Realtime coverage of U.S. EVA-21

CBS News

05:40 PM EDT, 05/11/13: NASA cautiously optimistic about leak repair, but making sure will take weeks

Replacing a pump assembly aboard the International Space Station during an impromptu spacewalk Saturday may have eliminated an elusive leak that took down a critical coolant loop. But mission managers say it will take several weeks of careful observation to make absolutely sure the system is leak free.

"We're very happy. We didn't see any obvious sign of a leak" after the replacement pump was installed, Joel Montalbano, deputy manager of the space station program at the Johnson Space Center, told reporters after the spacewalk repair job.

"But it's going to take some time, it'll take some weeks to look at the system, to evaluate the system and make sure we did indeed stop the leak. But again, today, no obvious signs (of trouble), so we're very happy with that."

Flight Director Ed Van Cise said the pressure of the ammonia in the coolant loop can fluctuate as solar heating increases and decreases due to periodic changes in the angle between the sun and the plane of the station's orbit.

"As the sun moves through its beta angle, it also effects the telemetry we see in the system," Van Cise said. "So we really need to get a lot of data to make sure it's fully stabilized."

In the wake of the leak, flight controllers re-configured the thermal control system, using a healthy loop to cool equipment normally serviced by the leaking 2B channel. The system will remain in that configuration until engineers can collect enough data to verify the integrity of the 2B loop.

"I expect it'll take a good four weeks, five weeks, maybe even a few weeks longer before we have a really 100 percent characterization," Montalbano said. "Obviously, the longer you go, the more confidence you get that you either have a leak or you've successfully solved that problem."

02:25 PM EDT, 05/11/13: Spacewalk ends

Astronauts Thomas Marshburn and Christopher Cassidy began repressurizing the space station's Quest airlock at 2:14 p.m. EDT, closing out an unplanned five-hour 30-minute spacewalk to fix a leak in one of the lab's ammonia coolant loops.

Astronaut Christopher Cassidy prepares to enter the Quest airlock module at the end of an apparently successful spacewalk to fix a coolant leak. (Credit: NASA TV)
While the ammonia leak did not re-appear after a replacement pump module was installed, flight controllers cautioned that it will take days, if not weeks, to make absolutely sure the coolant loop is leak free.

But there were no immediate signs of any trouble and mission managers were hopeful the pump replacement had, in fact, resolved the problem.

The spacewalk -- U.S. EVA-21 -- was carried out just two days after an ammonia leak was spotted on the far left side of the station's solar power truss, taking down one of the lab's critical coolant loops.

"I just have to say, it is incredible what we've done in just 48 hours," Marshburn radioed shortly before airlock repressurization began. "By 'we' I mean all of operations at Johnson (Space Center) and around the country, to do this in two days. We felt safe the whole time, didn't feel rushed, the (training) products were incredible. Thanks very much. We're just really proud to be a tiny part of it."

"The team sure appreciates your words," said Michael Fincke from the mission control center in Houston. "There were a lot of folks who stayed up really late ... to get this accomplished, and you guys performed excellently, the whole entire crew, and we're very proud of you."

Today's excursion was the 168th spacewalk devoted to station assembly and maintenance since construction began in 1998, the second so far this year and the fourth for both Marshburn and Cassidy, who were spacewalking crewmates during a 2009 shuttle mission.

Total station spacewalk time now stands at 1,061 hours and nine minutes , or 44.2 days. Marshburn's total now stands at 24 hours and 29 minutes while Cassidy's mark is 23 hours and 35 minutes.

Marshburn is scheduled to return to Earth Monday aboard a Soyuz ferry craft, along with cosmonaut Roman Romanenko and outgoing station commander Chris Hadfield.

Cassidy and cosmonauts Pavel Vinogradov and Alexander Misurkin will have the station to themselves until the end of the month when three fresh crew members -- Karen Nyberg, European Space Agency astronaut Luca Parmitano and cosmonaut Fyodor Yurchikhin -- arrive aboard another Soyuz.

If all goes well, Cassidy and Parmitano will carry out two already planned spacewalks July 9 and 16.

01:15 PM EDT, 05/11/13: New pump module installed; no signs of ammonia leak

Trying to fix an elusive ammonia coolant leak, astronauts Thomas Marshburn and Christopher Cassidy replaced a 260-pound pump assembly at the far left end of the International Space Station's main power truss Saturday and reported no signs of any leakage when the system was powered up and activated.

With the astronauts stationed safely to one side, on the lookout for any signs of escaping ammonia ice crystals, flight controllers activated the pump and reported normal operation and internal pressures.

Astronaut Thomas Marshburn (foreground) and crewmate Christopher Cassidy wrap up work to replace a suspect coolant pump aboard the International Space Station. (Credit: NASA TV)

The successful activation indicated the leak presumably was somewhere in the connections or internal plumbing of the pump assembly that was removed and that installation of the replacement unit had solved the problem.

If additional testing confirms the channel 2B coolant loop is tight, flight controllers will re-integrate the loop into the system, a process expected to take a few days to complete.

The leak was observed Thursday and the station's channel 2B power circuit was shut down as ammonia levels dropped toward minimum operating levels.

The leak was observed emanating from the integrated electronics assembly, or IEA, at the far left end of the power truss where batteries, coolant system components and other gear used by the port six, or P6, solar array are housed.

Inspecting the area today, Marshburn and Cassidy saw no signs of any obvious damage. So they pressed ahead with work to replace the ammonia pump assembly on the assumption that the leak could be in the component's internal plumbing.

The unit was pulled out of its housing with no problem and again, the spacewalkers reported no obvious signs of any problems. The new unit then was installed and ammonia quick-disconnect fittings were engaged. The old pump was stowed in the housing used by the replacement.

The new pump assembly powered up normally and with the crew safely out of the way, the pump was activated.

"We're flowing fluid through the pump," astronaut Michael Fincke called from Houston at 12:31 p.m., three hours and 48 minutes into the spacewalk. "Let us know if you see anything."

After a few moments of silence, Marshburn called down, saying "the negative reporting probably speaks for itself. We're not seeing anything."

"Yeah, negative reporting, I think, in this case is good," Fincke replied. "We have another 12-ish minutes of night and we're going to keep you out and see what it looks like during the day pass. But so far, so good, I guess."

"With the lights on it and headlamps and stuff, it might be easier to see at night," Marshburn said. "But copy."

The Thermal Systems Officer in mission control reported no problems with the pump as it continued to operate and a few minutes later, Fincke told the spacewalkers there were still no signs of trouble.

"Pump package is working as expected, we see no anomalies on the data we get, so it looks so far so good. It looks like, through your (helmet cameras), we're not seeing any signs of a leak."

"Good news!" one of the spacewalkers said.

"Yeah, good news," the other added.

And that remained the case as the space station sailed into orbital sunrise west of Hawaii.

After collecting their tools and running through an inventory, the astronauts planned to head back toward the space station's airlock.

As a routine safety precaution, they followed a "bake-out" procedure, taking the time needed to make sure any ammonia ice crystals that might have contaminated their suits had time to evaporate before re-entering the station.

10:35 AM EDT, 05/11/13: Suspect pump module removed; no obvious signs of damage

Astronauts Thomas Marshburn and Christopher Cassidy removed a suspect coolant pump assembly from its housing at the far left end of the International Space Station's power truss Saturday, reporting no signs of ammonia ice on the 260-pound component that might indicate the source of a leak.

One hour and 22 minutes into the impromptu spacewalk, Cassidy manually pulled the box out of its housing for a detailed inspection. Neither spacewalker saw anything unusual.

Astronaut Christopher Cassidy, wearing a spacesuit with red stripes, and Thomas Marshburn inspect the housing of a suspect ammonia pump assembly at the far left end of the International Space Station's power truss. (Credit: NASA TV)
"It looks really, really clean," Marshburn reported. "Surprisingly so. The only thing I can note is a little bit of metallic debris on the H4 bolt shaft."

The spacewalkers then used a mirror on the end of a shaft to look down inside the truss, below where the pump was mounted, and again reported "nothing glaring."

"All the pipes look shiny clean, no crud," Cassidy said. "I can't give you any good data other than 'nominal,' unfortunately."

After a bit more discussion on the ground, astronaut Michael Fincke in mission control told the spacewalkers to press ahead with installation of a replacement pump module on the assumption the leak might be inside the plumbing of the box they removed.

"And the big picture, we're going to put the new guy in the hole, put the bad guy in the temp stow location, pack up and go home," Cassidy said, referring to the station's airlock.

"In short, yes," Fincke replied.

A few moments later, the duo took a moment to marvel at the view as the station passed 255 miles above the southern tip of Africa.

"It's pretty amazing, huh?" Marshburn said.


"Did you see the moon?"

"I did, oh my God! Burn that in your memory!" Cassidy exclaimed. "Glad to be doing it with my friend."

"Same here," Marshburn said.

09:27 AM EDT, 05/11/13: No obvious signs of leak in initial visual inspection

After reaching their worksite at the far end of the International Space Station's left-side power truss, astronauts Thomas Marshburn and Christopher Cassidy carried out a quick inspection of an equipment bay where an ammonia coolant leak is suspected.

Neither spacewalker saw anything that might indicate where the leak is located.

"Our first glance, looking all around the box in question, I see no smoking guns or any evidence of drips or anything," Cassidy observed.

"Yeah, I'm not seeing flakes yet," Marshburn said, referring to leaking ammonia ice crystals.

"Me either," Cassidy agreed.

A few minutes later, however, he reported seeing what could be a brownish stain of some sort in the integrated electronics assembly, or IEA. But otherwise, he saw nothing unusual.

"Just looking visually, I see nothing off nominal. Just these smudges," Cassidy said.

Whether that has anything to do with the ammonia leak is not yet known.

The spacewalkers are working to remove and replace an ammonia pump assembly in the IEA just in case the leak is somewhere inside the complex 260-pound component.

09:09 AM EDT, 05/11/13: Spacewalk begins

Floating in the International Space Station's Quest airlock, astronauts Thomas Marshburn and Christopher Cassidy switched their spacesuits to battery power at 8:44 a.m. EDT (GMT-4) Saturday, kicking off a hurriedly planned spacewalk to look for the source of an ammonia coolant leak.

Whether or not they find any visible signs of damage, the astronauts are expected to replace a 250-pound pump assembly mounted in the area where the leak was observed, just in case something inside is the culprit.

For identification, Cassidy, call sign EV-1, is wearing a suit with red stripes while Marshburn, EV-2, is wearing an unmarked suit.

This is the 168th spacewalk devoted to station assembly and maintenance since construction began in 1998, the second so far this year and the fourth for both Marshburn and Cassidy, who were spacewalking crewmates during a 2009 shuttle mission.

During that flight, the astronauts replaced batteries used to store power generated by the station's far left, or port 6, solar arrays. During Saturday's outing, Marshburn and Cassidy planned to work in the same area to replace the ammonia pump used by one of the P6 array's two coolant loops.

The coolant loop in question is part of power channel 2B, used to carry heat away from electronics associated with one of the two outboard arrays on the left side of the station's main truss.

On Thursday, the station crew reported a stream of frozen ammonia streaming away from the P6 truss segment. Analyzing video shot from different angles, engineers concluded the leak most likely was coming from an area where the pump assembly, used to push ammonia through the system, is mounted.

Engineers already had been monitoring a very small leak in the 2B system, amounting to about 5 pounds of ammonia per year. But on Thursday, a leak rate surged to about 5 pounds per day, likely the result of a component failure or a micrometeoroid impact.

The station gets most of its electrical power from eight huge solar arrays, four on each end of a long truss. The electronics for each array are cooled by independent loops that use ammonia flowing through radiators to dissipate heat.

The station can safely operate with one of the eight loops out of action, but mission managers want to find and fix the leak in the 2B power channel to restore redundancy and avoid possible equipment shutdowns if a subsequent failure occurs.

"Our primary objective really is to try to get a look at the leak," said space station Program Manager Mike Suffredini. "When it's at this pressure, and we're single phase in the lines, we produce this snow when we leak.

"But these cracks, or holes or whatever is leaking is very, very small. So once the leak goes down to a point where you don't see the snow anymore, it would be very hard to find. Since the crew is prepared and the ops team is ready to go, we're going to get them outside and see if we can't lay eyes on the actual leak source."

Wnile the spacewalkers are in the area, "the next most likely cause really is the pump itself and the QDs (quick disconnects) associated with the pump. So we're going to go ahead and change out that pump and see if we can't recover the system."