Realtime coverage of U.S. EVA-24

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07:15 PM, 12/21/13 Update: Second coolant repair spacewalk delayed to Tuesday; revising 6:30 p.m. update to include earlier air-to-ground exchange, updated NASA statement

A NASA statement released late Saturday said the decision to delay a second space station repair spacewalk from Monday to Tuesday would give the crew time to assemble a backup spacesuit for astronaut Rick Mastracchio after a "configuration issue" raised questions about whether the original suit could be used.

"The extra day will allow time for the crew to resize a spare spacesuit on the space station for use by Mastracchio," the agency said. "During repressurization of the station’s airlock following the spacewalk, a spacesuit configuration issue put the suit Mastracchio was wearing in question for the next excursion -- specifically whether water entered into the suit's sublimator inside the airlock."

The sublimator is a device in the spacesuit's backpack that helps dissipate excess heat.

In any case, NASA said, "the flight control team at NASA's Johnson Space Center in Houston decided to switch to a backup suit for the next spacewalk."

"This issue is not related to the water leak that was seen during a July spacewalk by European Space Agency astronaut Luca Parmitano and NASA's Chris Cassidy," the agency said. "Both Mastracchio and Hopkins reported dry conditions repeatedly throughout today’s activities."

During an earlier post-spacewalk exchange between Japanese astronaut Koichi Wakata and mission control, it appeared an inadvertent switch throw might have played a role.

"And Houston, (this is the) airlock, on one," Wakata called down. "EV-1, he inadvertently moved the water switch to on and it was quickly returned to off, o-f-f."

"Copy, standby," astronaut Akihiko Hoshide replied from Houston. "And Koichi, just to confirm, that was a very brief moment?"

"Yeah, it was one or two seconds, Aki."

"And Koichi copy your last, you can press with the rest of the procedure. We may have to take a look at the sub water later, but you can continue with the procedure."

In a later exchange, the crew was given a procedure to dry out excess moisture in the suit.

As it now stands, a second spacewalk by Mastracchio and Mike Hopkins will begin around 7:10 a.m. EST (GMT-5) Tuesday. A third spacewalk, if required, presumably would be carried out the day after Christmas. But given how much the crew accomplished Saturday, a third spacewalk may be unnecessary.

05:45 PM, 12/21/13 Update: Second coolant repair spacewalk delayed to Tuesday

Spacesalkers Rick Mastracchio and Mike Hopkins completed phase one of a complex coolant system repair job aboard the International Space Station Saturday, but an unexplained problem with Mastracchio's suit prompted flight controllers to delay a second spacewalk for 24 hours, from Monday to Christmas Eve.

"We are going to slip EVA-25 one day and so that's going to move to Tuesday, Dec. 24. Merry Christmas Eve," radioed astronaut Kate Rubins from mission control. "We are going to cancel the morning DPC (daily planning conference) tomorrow so you guys can sleep in a few extra hours."

The problem apparently involved excess water in the sublimator of Mastracchio's spacesuit, a device used to dissipate heat. The issue is not believed to be related to water leakage in July that flooded another spacewalker's helmet and forced a frightening abort. But NASA did not provide any immediate details regarding the cause of the excess water in Mastracchio's suit.

"Due to come concern about the water in the sublimator of the 3010 suit, we want to dry that suit out overnight and so if you guys don't mind, if one person could take about 15 minutes I'm going to give you some steps to get the suit in a good config," Rubins called. "We're going to power the suit up and turn the radio on as a heat source and take a few pictures of the config, and that should help with the drying out."

"OK, I'll be happy to do that," Japanese astronaut Koichi Wakata replied.

Assuming no other problems crop up, the second spacewalk will begin at 7:10 a.m. EST (GMT-5) Tuesday.

Despite the delay, Mastracchio and Hopkins were able to complete the primary objectives of Saturday's spacewalk, along with a major item on the list of tasks planned for their second excursion. Barring major problems, it appears likely the astronauts will be able to complete the coolant system repair work in two spacewalks without the need for a third excursion.

In any case, the delay from Monday to Tuesday takes a Christmas spacewalk off the table. If a third EVA is required for some reason, it is unlikely to take place before Thursday.

In the meantime, Rubins said, "we're working the plan to let you guys sleep in tomorrow."

"Sounds good. Thank you very much," Wakata replied.

Mastracchio and Hopkins switched their spacesuits to battery power at 7:01 a.m. to officially kick off the first of at least two spacewalks to remove a suspect ammonia pump module and install a replacement. Trouble with a valve inside the pump assembly partially disabled one of the station's two cooling loops, which are needed to dissipate the heat generated by the lab's electrical systems.

While the failure did not put the crew in any danger, the partial loss of cooling forced flight controllers to power down non-essential equipment in the forward modules of the outpost, including equipment used for scientific research. Repairing coolant loop A is a high-priority for the crew, both to resume normal operations and to restore lost redundancy in a critical system.

The pump module in question was installed during a series of spacewalks in August 2010. Major problems getting large ammonia lines disconnected and re-attached delayed the repair work and forced the crew to carry out a third, unplanned EVA to finish the work.

This time around, flight planners built a third spacewalk into the timeline right off the bat in case similar problems developed. But Mastracchio and Hopkins had no problems disconnecting the pump module today and, running well ahead of schedule, they were able to remove it from the station's solar power truss and mount it on a storage fixture.

The original plan called for the spacewalkers to simply disconnect the pump module and to remove it during a second spacewalk Monday. But Mastracchio and Hopkins got the pump out well ahead of schedule, prompting flight planners to ask the astronauts to extend the spacewalk in order to complete a few other get-ahead tasks on Monday's timeline.

Astronauts seldom object to such extensions, but Mastracchio told Douglas Wheelock in mission control that he preferred ending the spacewalk at that point.

"My vote would be to call it (off) for today, but it's up to you guys if you really want to go out there," Mastracchio radioed shortly after mounting the old pump on a storage fixture.

"And Rick, could you give us some ideas if it's temperature, your (suit) temperature?" Wheelock asked. He was referring to an earlier conversation when Mastracchio reported his feet were unusually cold.

"It's just, more a couple of things," Mastracchio replied, declining to provide details on the open space-to-ground audio loop.

12:45 PM EST, 12/21/13 Update: Astronauts complete pump removal ahead of schedule, wrap up spacewalk

Astronauts Rick Mastracchio and Mike Hopkins returned to the International Space Station's airlock Saturday to wrap up a successful five-hour 28-minute spacewalk, the first of at least two excursions needed to repair a critical coolant loop.

Running well ahead of schedule, the astronauts decided to call it a day instead of pressing ahead with additional work planned for a second spacewalk Monday and began repressurizing the station's Quest airlock at 12:29 p.m. EST (GMT-5) to officially end a virtually problem-free spacewalk.

Astronaut Rick Mastracchio, anchored to the end of the International Space Station's robot arm, holds a 780-pound ammonia pump module while arm-operator Koichi Wakata maneuvers them to a storage fixture. (Credit: NASA TV)

Mastracchio and Hopkins successfully disconnected a faulty ammonia pump module, removed it from its rack in the station's solar power truss and maneuvered it to a mounting fixture to clear the way for installation of a replacement pump module Monday. There were no indications of any water leaks in either astronaut's spacesuit like one that triggered a frightening abort during a spacewalk in July.

The original flight plan called for Mastracchio and Hopkins to simply disconnect the old pump during the initial spacewalk and to remove it during the second outing. But the spacewalkers had no problems disconnecting ammonia lines and power cables and flight controllers told them to go ahead and remove the refrigerator-sized pump module.

All of that went off without a hitch. Anchored to the end of the International Space Station's robot arm, Mastracchio had no trouble pulling the 780-pound ammonia pump module from the power truss, holding it in place over his head while arm-operator Koichi Wakata, working inside the station, moved him to the mounting fixture.

A few minutes later, the pump assembly was safely locked in place, completing the first phase of work to repair coolant loop A, one of two used to dissipate the heat generated by the station's electrical systems.

Other than an early comment about cold toes, Mastracchio and Hopkins had no complaints and nearly five hours into the excursion, mission control asked how the spacewalkers felt about moving outboard along the station's solar power truss to prepare a new pump module for installation Monday. The astronauts earlier agreed to extend the spacewalk an hour or so.

Without giving any reasons, Mastracchio said he would prefer to end today's spacewalk after securing tools and tethers.

"We wanted to get your take before you start moving, we're four hours and 46 minutes into the EVA, and we would like to get some work done on the spare out on ESP-3, we'd like to get your thoughts on that," astronaut Douglas Wheelock radioed from mission control

"OK, so basically all we could do is open MLI (insulation blankets) and break torque (on attachment bolts), is that it?" Mastracchio asked.

"Possibly cap removal and removing the tape from the electrical connectors as well, and we're breaking torque on the bolts as well, yes, and it'll help us with some get aheads and put us in a good config for the second EVA," Wheelock said.

Before and after: Four thick ammonia coolant lines can be seen attached to the loop A coolant pump module in the top photo, along with five electrical connectors. All of the lines were removed, bottom photo, so the pump could be pulled from its housing on the right side of the space station's main power truss. (Credit: NASA TV)

"Yeah, my vote would be to call it (off) for today, but it's up to you guys if you really want to go out there," Mastracchio replied.

"And Rick, could you give us some ideas if it's temperature, your (suit) temperature?" Wheelock asked.

"It's just, more a couple of things," Mastracchio replied, without providing any details.

"Copy all, Rick. And standby, we'll have further words for you in just a minute."

"I assume we could add those tasks and still get them all done on EVA-2, correct?" Mastracchio asked.

"Copy that, Rick."

A few moments later, Wheelock told the astronauts, "OK, guys, we're going to wrap it up and just do some cleanup here now."

Given how far ahead of the timeline the crew got, flight controllers are still hopeful the astronauts can complete the pump swap out Monday, avoiding a third spacewalk on Christmas day.

This was the 175th spacewalk devoted to station assembly and maintenance since construction began in 1998, the ninth so far this year, the seventh for Mastracchio and the first for Hopkins.

As it now stands, 114 astronauts and cosmonauts representing nine nations have logged 1,100 hours and seven minutes of spacewalk time outside the International Space Station, or 45.8 full days. Mastracchio's time outside during his seven spacewalks totals 43 hours and 58 minutes, moving him up to 15th on the list of most experienced spacewalkers.

10:25 AM EST, 12/21/13 Update: Astronauts ahead of schedule, plan to extend spacewalk

Astronauts Rick Mastracchio and Mike Hopkins are running about an hour and a half ahead of schedule with work to replace a faulty ammonia pump module on the International Space Station. As a result, flight controllers plan to extend the spacewalk by an hour or so to remove the pump assembly from its rack and mount it on a nearby stowage fixture, a task originally planned for a Monday spacewalk.

The original flight plan called for the spacewalkers to disconnect the coolant loop A ammonia pump assembly during a 6.5-hour outing and to make initial preparations for installing a spare unit. Under that plan, the astronauts would remove the faulty pump during a second spacewalk Monday and begin the installation of the replacement.

Astronaut Rick Mastracchio, anchored to the end of the International Space Station's robot arm, works at an ammonia pump assembly on the forward face of the lab's solar power truss to disconnect electrical cables. (Credit: NASA TV)

During a third spacewalk on Christmas day, the crew would complete connections to the new pump and finish stowing the old unit.

NASA is still holding open the option of a third spacewalk to finish the work, but if Mastracchio and Hopkins can keep up this pace, and if no major problems develop, it may be possible to complete the pump swap out in two EVAs.

But that will depend on a variety of factors, including how the installation of the replacement pump goes during the second spacewalk Monday. For their part, the crew had no objections to a spacewalk extension, although Mastracchio said his feet were quite cold and that he hoped sunlight might warm him up a bit.

"Houston, the only issue that I personally am having is it's very, very cold. Because I'm just floating here on the arm, I've got very, very good airflow in my boots but my toes are quite cold. They're fine right now, I just would, uh, let's see, what, it's 7:15? Is that, what, three or four more hours?"

"About four more hours, Rick" astronaut Doug Wheelock replied from mission control.

"Right. That's my only concern, are my toes. I've got my glove heaters on because my fingers were cold, but they're starting to warm up nicely now. So that's my biggest concern right now. Other than that, everything else is OK. Now that the sun's out, maybe it'll warm up a little bit."

A few minutes later, Wheelock said "with these adjustments on your suit, we're waiting for your go, if you feel you can go to the seven (hours), 7:15 EVA."

"Yeah, I think I can do it, Houston, I think my feet will warm up now that the sun's up," Mastracchio replied. "I've got the suit warmer now."

"OK, that sounds great, guys," Wheelock said.

09:20 AM EST, 12/21/13 Update: Spacewalkers work to disconnect ammonia lines

Two hours and 20 minutes into a planned 6.5-hour spacewalk, astronauts Rick Mastracchio and Mike Hopkins are in the process of disconnecting ammonia coolant lines from a suspect pump module they plan to replace.

The coolant loop A pump module is located on the right side of the station's solar power truss. Mastracchio is anchored to the end of the station's robot arm while Hopkins is free floating.

A helmet cam view of the ammonia lines and electrical connectors feeding into a large pump module that must be replaced to restore coolant loop A to normal operation. In this view, the four ammonia lines can be seen -- from left to right, M1, M2, M3 and M4 --in the center of the frame, lanked on the left and right by electrical cables. (Credit: NASA TV)

The refrigerator-size pump module is plumbed into the coolant loop with three 1.5-inch-wide ammonia lines, referred to as M1, M2 and M3, and a one-half-inch line, known as M4. All four had to be disconnected, along with five electrical cables.

The ammonia quick-disconnect fittings are locked in place with complex mechanisms that feature so-called "spool positioning devices," or SPDs, that ensure proper alignment or both sides, levered handles that can pull the connector components together or force them apart, locking collars and safety buttons that must be depressed before the components can be disengaged.

During a pump module replacement in August 2010, another team of astronauts ran into major problems getting one of the ammonia lines disconnected, presumably because of pressure in the system. This time around, flight controllers reduced the pressure in the loop before the spacewalk began to prevent any similar problems.

Ammonia lines M3 and M4 were the first to get disconnected. Both needed to be plugged into a so-called "jumper" box to allow the ammonia in coolant loop A to expand and contract as the station moves into and out of sunlight during the course of the pump replacement work.

Mastracchio had no problems disconnecting M3 and M4, reporting a small amount of ammonia ice crystals, or flakes, floating out of the connectors.

"I do see some snow, very little," he said, "very small flakes. Coming from the forward side of the QD. Very small flakes, if you will, and now I don't see them any more. Very, very small particles."

"Copy that, Rick, and can you tell if any of that hit your suit?" astronaut Doug Wheelock asked from mission control.

"I think yes," Mastracchio replied. A few moments later, after disconnecting the M3 line, he reported "a few more flakes coming out, not too bad though. ... Looks like the male QD is kind of iced up a little bit."

A view of the space station's right side power truss, where Rick Mastracchio and Mike Hopkins are working to remove a suspect ammonia pump module. (Credit: NASA TV)

The flakes apparently came from ammonia trapped in the connectors and were not the result of a leak. But contact with ammonia ice can trigger a lengthy decontamination procedure at the end of a spacewalk to make sure any traces on the spacesuits have evaporated before the crew re-enters the station.

Whether or not those procedures will be required today is not yet known.

In any case, Mastracchio had no problems connecting M3 and M4 to the jumper box and the crew is pressing ahead with the other lines and cables. As of 9:20 a.m., the crew was running about an hour ahead of schedule.

07:15 AM EST, 12/21/13: Station astronauts begin high-stakes spacewalk

Astronauts Rick Mastracchio and Mike Hopkins switched their spacesuits to battery power early Saturday, kicking off the first in a series of spacewalks to replace a refrigerator-size ammonia pump assembly aboard the International Space Station in a high-stakes attempt to restore a critical coolant loop to normal operation.

Floating in the station's Quest airlock module, the astronauts began the planned six-and-a-half-hour excursion at 7:01 a.m. EST (GMT-5) as the space station sailed 260 miles above the Atlantic Ocean approaching Africa.

"Quite a view," Hopkins marveled as he floated outside the airlock to begin his first spacewalk.

"Yeah, watch that first step," joked Mastracchio, making his seventh EVA.

Engineers are trying to figure out how to fix a balky flow control valve used to regulate the temperature of ammonia coolant in an external thermal control system loop aboard the International Space Station.The valve in question is located inside a pump module, noted above, that is part of coolant loop A, one of two critical systems used to keep station components from over heating. (Credit: NASA)

For identification, Mastracchio, call sign EV-1, is wearing a suit with red stripes and using helmet camera No. 20. Hopkins, EV-2, is wearing an unmarked suit with helmet camera No. 18. This is the 175th spacewalk devoted to station assembly and maintenance since construction began in 1998 and the ninth so far this year.

Hopkins' spacesuit -- serial No. 3011 -- is the same one that suffered a water leak during a July spacewalk, flooding the helmet of European Space Agency astronaut Luca Parmitano in a frightening emergency that forced the crew to stop work and beat a hasty retreat to the safety of the station's airlock.

An exhaustive investigation blamed the leakage on contamination that clogged one or more filters in the suit's cooling water recirculation system. While the root cause of the contamination has not yet been determined, the suspect hardware in suit No. 3011 was replaced and engineers are confident the problem has been resolved.

Just in case, the astronauts positioned water-absorbing pads behind their heads, where the water entered Parmitano's helmet in July, and used velcro to secure snorkel-like plastic tubes within easy reach of their mouths. The tubes extend down into the body of the suit, giving the spacewalkers an unobstructed source of air if water somehow makes it into either helmet.

The water-absorbing pads and snorkels should provide more than enough time to reach the safety of the station's airlock if another leak does, in fact, develop.

The spacewalks were ordered after a critical valve in one of the space station's two coolant loops malfunctioned last week, resulting in lower-than-allowable temperatures.

While coolant loop A remained partially operational, flight controllers were forced to shut down a variety of systems in the station's forward modules, including experiment hardware, to keep those systems from over heating. Coolant loop B remained fully functional.

Engineers attempted to resolve the problem using a software patch to precisely control the position of another valve in the coolant system, and thus the temperature of the ammonia in loop A. But NASA managers ultimately opted for a series of spacewalks to replace the ammonia pump module where the suspect flow control valve is located.

The pump module in question was installed during three 2010 spacewalks after the pump in the original assembly broke down, taking out coolant loop A in its entirety. This time around, the loop A pump is working normally, cooling components mounted outside the station's habitable modules. But the faulty flow control valve is preventing the loop from cooling components mounted inside the habitable compartments.

The loop A pump module is mounted on the right side of the station's main solar power beam in truss segment S1.

Mastracchio and Hopkins plan to replace the 780-pound assembly with one of three spares mounted on external storage platforms. During Saturday's spacewalk, they plan to set up the tools they'll need, open insulation blankets and disconnect four ammonia lines from the balky pump unit.

The first two ammonia lines to be disconnected - M3 and M4 - will be connected to a "jumper box" that will prevent the loop A coolant system from over pressurizing during temperature swings in orbit. With the jumper box hooked up, the astronauts will disconnect the other two ammonia lines, along with five electrical connectors.

During a second spacewalk Monday, the astronauts plan to remove the faulty pump assembly and temporarily stow it on a nearby mounting fixture. The replacement pump module then will be installed in its place and the astronauts will re-connect the electrical lines.

During a third spacewalk Christmas day, Mastracchio and Hopkins will re-connect the fluid lines and close out the replacement module. They also will move the old pump assembly to the same storage pallet where the replacement pump was mounted.

If the work goes smoothly, it may be possible to complete the pump module swap out in two spacewalks. But during the 2010 replacement work, Douglas Wheelock and Tracy Caldwell-Dyson ran into problems getting the ammonia lines disconnected and a third spacewalk was required.

Given their past experience, NASA planners say there's a good chance the Christmas day spacewalk will be needed and time has been set aside just in case.

Here is a timeline of major spacewalk tasks planned for Saturday (in EST and event elapsed time; best viewed with fixed-width font):

07:01 AM...00...00...Suits to battery power (spacewalk begins)
07:06 AM...00...05...Egress/setup
07:46 AM...00...45...EV1: Robot arm (SSRMS) setup
.....................Install WIF adapter
.....................Install portable foot restraint
.....................Safety tether swap to SSRMS
07:46 AM...00...45...EV2: Failed pump module preps
.....................Open large ORU bag; stage crewlock bag 1
.....................Stage fish stringer
.....................Open and restrain pump module insulation shroud
.....................Release TA clamps from pump module fluid lines
.....................Assist EV1 with spool positioning device (SPD) removal
08:26 AM...01...25...EV1: Failed pump module preps
.....................Remove SPDs
08:56 AM...01...55...EV1: SSRMS ingress
08:56 AM...01...55...EV2: Ammonia line quick disconnect demate (assist EV1)
09:06 AM...02...05...EV1: QD demates (M4, M3,M1,M2)
.....................Close/demate M4,M3
.....................Install pump module jumper
.....................Attach insulation to jumper
.....................Close/demate M1,M2
.....................Install insulation cover over M1,M2
11:41 AM...04...40...EV2: Prep failed pump module for removal
.....................Demate electrical connectors (5)
12:11 PM...05...10...EV2: Spare pump module prep
.....................Open pump module insulation
.....................Remove tape/velcro from pump module handrails
12:21 PM...05...20...EV1: Egress SSRMS
12:41 PM...05...40...Clean up/ingress
.....................EV1 close and stow large ORU bag on CETA cart
01:26 PM...06...25...Pre-repress
01:31 PM...06...30...Spacewalk ends

Going into Saturday's EVA, 113 astronauts and cosmonauts representing nine nations had logged 1,094 hours and 39 minutes of spacewalk time outside the space station, or 45.6 full days. Mastracchio's time outside during his previous six spacewalks totals 38 hours and 30 minutes, putting him 23rd on the list of most experienced spacewalkers.