posted 07-31-2010 11:10 PM               

As a result, several systems have had to be powered down and crew members, kept awake beyond their scheduled sleep period, needed to move frozen science results between MELFI (Minus Eighty-Degree Laboratory Freezer for ISS) cold storage facilities.

Status update:

One of two ammonia-fed cooling loops on the International Space Station shut down at 7 p.m. CDT on Saturday after a power spike was detected on the loop's Pump Module.

The Pump Module, which enables ammonia to flow through the loops, remains down, and with the loss of the cooling loop, several systems were forced to shut down, including two of the station's four Control Moment Gyroscopes (CMGs), one of two redundant strings of S-band communication, one of two redundant strings of the station's Global Positioning System (GPS), several DC to DC current converters in Tranquility Node 3 and a set of components that route commands to station equipment.

When the cooling loop went down, the crew was awakened by caution and warning alarms and began powering down equipment to balance the cooling loads that were transferred to Cooling Loop B, which is active and in good condition.

Tracy Caldwell Dyson, joined first by Doug Wheelock and later by Shannon Walker, remained awake into the early hours on Sunday to work troubleshooting procedures while the rest of the crew slept. The crew is in no danger, but will need to work additional troubleshooting on Sunday to keep the station in a stable configuration, including the installation of a jumper cable to maintain proper cooling to the Zarya functional cargo block (FGB).

The station is equipped with at least two spare pump modules that are housed on external platforms on the station's truss should a replacement of the Loop A Pump Module be required.

posted 08-01-2010 09:39 AM               

Repair spacewalks being discussed

One of two ammonia-fed cooling loops on the International Space Station remains shut down after an attempt to restart its pump module failed early Sunday morning.

The pump module's circuit breaker initially tripped Saturday night, but had successfully been reset overnight. It tripped again Sunday when the pump module was restarted.

Teams in Mission Control in Houston are reviewing data to determine whether to attempt another restart and are discussing options to perform two spacewalks later this week to replace the pump module. Two spare pump modules are housed on platforms on the outside of the station.

The space station crew is in no danger and all systems are stable, though operating in a "single string" mode without redundancy.

Two of the four main buss switching units are cooled by the failed loop. The units switch and route power to various station systems. Mission Control is evaluating whether the heat generation may be low enough to not require powering off these switching units.

With the loss of the cooling loop, several systems have been and remain shut down, including one of two S-band communication circuits, one Global Positioning System (GPS) circuit, several DC to DC current converters in Tranquility Node 3 and a set of components that route commands to station equipment.

Though two of the four station Control Moment Gyroscopes (CMGs) initially were turned off, the crew installed a jumper cable to regain a third gyro. The gyroscopes maintain the station's attitude in space.

posted 08-01-2010 11:30 AM               

Thermal Loop A ETCS Pump Shutdown

After losing one cooling system pump (of two), ISS is currently stable, although thermally on single string (no redundancy). Last night (at 7:48pm EDT) RPC1 (Remote Power Controller 1) in RPCM (RPC Module) S11A_D tripped open, powering off the Loop A ammonia pump of the S1 ETCS (External Thermal Control System), resulting in the loss of one half of the cooling to ISS.

This required a number of powerdowns (i.e., turning off selected systems for thermal protection), including redundant power to four CQs (Crew Quarters), three in Node-2, one in Kibo JPM, with both fans in each CQ remaining functional but zero fault-tolerant (crew is still Go for CQ use). Due to loss of heater power, MBS (Mobile Base System), SSRMS (Space Station Remote Manipulator System), and SPDM (Special Purpose Dexterous Manipulator) are currently zero fault-tolerant.

T2/COLBERT treadmill alignment guides are installed and should remain installed during crew sleep periods to protect against LOAC (Loss of Attitude Control) while attitude control is degraded; the latter also renders use of unisolated exercise (Russian Velo) No-Go to reduce momentum perturbations; all other exercise is currently permitted.

S-band is operating nominally (on String 2, with String 1 in hot backup); Ku-band is operating nominally; audio subsystem is operating nominally (Node-2, COL and JPM are zero fault tolerant for C&W/Caution and Warning annunciation and voice comm); no video from Node-2, Node-3, COL and JPM modules; HCOR (high-rate communications outage recorder) is operating nominally; Russian UHF: no issues, both radios are deactivated.

Possible causes of the RPC trip include an electrical fault in the cabling between the RPCM and ammonia pump, electrical fault in the EMI (electromagnetic interference) filter, electrical fault in the pump control board, or an electrical fault in the pump.

The RPC tripped because of an overcurrent; this points toward hardware components that do not have internal current limiting, such as the EMI filter, pump control board, and the pump itself (they get their current limiting function from the RPCM). A mechanical failure of the pump is considered unlikely.

After a pump restart attempt this morning, RPC1 tripped again. There will be more attempts.

There are two spare pumps on orbit. It will take two EVAs to remove and replace the pump. ISS is stable, but single string, and it is desired to get Loop A back as soon as possible.

The teams are looking at replacing the Thursday (8/5) spacewalk with the first of two EVAs, followed by the second EVA on 8/7 (Saturday). The spacewalks need special planning since the system is in a reduced power configuration.

posted 08-01-2010 11:50 AM               

Station in stable configuration after loss of cooling loop

Mission Operations personnel and ISS Program management met Sunday morning to discuss the status of the loss of cooling loop A Saturday night on the International Space Station when a circuit breaker tripped just before 7 p.m. Central time that resulted in the failure of the Pump Module for loop A that feeds ammonia to maintain the proper cooling for systems and avionics.

An attempt overnight Sunday to close the circuit breaker and restart the Pump Module was not successful.

The station is in a stable configuration with most systems receiving cooling and many systems operating with redundancy following the installation of jumper cables from the Destiny Lab's power system overnight. The crew is not in any danger and is monitoring systems and relaxing on an otherwise off duty day.

One of two Control Moment Gyroscopes (CMG 1) that was taken off line Saturday night was spun up once again this morning, enabling the station to operate with three of four CMGs to electrically control the orientation of the outpost. Temperatures on the Main Bus Switching Units, which route power to various systems, are a little higher than normal, but well within normal parameters and are stable.

The flight control and management teams approved a preliminary plan to replace a planned spacewalk Thursday by Expedition 24 crew members Doug Wheelock and Tracy Caldwell Dyson with at least two spacewalks to swap out the failed Pump Module that resides on the station's S1 truss. There are two spare Pump Modules on stowage platforms on the station's truss. The replacement module under consideration for replacement resides on External Stowage Platform 2, which is adjacent to the Quest airlock. The crew is being informed that replanning for alternate spacewalk activity is underway.

Although a final decision on a new spacewalk plan is still pending engineering and timeline analysis, the most likely scenario would call for an initial spacewalk no earlier than Thursday by Wheelock and Caldwell Dyson to replace the Pump Module and structurally bolt it into place on the S1 truss, with an additional spacewalk by the duo two or three days later to mate fluid and electrical connections.

To protect for spacewalk activity later this week, the flight control team plans to vent residual ammonia in the lines between the Ammonia Tank Assembly on the S1 truss and the failed Pump Module Tuesday to prepare for the module's eventual replacement.

The tasks originally planned for Thursday's previously scheduled spacewalk by Wheelock and Caldwell Dyson to install a power extension cable to the Unity module prior to the delivery of the Permanent Multipurpose Module (PMM) on the STS-133 mission in November and to install a Power and Data Grapple Fixture to the Zarya module to support future robotics work will be deferred to a later date.

A briefing to discuss the station's status and spacewalk replanning efforts is scheduled Monday on NASA Television at 3 p.m. Central time originating from the Johnson Space Center with Mike Suffredini, ISS Program Manager and Courtenay McMillan, Expedition 24 Spacewalk Flight Director.

posted 08-02-2010 05:38 PM               

Analysis, spacewalk preparations continue after loss of cooling loop

Teams of flight controllers are continuing engineering analysis and refining spacewalk procedures to replace a failed International Space Station ammonia pump module later this week. Expedition 24 astronauts Doug Wheelock and Tracy Caldwell Dyson currently are scheduled to start the repairs on the station's starboard truss Thursday.

The pump failed Saturday night after a spike in electrical current tripped a circuit breaker. When the 780-pound pump failed, it shut down half of the station's cooling system. Efforts to restart the pump, which feeds ammonia coolant into the cooling loops to maintain the proper temperature for the station's electrical systems and avionics, were not successful. The station's crew worked with Mission Control to put the station in a stable configuration.

The crew, which is in no danger, has resumed normal work activities. A tag-up to review procedures with spacewalk specialists is planned later today.

On Wednesday, flight controllers plan to move the Mobile Transporter, which will be used to support robotics operations for the spacewalks, into position at the replacement worksite.

Current planning continues to support an initial spacewalk on Thursday beginning just before 6 a.m. CDT by Wheelock and Caldwell Dyson to unbolt and remove the failed pump module, and to install the spare. NASA TV coverage will begin at 5 a.m.

A second spacewalk to hook up a variety of electrical and fluid connections for the new pump module could occur two or three days after the first spacewalk.

posted 08-02-2010 07:06 PM               

Repair spacewalk planned for Friday

NASA has decided to wait until Friday to conduct a spacewalk to replace a failed ammonia pump module on the International Space Station.

Mission managers, program managers, flight controllers, engineers, astronauts and spacewalk experts made the decision on Monday evening after further analyzing and refining engineering requirements, and reviewing the results of an underwater practice session.

Expedition 24 astronauts Doug Wheelock and Tracy Caldwell Dyson currently are scheduled to start the repairs on the station's starboard truss Friday.

Fellow astronauts Cady Coleman and Suni Williams spent Monday afternoon in the Johnson Space Center's Neutral Buoyancy Lab (NBL) practicing underwater the tasks needed to restore the cooling loop over the course of two spacewalks.

Meanwhile, robotics experts are continuing to refine the procedures that will be used by Expedition 24 flight engineer Shannon Walker to guide Canadarm2 as she moves Wheelock into position to swap the failed unit with a spare unit currently stored on External Stowage Platform 2, which in turn is attached to the Quest airlock.

The station today remains in a stable configuration. The crew is on a normal sleep shift, and supporting a normal workday, but most of their planned activities this week have been canceled or deferred in order to support spacewalk preparations.

Plans are in work to move the station's Mobile Transporter into position on the Starboard 1 truss on Tuesday. With the Mobile Transporter positioned early, the team will be able to gather additional data to confirm power resources are sufficient to use the arm to support the spacewalk.

Each pump module weighs 780 pounds and is 5.5 feet long (69 inches) by 4 feet wide (50 inches), and is 3 feet tall (36 inches). The spacewalkers will need to disconnect and reconnect five electrical connectors, four fluid quick-disconnect devices, one fixed grapple bar and four bolts.

The spare pump module that will be used to replace the failed unit was delivered to the station on the STS-121 mission in July 2006.

The pump failed Saturday night after a spike in electrical current tripped a circuit breaker. When the pump failed, it shut down half of the station's cooling system.

Efforts to restart the pump, which feeds ammonia coolant to maintain the proper temperature for the station's electrical systems and avionics, were not successful. The station's crew worked with Mission Control to put the station in a stable configuration.

NASA TV coverage will begin at 5 a.m. CDT and Wheelock and Caldwell Dyson will begin the spacewalk at 5:55 a.m. Friday. In the first spacewalk, they will unbolt and remove the failed pump module, and install the spare.

A second spacewalk to hook up a variety of electrical and fluid connections for the new pump module could occur two or three days after the first.

posted 08-05-2010 11:02 AM               

Spacewalks delayed to Saturday and Wednesday

Teams of NASA flight controllers, engineers, and spacewalk and robotics experts have made significant progress in preparing for two spacewalks to replace a faulty cooling system component on the International Space Station.

To allow teams additional preparation time, the spacewalks now are scheduled for Saturday and Wednesday.

For both days, NASA TV coverage remains at 5 a.m. CDT, and the spacewalks will begin at 5:55 a.m. The spacewalks are expected to take 6.5 to 7 hours each.

posted 08-07-2010 06:47 AM               

Spacewalkers to replace failed ammonia pump module

Expedition 24 Flight Engineers Doug Wheelock and Tracy Caldwell Dyson switched their spacesuits to battery power at 6:19 a.m. CDT on Saturday, signaling the start of the first of two spacewalks that will focus on removing and replacing a failed ammonia pump module.

The repair tasks, which include removing the failed pump module from the S1 Truss and retrieving a spare from an external stowage platform, are expected to take between six-and-a-half to seven hours.

Installation and activation of the new pump module is scheduled for a second spacewalk planned for Wednesday.

Wheelock, designated as EV1, or extravehicular crew member 1, is wearing a spacesuit with red stripes and conducting his fourth career spacewalk.

Caldwell Dyson, as EV2, is wearing an unmarked suit and making her first spacewalk.

ISS Flight Engineer Shannon Walker is operating Canadarm2, the station's robotic arm, and assisting the spacewalkers from inside the station.

posted 08-07-2010 10:14 AM               

First of four umbilical lines disconnected

Three hours and 47 minutes into today's spacewalk, Doug Wheelock has released M4, the first of four 'quick disconnect' umbilical lines leading into the failed ammonia pump module he and Tracy Caldwell Dyson are working to replace.

posted 08-07-2010 11:24 AM               

Astronauts encounter trouble freeing second line

After releasing the first umbilical line (M4) with little trouble, Doug Wheelock moved onto unlatching the second (M3) 'quick disconnect' cable feeding ammonia into the failed pump module and ran into considerable difficulty.

Reporting seeing "snowflakes" -- bits of frozen ammonia -- drift free from the line, Wheelock was told by flight controllers to try cycling the quick-disconnect latch. Doing so, he found he was unable to open it again, even exerting appreciable force, due to what was assumed to be pressure in the line.

As the space station passed from day into night, Mission Control had hoped cooling the line would release some of the pressure, but to no avail. Ultimately, it was decided to move to the next line (M2) in the series, the idea being that by connecting it to a jumper box, it would help release the pressure from the M3 line.

A little over five hours into today's spacewalk, Wheelock and Tracy Caldwell Dyson are running significantly behind in their planned work due to the time they have spent troubleshooting the stubborn valve.

posted 08-07-2010 11:46 AM               

Second (M2) quick-disconnect demated

Encountering only initial resistance, Doug Wheelock detached the M2 quick-disconnect ammonia line from the failed pump module that he and Tracy Caldwell Dyson are attempting to replace.

The valve came free at 11:31 a.m. CDT, more than five hours into today's planned seven hour spacewalk, after the astronauts struggled and failed to free the M3 line.

Flight controllers hope that by connecting the M2 and previously-freed M4-pressurization lines to a jumper box, pressure in M3 line will then lower, enabling its release.

posted 08-07-2010 12:19 PM               

Third (M1) quick-disconnect demated

Doug Wheelock released the M1 quick-disconnect ammonia line just as easily as he did the M2 valve before it.

Now approaching the six-hour mark in this spacewalk, Wheelock and Tracy Caldwell Dyson have returned to the stuck M3 line to reattempt its release. Flight controllers reported seeing a drop in pressure in that line after the M2 and M1 lines were freed.

posted 08-07-2010 12:27 PM               

Spacwalkers bang free stuck line, releasing 'snowflake storm'

At 12:20 p.m. CDT, using a scoop tool as a hammer, Doug Wheelock successfully hammered open the previously stuck M3 line, enabling its demate from the pump module that he and Tracy Caldwell Dyson have been working to replace.

Before unlatching the line however, Wheelock and Caldwell Dyson encountered a steady stream of ammonia emanating from the valve, effectively creating a storm of frozen-ammonia snowflakes enveloping the spacewalkers' workspace.

As a result, flight controllers directed the astronauts to secure the line in place and reinstall a spool positioning device in an effort to halt the leak.

Approaching seven hours and the end of this spacewalk, the release of the final quick-disconnect line, as well as electrical connectors, will be deferred to the next excursion, planned now for Wednesday. Both tasks need to be completed before the failed pump module can be removed, which had been the primary goal for today.

posted 08-07-2010 02:42 PM               

Longest station spacewalk in history comes to its end

Repressurization of the Quest airlock began at 2:22 p.m. CDT, ending today's spacewalk by Doug Wheelock and Tracy Caldwell Dyson. At eight hours and three minutes, it was the longest spacewalk in the International Space Station's (ISS) history and the sixth longest of all time.

The previous station record of seven hours and 55 minutes was set in 2007 during ISS Expedition 14 by Michael Lopez-Alegria and Sunita Williams.

The longest spacewalk to date was conducted on March 11, 2001. STS-102 astronauts James Voss and Susan Helms spent eight hours and 56 minutes configuring cables for a pressurized mating adapter and installing a lab cradling assembly for the station.

Today's spacewalk was the 148th devoted to space station assembly and maintenance since 1998, the 240th by U.S. astronauts, and the 337th in history.

posted 08-09-2010 10:43 PM               

Second of three spacewalks planned for Wednesday

NASA's flight controllers and engineers continue meetings to review the results from the first spacewalk and to plan for the second of what now will be three excursions to complete the replacement of a failed pump module on the station's starboard truss.

Station program managers have laid out procedures for Expedition 24 flight engineers Doug Wheelock and Tracy Caldwell Dyson to perform at the start of the next spacewalk, now planned for Wednesday. The tasks are designed to greatly reduce, or eliminate the possibility of ammonia coolant leaking from the final fluid connector, M3, when it is demated, setting the stage for the failed pump module to be removed.

The plan calls for the astronauts to close other quick disconnect lines where the S1 and S0 trusses meet, which should isolate ammonia upstream from the final connector, in turn preventing any recurrence of leakage while the new pump module is being installed.

Wheelock and Caldwell Dyson will then configure lines and valves prior to demating the final electrical connections from the old pump so that it can be parked on a stowage bracket on the ISS's mobile base system.

NASA's mission management team will meet on Tuesday morning to provide its final approval for the spacewalk as planned to proceed.

Station systems remain in good condition operating on the second of two cooling loops. NASA says the astronauts are well rested following Saturday's spacewalk, having spent the last two days recharging spacesuit batteries, reviewing spacewalk procedures and configuring tools for their next excursion.

NASA TV coverage of the second spacewalk will begin at 5 a.m. CDT Wednesday. The spacewalk is scheduled to begin around 5:55 a.m.

posted 08-10-2010 05:39 PM               

Crew wraps up preparations for Wednesday spacewalk

Doug Wheelock and Tracy Caldwell Dyson spent Tuesday completing preparations for their spacewalk Wednesday to replace an ammonia coolant pump module that failed on July 31.

Ground controllers activated the gaseous pressure regulator valve for the Loop A nitrogen tank assembly, lowering the overall pressure for the inactive cooling loop. The reduced pressure will facilitate closing quick disconnect valves, enabling removal of the fourth and final fluid line connector holding the failed pump module in place.

The lowered pressure will also facilitate the remate of all of the fluid lines once the new pump is installed. That activity is targeted for the third spacewalk no earlier than Sunday.

Wheelock and Caldwell Dyson conducted final timeline reviews on Tuesday with flight controllers and moved into the Quest airlock just before 3 p.m. CDT to begin their overnight "campout" to reduce the nitrogen in their bloodstreams. They will be awakened at 1 a.m. on Wednesday to complete preparations and to don their spacesuits.

To complete some fine-tuning to robotic procedures and to allow the crew and the flight control team a little additional rest, the spacewalk Wednesday will begin one hour later than originally planned.

NASA Television coverage of the spacewalk will now begin at 6 a.m. CDT. The spacewalk is scheduled to begin just before 7 a.m.

posted 08-11-2010 07:43 AM               

Spacewalkers begin second spacewalk to replace failed pump

Expedition 24 flight engineers Doug Wheelock and Tracy Caldwell Dyson switched their spacesuits to battery power at 7:27 a.m. CDT Wednesday, signaling the start of today's spacewalk, the second of at least three spacewalks focused on removing and replacing a pump module that failed on July 31. Today's spacewalk is expected to last about six hours.

This is the fifth spacewalk for Wheelock, who is wearing a spacesuit bearing red stripes, and the second for Caldwell Dyson, whose suit is unmarked.

After setting up tools and moving to their worksite, the spacewalkers will first isolate the Loop A cooling line, closing quick disconnect lines, and then will use a tool to vent residual ammonia before proceeding to disconnect the M3 fourth and final umbilical they left mated during the first spacewalk.

They will then demate five electrical and data cables and loosen four bolts from the old pump so that it can be extracted and parked on a payload bracket on the station's mobile base system.

If all goes as planned, the spare pump will be installed during the third spacewalk targeted for no earlier than Sunday.

posted 08-11-2010 09:37 AM               

Brute force frees final coolant line from failed pump

Returning to where he left off at the end of Saturday's spacewalk, Doug Wheelock was able to close and demate the quick disconnect valve on the final fluid connector leading into the failed pump module.

Seeing only some small ammonia "snowflakes" and no visible leaks, Wheelock was given the go by Mission Control to press ahead with demating the M3 connector rather than proceed first with isolating the coolant flow upstream, as had been the plan prior to the spacewalk beginning.

Shaking the connector to break free ice that had built up inside the valve and using a tool to forcefully pry it off the pump, Wheelock successfully demated the M3 line at one hour and 56 minutes into the spacewalk.

Having averted the need to first isolate and then vent the line, the spacewalkers were about 35 to 45 minutes ahead of schedule. They are now focusing on disconnecting five electrical and data cables and four bolts from the failed pump, enabling its removal.

posted 08-11-2010 10:30 AM               

Data, electrical cables removed, bolts loosened

Half way through their planned six hour spacewalk, Doug Wheelock and Tracy Caldwell Dyson have completed releasing five data and electrical cables and loosening the torque on four bolts holding the failed pump module in place.

Wheelock will now retrieve a grapple bar that will be used to move the pump module from its place on the truss to a payload bracket on the station's mobile base system.

Caldwell Dyson, after stowing the electrical and data connectors for their reconnection during Sunday's spacewalk, will start removing the pump module bolts.

posted 08-11-2010 12:32 PM               

Failed pump module extracted from station's truss

Having removed the four coolant lines, five electrical and data cables and four bolts holding it in place, and having installed (with a bit of trouble) an adjustable grapple bar, Doug Wheelock, riding the robotic arm, and Tracy Caldwell Dyson have extracted the failed ammonia pump module from the station's truss and are moving it to a payload bracket on the mobile base system where it be stored.

The failed pump module, which measures 69 by 50 by 36 inches and has a mass of about 780 pounds, will remain on the bracket for the foreseeable future, until plans are made to move it or return it to Earth for analysis.

posted 08-11-2010 01:02 PM               

Failed pump module stowed on payload bracket

Riding the station's Canadarm2 robotic arm as controlled from inside the space station by Shannon Walker, spacewalker Doug Wheelock carried the 780 pound, failed pump module from the station's truss from where it was removed to a payload bracket on the mobile base system.

The module was installed on the stowage platform at 12:55 p.m. CDT, five hours and 28 minutes into today's spacewalk.

Wheelock is now joining Tracy Caldwell Dyson at External Stowage Platform 2, adjacent to the Quest airlock, to prepare the replacement pump module, setting the stage for its removal and installation during Sunday's spacewalk.

posted 08-11-2010 02:08 PM               

Spacewalkers wrap up work configuring replacement pump

Six hours and 34 minutes into the spacewalk, Doug Wheelock and Tracy Caldwell Dyson are now heading back to the Quest airlock in preparation of ending their work outside the space station.

As their final set of tasks today, the spacewalkers disconnected three electrical connectors from the replacement coolant pump on External Stowage Platform 2 and reconfigured some of its insulation blankets in preparation for installing it on the station's truss during their next spacewalk, planned for Sunday.

posted 08-11-2010 03:02 PM               

Second pump module repair spacewalk ends

Repressurization of the Quest airlock began at 2:53 p.m. CDT, ending today's spacewalk by Doug Wheelock and Tracy Caldwell Dyson seven hours and 26 minutes after it began.

Wheelock and Caldwell Dyson have now logged a total of 15 hours and 29 minutes over the course of two spacewalks working to repair the International Space Station's Loop A coolant system. The pair have planned at least one more spacewalk to complete replacing the ammonia coolant pump module that failed on July 31.

The spacewalkers today successfully removed and stowed that failed pump and began configuring its replacement.

Today's spacewalk was the 149th devoted to space station assembly and maintenance since 1998, the 241st by U.S. astronauts, and the 338th in history.

Earlier targeted for Sunday, the next spacewalk has been postponed one day to Monday to provide an additional day of rest for the crew and flight controllers.

posted 08-15-2010 10:13 PM               

Third repair spacewalk set for Monday

After completing several days of analysis and refining procedures, space station managers met Sunday and gave final approval for a third spacewalk Monday to install a replacement ammonia pump as well as to restart and activate half of the station's cooling system that became inoperable two weeks ago.

The spacewalk, which will be the third aimed at bringing the station's cooling system back to full capability, is scheduled to begin at 5:55 a.m. CDT. NASA TV coverage will begin at 5 a.m. CDT.

To prepare for their planned 6.5 hour excursion, spacewalkers Doug Wheelock and Tracy Caldwell Dyson are spending Sunday night in the Quest airlock, "camping out" within a lower pressure atmosphere to purge the nitrogen from their bloodstreams as a means of avoiding the bends.

posted 08-16-2010 06:04 AM               

Third repair spacewalk gets underway

Doug Wheelock and Tracy Caldwell Dyson are conducting the third spacewalk to replace a failed ammonia pump module on the station's S1 Truss. They set their spacesuits to internal battery power at 5:20 a.m. CDT, starting their excursion 35 minutes earlier than planned.

Their goal is to install a spare pump module, replacing the unit that failed on July 31 that resulted in one of the station's two coolant loops to shut down. The failed pump was removed from the truss during the spacewalk last Wednesday.

The spare to be installed was delivered to the station in July 2006 on the STS-121 mission and was mounted to a spare parts platform adjacent to Quest from where Monday's spacewalk is being staged.

Wheelock and Caldwell Dyson will maneuver the spare pump from the External Stowage Platform 2 to the S1 truss, where they will then mechanically attach it by driving a set of four bolts and mating five electrical connectors. At that point, if all goes as planned, ground controllers should see the replacement pump come to life by verifying electrical continuity and a brief spin up of the pump's driveshaft.

With the new pump checked out, Wheelock and Caldwell Dyson will then turn their attention to demating two fluid lines that are attached to a jumper box temporarily maintaining the proper pressure in the ammonia reservoir that feeds coolant to the pump itself. Nitrogen will be vented from the replacement pump after the first fluid line is connected, followed by the connection of three other fluid lines to permit the pump to finally be filled with coolant from an Ammonia Tank Assembly.

If time permits, the spacewalkers will close out their work by attaching a cable extension from Quest to an existing power cable on the Unity node. The extension is required to avoid interference when the Permanent Multipurpose Module is mated to the earth-facing side of Unity during the STS-133 mission in November.

The cable installation was one of the tasks scheduled for the original spacewalk Wheelock and Caldwell Dyson were to have conducted before the pump module failed two weeks ago.

posted 08-16-2010 08:19 AM               

Replacement pump relocated

Two hours and 44 minutes into today's spacewalk, Doug Wheelock and Tracy Caldwell Dyson have completed relocating, securing four bolts and mating five electrical connectors to the replacement pump module now in place on the station's S1 truss.

Working efficiently to stay on their timeline, the spacewalkers are now preparing to connect fluid lines to the newly installed pump module as ground controllers verify electrical continuity to the unit.

posted 08-16-2010 09:51 AM               

Fluid lines mated to replacement pump

At 9:41 a.m. CDT, four hours and 21 minutes into today's spacewalk, Doug Wheelock and Tracy Caldwell Dyson completed mating the four quick disconnect lines that will feed ammonia into the replacement pump module.

The fluid lines are the same that they removed from the failed pump module on previous spacewalks, but unlike their earlier experience, the astronauts encountered no issues with mating each connector.

Before they began installing the fluid lines, flight controllers confirmed that the pump module is in healthy condition after being delivered to the space station four years ago.

posted 08-16-2010 11:43 AM               

Spacewalkers complete pump replacement

More than six hours into today's spacewalk, Doug Wheelock and Tracy Caldwell Dyson have successfully replacing a failed ammonia coolant pump module with a spare unit and are now cleaning up their worksite.

The spacewalkers' work complete, flight controllers commanded from Mission Control in Houston for the newly-installed pump module to be filled with ammonia, beginning a multi-day process of restoring use of the station's cooling system Loop A.

posted 08-16-2010 12:52 PM               

150th station-dedicated spacewalk ends

Completing one of the most complex repairs in the history of the International Space Station (ISS), Expedition 24 flight engineers Doug Wheelock and Tracy Caldwell Dyson began re-pressurizing the Quest airlock at 12:40 p.m. CDT, marking the end to the 150th spacewalk devoted to ISS assembly and maintenance since 1998.

During their seven hours and 20 minutes outside, Wheelock and Caldwell Dyson relocated, secured and ran cables to a spare pump module, replacing the 780-pound unit that failed on July 31 and was removed during two prior spacewalks last week. The spacewalkers' work will lead over the next couple of days to the restoration of the station's Loop A cooling system.

"We got our station back," said Wheelock, after hearing from Mission Control that the pump was "looking good."

The astronauts had planned to get to one "get-ahead" task, attaching a cable extension from Quest to an existing power cable on the Unity node to avoid interference when the Permanent Multipurpose Module is added during the STS-133 mission in November. Running short on time however, flight controllers decided to defer that activity to a later spacewalk.

Today's spacewalk was the sixth for Wheelock, who with a total of 43 hours and 30 minutes accumulated now ranks tenth in the world for time spent working in the vacuum of space.

This was Caldwell Dyson's third spacewalk. It was the 242nd by U.S. astronauts and the 339th in history.

During the course of the space station's 150 spacewalks, astronauts and cosmonauts have spent 944 hours and 24 minutes outside since the first ISS-devoted excursion in December 1998.

posted 08-19-2010 09:05 AM               

Cooling system fully operational

The Expedition 24 crew had a light-duty day Wednesday as the Loop A cooling system reactivation continued aboard the International Space Station. The reactivation followed Monday's spacewalk, the third in a series of excursions that began Aug. 7 to remove and replace an ammonia coolant pump module that failed July 31.

Power restoration to systems affected by the pump module failure is complete and Loop A has been reintegrated back into the station's thermal control system. The Japanese Kibo and European Columbus laboratories also have been reconfigured back into the Loop A cooling system.

The space station is scheduled to be fully restored to a normal cooling configuration Thursday when flight engineer Doug Wheelock disconnects a jumper cable that was hooked up to provide backup cooling for the Russian segment.