVA.

Work on the replacement pump is complete for today. All three electrical connectors were unplugged as part of the prep work for the next EVA. The astronauts are proceeding into cleanup tasks and tool inventories before completing this excursion.

Tracy has removed those strips of tape on the new pump module's handrails. Both spacewalkers will work together now to disconnect the three electrical umbilicals. However, there won't be enough time to deal with the bolts today.

Among the possible activities that could be accomplished on the new pump today, Tracy and Doug can remove protective tape, demate electrical umbilicals and loosen bolts. All of that is meant to speed up the next's spacewalk timeline.

Now at the six-hour mark of the EVA. Mission Control opted to use the remaining time available in spacesuit consumables to tackle some get-ahead tasks with the new pump.

Doug has exited the robotic arm's work platform, completing his ride for this spacewalk. He'll soon join Tracy at ESP-2 where the new pump is being prepped for Sunday's installation.

Read our update story on the old pump's removal.

The old pump has been deposited on the mobile transporter's hand-like anchor where it will remain at least a few days. This gets the pump out of the astronauts' way when they relocate the replacement from the spares deck to the station truss on Sunday.

Doug has the grapple fixture on the pump lined up with the receptacle known as POA on the Canadian-made Mobile Base System. POA is the Payload Orbital replacement unit Accommodation.

With the pump module in his hands, Doug is being maneuvered by the space station's robot arm to the mobile transporter parked on the rail tracks.

While Doug puts the old pump in its temporary home, Tracy is moving to the External Stowage Platform No. 2 to start working on the replacement pump that will be the focus of attention on Sunday's spacewalk.

Some 11 days after the ammonia pump module inside the Starboard 1 truss failed, spacewalkers Doug Wheelock and Tracy Caldwell Dyson have pulled the 780-pound device out of the International Space Station.

The spacewalkers have attached the handling device onto the old pump. This fixture will enable the pump module to be stowed on the station's mobile transporter until a future spacewalk.

Tracy and Doug continue efforts to get the grapple bar aligned and snapped on the pump module. The pump is about half-way out of the truss.

Doug has gotten the final bolt undone after applying extra effort. The pump is ready to depart the International Space Station's Starboard 1 truss -- a girder that is part of the outpost's backbone structure.

The coolant pump is original equipment launched within the truss on the STS-112 shuttle mission in October 2002 and then fully activated during STS-116 in December 2006.

The pump experienced some sort of electrical short on July 31, knocking out half of the space station's thermal cooling system. It's being replaced by one of four available spares prepositioned aboard the station.

Read our update story on the pump removal preps.

Now back at the Starboard 1 truss worksite, Doug is using the cordless power tool to release that last bolt. Next, he will hand the grapple bar to Tracy for installation onto the pump and then they'll then work in tandem to secure the two pieces of hardware together.

Mission Control says the spacewalkers are running about 30 minutes ahead of the EVA timeline.

Three of the four structural bolts have been disengaged from the pump by Tracy. The last bolt will be removed when the spacewalkers are ready.

Tracy has started the final unbolting of the failed pump module.

The station's robotic arm has flown Doug over to the nearby External Stowage Platform No. 2 to retrieve the so-called Adjustable Grapple Bar that will used in handling the pump after removal.

Passing the three-hour mark into today's spacewalk. To recap, the ammonia hose that created trouble on Saturday did not leak today and the astronauts were able to disconnect it from the failed pump module without performing a lengthy venting procedure. They've since loosened the bolts holding the pump in place and removed the electrical cables.

Still to come for Tracy is fully releasing the bolts. And Doug will retrieve a grapple device to be mounted onto the pump, enabling an external space station bracket to grip the pump after removal.

And success reported by Tracy as she gets the fifth and final of the power lines unhooked from the pump module.

Doug has completed the torque breaking on the four bolts. Tracy will finish releasing them later.

Tracy is having some difficulty getting the last couple of cables released from the pump.

It's time for Doug to start breaking torque on the bolts that hold the pump module inside the Starboard 1 truss of the International Space Station. Meanwhile, Tracy continues working on the power and data cable removal.

A spacesuit inspection by the spacewalkers didn't reveal any ammonia contamination directly on the fabric.

Astronaut Doug Wheelock, applying brute force to shake a balky connector loose, successfully demated a final ammonia line from a failed coolant pump module today, setting the stage for the pump's removal and eventual replacement.

Read our full story.

Tracy has begun unplugging the five electrical and data wires from the failed pump module.

Next up, Doug will close a valve on another ammonia hose -- the M2 connector -- that was removed from the pump and fitted to a jumper box Saturday. This will enable ground controllers to prep the line for its eventual mating to the new replacement pump.

Brute strength has freed the M3 connector at last. The spacewalkers have now progressed beyond Saturday's troubles, successfully getting the final ammonia hose disconnected from the failed pump module. Still left to remove are power cables and the bolts holding the pump in place.

The connector isn't coming off easily. There could be ice inside, so Doug has permission to try shaking the connector.

The "go" has been given to demate the M3 connector from the pump. This is the point where the station repair work was halted Saturday because of the connector's leak. But it's not leaking today.

Now under discussion is whether the spacewalkers need to do any venting of residual ammonia from the system before the pump is removed.

Engineers in Mission Control have determined it is no longer necessary for Tracy to close that upstream connector after all. With no significant leakage around the pump's ammonia hose experienced today, the spacewalkers will alter the workaround plan.

The M3 connector bail has been opened by Doug to see if the leak reappears. A few snowflakes of ammonia can be seen in his helmet-mounted camera. But it's nothing like the notable leak rate that happened Saturday.

Doug has arrived at the fail pump module where that troublesome M3 umbilical remains attached. An ammonia leak from that connector prevented the spacewalkers from demating the hose and removing the pump as planned on Saturday.

Some struggles getting this quick-disconnector closed between the Starboard 1 and Starboard 3 trusses.

Tracy is performing her task to close the cooling fluid line to the DC-to-DC Converter Unit cold plate in the interface between the S1 and S3 trusses. Meanwhile, Doug is boarding the station's arm now.

Now one hour into the EVA and the astronauts are reported by Mission Control to be right on the prescribed timeline.

Doug's initial activities include fetching a bag containing the vent tool extender and then jumping aboard the space station's robotic arm work platform. Tracy will stow equipment bags and then move one of the CETA carts on the station rail tracks from the S1 to the S3 truss and gain access to a quarter-inch umbilical connector for closing. That's part of the plan to isolate the cooling line for venting residual ammonia today.

Both spacewalkers have emerged from the airlock. This is Doug's fifth EVA. In addition to Saturday's spacewalk, he did three excursions to install the Harmony module and relocate the P6 solar array power tower on the STS-120 shuttle flight to the station in 2007. And this is Tracy's second spacewalk.

Read our full story on the spacewalk's start.

This is the 149th spacewalk devoted to station assembly and maintenance and the 13th so far this year. Total space station EVA time since construction began in 1998 stood at 929 hours and 38 minutes going into today's spacewalk.

EVA BEGINS. The spacewalkers switched their suits to internal battery power at 8:27 a.m. EDT, marking the official start time for today's EVA by Doug Wheelock and Tracy Caldwell Dyson. It's the second of three spacewalks to replace a failed cooling pump outside the International Space Station.

The depressurization just finished and the Quest airlock's outer hatch leading to space is now open.

Depressurization is pausing at 5.0 psi for planned leak and spacesuit checks.

Airlock depressurization is in work.

Doug and Tracy have moved into the section of the airlock that will be depressurized shortly. Their crewmates assisting in today's spacewalk preps have closed the inner hatch.

Final steps in the suitup process are being completed now. The spacewalkers are being outfitted with the SAFER backpacks that would enable an untethered astronaut to fly back to the station.

The Expedition 24 astronauts are getting ready to head outside for the second in a series of repair spacewalks to fix the International Space Station's broken ammonia coolant pump. Today's EVA is scheduled to start around 7:55 a.m. EDT, but preps in the airlock are running about 30 minutes behind schedule at the moment.

NASA managers Tuesday approved plans for a second spacewalk Wednesday to deal with an unexpected ammonia leak that blocked removal of a faulty coolant system pump aboard the International Space Station during a spacewalk Saturday. If the plan works, the old pump will be removed and a new pump installed and hooked up during a third spacewalk Sunday.

Astronauts Douglas Wheelock and Tracy Caldwell Dyson planned to spend the night in the station's Quest airlock module at a reduced pressure of 10.2 pounds per square inch to help purge nitrogen from their bloodstreams before the planned 6:55 a.m. EDT start of the six-hour spacewalk.

NASA managers decided late Tuesday, however, to delay the start of the EVA by one hour, letting the astronauts get a bit more sleep and giving engineers more time to fine-tune robotics procedures. Under the revised timeline, Wheelock and Caldwell Dyson will switch their spacesuits to battery power at 7:55 a.m. to officially kick off the excursion.

Read our full story.

Flight controllers are revising plans for a second spacewalk Wednesday to replace a coolant pump aboard the International Space Station, adding work to isolate an ammonia leak that should clear the way for installation of a new pump during a third spacewalk Sunday.

Read our full story.

An unexpected ammonia leak and a jammed quick-disconnect fitting disrupted an eight-hour three-minute spacewalk Saturday to replace a faulty coolant system pump on the International Space Station. Assuming engineers can come up with a quick fix for the leak, the astronauts may attempt to replace the pump during a second spacewalk Wednesday.

Read our full story.

Archived video coverage from today's spacewalk for our Spaceflight Now+Plus subscribers:

Additional coverage for subscribers:
VIDEO: PROBLEM CONNECTOR FINALLY FREED BUT THEN LEAKS PLAY
VIDEO: LEFT-HAND AMMONIA UMBILICAL COMES OFF WITH EASE PLAY
VIDEO: SKIPPING OVER PROBLEM CONNECTOR TO DO ANOTHER PLAY
VIDEO: TROUBLES AND ADVICE FOR STUCK AMMONIA LINE PLAY
VIDEO: PUMP'S SMALL COOLING LINE UNPLUGGED PLAY
VIDEO: SPACEWALKER DOUG WHEELOCK WORKS ON OLD PUMP PLAY
VIDEO: BRIEFING ON PUMP FAILURE AND SPACEWALKS PLAY
SUBSCRIBE NOW

And with good news from the contamination checks, the crew is pressing ahead with airlock repressurization.

Continuing with the ammonia checks, Doug is using an instrument now to check for any remaining contamination.

This was the 148th spacewalk devoted to station assembly and maintenance and the 12th so far this year. Total space station EVA time since construction began in 1998 stands at 929 hours and 38 minutes.

This was the fourth EVA in the career of Doug Wheelock and first for Tracy Caldwell Dyson. Wheelock has accumulated 28 hours and 44 minutes of spacewalking time on his previous excursions.

EVA ENDS. Repressurization of the Quest airlock module began at 3:22 p.m. EDT, marking the official end of today's spacewalk. The EVA lasted 8 hours and 3 minutes.

The next spacewalk to continue with the ammonia pump removal and replacement is scheduled for Wednesday.

Now passing the 8-hour mark of the EVA.

The "go" has been given from Houston to close the hatch after the spacewalkers' decontamination procedures in the open airlock.

Doug and Tracy are back inside the airlock. But they'll need to complete some ammonia decontamination steps just in case.

To recap, the spacewalkers began the EVA at 7:19 a.m. EDT and immediately got busy with the worksite setup activities. They removed hose clamps and then started disconnecting umbilicals from the broken space station coolant pump. However, difficulty unplugging one of the ammonia lines consumed a couple of hours and a leak seen from that connector once it finally cooperated prevented the astronauts from progressing any further in their work.

Original plans had called for the pump's removal and installation of the new pump today, with a second spacewalk on Wednesday to finish hooking up the replacement and stowing the old one.

That next EVA will resume where today's work stopped. Whether a third spacewalk will be needed is not yet clear.

No visual indications of ammonia on the suits, Doug says. They'll be ingressing the airlock now.

The spacewalkers are inspecting their suits to look for any signs of ammonia contamination such as waxy patterns blotching the fabric. Sublimation is the best method to ridding the suits of ammonia, which can be accomplished by the crew members "baking out" in sunlight.

Read our full story on the ammonia leak that prevented the failed pump from being removed today.

The spacewalkers are moving into cleanup chores to stow tools. They'll also be inspecting their suits and performing the precautionary decontamination steps to ensure they don't track ammonia inside the station.

With time running out in this EVA, the astronauts and ground controllers are working to get the umbilicals secured and shaded. Removal of the old pump originally planned for today won't be possible.

The crew believes the leak has stopped.

Doug is reinstalling the so-called spool positioning device around the ammonia hose connector. He'd taken the SPD off earlier in the EVA in preparation for demating the line. The positioning devices are used on quick-disconnects throughout the station.

Officials had hoped this was just some residual ammonia in the connector. But the concern blossomed into worries about losing too much coolant from either the hose or the pump itself as the leak continued.

Engineers are looking at options given the continuing leak of ammonia from the gap between the pump and unlocked connector.

A fair amount of ammonia is leaking from the connector, which remains freed but still seated to the pump.

He got it! After a couple hours of work, Doug has gotten all ammonia umbilicals freed from the failed pump module on the Starboard 1 truss. He had experienced significant troubles with one of the lines in particular. But success at last with muscle power and a makeshift hammer, prompting applause in Houston.

Ground controllers have seen a slight drop in line pressure for the troublesome M3 connector. So Doug will give another try at demating that umbilical.

The second of three 1.5-inch ammonia umbilicals has been disconnected from the old pump module by spacewalker Doug Wheelock. The M1 connector, positioned on the left-side of Doug's workspace, was released with relative ease.

Next up -- the spacewalkers will try removing the M1 ammonia line from the pump. That's the fourth of the four hoses and the one not attempted yet.

The jumper box has been configured with one ammonia umbilical and a cooling line. They spacewalkers will slip a thermal cover around the box to protect the unit for the next few days.

The umbilical removed a few minutes ago -- called M2 -- has been seated and mated to the vent box that the spacewalkers installed at the worksite earlier in the EVA. And the first cable disconnected from the pump -- the M4 cooling line -- has been attached to the box as well. This is a jumper system meant to keep the ammonia pressure stable during the pump replacement.

The troublesome M3 umbilical and the not-yet-touched M1 connector remain secured to the pump.

Success with the other umbilical line. Doug has demated the middle ammonia line of three that run from the station to the broken pump. He skipped over to this line after being unable to get the previous umbilical to budge.

The EVA has passed the five-hour mark with only two hours left. Mission Control is assessing what more can be accomplished in time remaining.

The team is moving to Plan B. Doug is going to try removing one of the other umbilicals and attach it to the jumper vent box in hopes that decreases the overall pressure in the system.

Flight controllers aren't sure what is causing all this trouble getting the connector unhooked. Repeated efforts by the spacewalkers haven't worked.

Astronaut Dave Wolf, who installed on this Starboard 1 truss during the STS-112 mission in 2002, has joined the ground team in Mission Control. He's radioing some tips and advice to spacewalker Doug Wheelock on handling these ammonia connectors.

Read our full story on the spacewalk's ongoing progress and the umbilical troubles.

The EVA has fallen well behind the timeline. And since some ammonia snowflakes were seen, the spacewalkers will need to reserve time for decontamination procedures.

Doug is going to take a break while the space station heads into an orbital night pass. Mission Control is hoping the cold darkness will ease some of the pressure in the umbilical line.

Thus far, the spacewalkers have removed a half-inch-diameter cooling line from the pump but then ran into troubles getting the first of three 1.5-inch ammonia umbilicals released.

Still no luck trying to open the connector.

Doug is really struggling to get the second of four umbilicals unhooked from the pump.

A few tiny snowflakes can be seen flying away as Doug works to remove the second umbilical. Some trace amounts of ammonia escaping from the lines isn't a surprise.

Unplugging the old pump module has begun.

Robot arm operator Shannon Walker is inching Doug into the best orientation to reach the pump umbilicals for disconnecting them. And Tracy is getting tethers and equipment prepositioned to restrain the umbilicals once removed.

Now at the 2-hour, 50-minute mark of the EVA, the spacewalkers are in place to install a temporary box to serve as an umbilical vent. This will keep the ammonia system pressure stable while the pump is removed and later replaced.

Doug has gotten himself perched on the station's robot arm, which is being operated from inside the Destiny laboratory by fellow astronaut Shannon Walker.

And Tracy has moved one of the CETA cars along the truss rail tracks to help the spacewalkers today. A foot platform protruding from the cart will give Tracy a secure place to work during the pump replacement.

The next block of time for Doug is prepping the space station's robotic arm to install a foot platform and then hopping aboard. The arm will be maneuvering the spacewalker during the upcoming pump removal.

After Tracy finishes the CETA cart preps, she will install a jumper vent on the pump module.

Doug has completed removing the spool positioning devices, SPDs, on the ammonia line connectors at the pump.

Mission Control says Doug is running a bit ahead of his timeline and Tracy is a few minutes behind.

Tracy is getting a foot platform configured for later use during the pump removal and replacement.

The three hose positioning clamps have been released by Doug in advance of the ammonia lines being disconnected from the pump.

After performing an inspection of the failed pump module and the associated electrical connectors, Doug is proceeding onward to the next step to start releasing clamps that hold the ammonia hoses. He didn't see any discoloration or obvious signs of distress during the external examination of the pump.

Doug is moving into the next phase of his spacewalk timeline -- initial preps for removing the failed pump. Meanwhile, Tracy's upcoming tasks include readying one of the crew equipment "CETA" carts for use today.

The crew is in the "setup" portion of the spacewalk, as the astronauts gather the equipment they will need and move out to the Starboard 1 truss worksite.

This is the 148th spacewalk devoted to station assembly and maintenance and the 12th so far this year. Total space station EVA time since construction began in 1998 stood at 921 hours and 35 minutes going into today's spacewalk.

Both spacewalkers have emerged from the airlock. This is Doug's fourth EVA. He accumulated 20 hours and 41 minutes of spacewalking time during three excursions to install the Harmony module and relocate the P6 solar array power tower on the STS-120 shuttle flight to the station in 2007. And this is Tracy's first spacewalk.

Read our full story on the spacewalk's start.

This is the first American-led spacewalk to occur at the space station without a space shuttle present since January 2008.

EVA BEGINS. The spacewalkers switched their suits to internal battery power at 7:19 a.m. EDT, marking the official start time for today's EVA by Doug Wheelock and Tracy Caldwell Dyson. This is the first of two spacewalks to replace a failed cooling pump outside the International Space Station.

The depressurization has been completed and the Quest airlock's outer hatch leading to space is now open.

Airlock depressurization has commenced.

Doug Wheelock and Tracy Caldwell Dyson have moved into the section of the airlock that will be depressurized shortly.

Final steps in the suitup process are being completed now. The spacewalkers are being outfitted with the SAFER backpacks that would enable an untethered astronaut to fly back to the station.

Mission Control projects the spacewalk will begin around 7:30 a.m. EDT or so. Preps have fallen a bit behind schedule this morning.

With help from their fellow crewmates, Doug Wheelock and Tracy Caldwell Dyson are getting suited up inside the Quest module for today's spacewalk.

The International Space Station crew just started the spacewalk preparation timeline for today's EVA. They were awakened about 90 minutes ago.

Astronauts Douglas Wheelock and Tracy Caldwell Dyson made final preparations Friday for a planned spacewalk Saturday to replace a faulty ammonia pump module in the International Space Station's external cooling system.

Wheelock and Caldwell Dyson planned to shut themselves in the Quest airlock module around 4 p.m., lowering the pressure to 10.2 pounds per square inch overnight to help purge nitrogen from their bloodstreams. Crew sleep was scheduled to begin at 5:30 p.m. with wakeup at 2 a.m. Saturday.

"I just want to let everyone know we have unanimous concurrence on a 'go' for EVA tomorrow," Kathy Bolt radioed the crew from mission control at the Johnson Space Center in Houston.

"And you've got a 'go' from the crew," Wheelock replied. "Thanks, we're very excited, Kathy, that's great news. We're ready to perform."

Read our full story.

NASA managers, assessing timelines for two planned spacewalks to replace a faulty ammonia coolant pump aboard the International Space Station, decided Thursday to delay the first of two repair spacewalks, from Friday to Saturday, to give planners more time to fine-tune the required procedures.

The second spacewalk, which had been tentatively targeted for Monday, will slip an additional two days to Wednesday. Both spacewalks will still begin around 6:55 a.m. EDT.

Read our full story.

NASA astronauts and engineers are refining plans for two spacewalks by astronauts Douglas Wheelock and Tracy Caldwell Dyson to replace a large ammonia pump module that shut down Saturday, knocking out one of the International Space Station's two cooling loops.

The astronauts hope to carry out the first spacewalk Friday morning, starting at 6:55 a.m. EDT, and a second excursion Monday to finish the job, one of the so-called "big 14" on a list of critical components that require spacewalk repair if problems crop up.

NASA managers initially targeted Thursday for the first spacewalk and Sunday for the second, but decided late Monday they needed more time to review procedures.

Read our full story.

Trouble with one of the International Space Station's external coolant loops, used to dissipate the heat generated by the lab's electronics systems, triggered an extensive powerdown late Saturday. NASA managers met Sunday and gave preliminary approval to a difficult two-spacewalk repair job, starting as early as Thursday, to restore the critical system to normal operation.

It is not yet clear what went wrong, but the ammonia pump module that is part of coolant loop A, mounted on the right side of the station's main power truss, failed around 8 p.m. EDT Saturday. A problem somewhere in the system caused a circuit breaker to trip, setting off multiple alarms and waking the crew.

With half the station's cooling gone, flight controllers were forced to shut down two of the station's four U.S. control moment gyroscopes, used to help maintain the lab's orientation in space, one communications channel, several solar power current converters and a variety of computer control boxes known as multiplexer-demultiplexers.

Read our full story.