Engineers have not yet figured out the root cause of a leak in a spacesuit that flooded an astronaut's helmet during a spacewalk last summer. But NASA managers said Wednesday they are confident the suit in question, and another that will be used for urgent space station coolant repair work starting Saturday, will operate safely without any dangerous leakage.

Astronauts Rick Mastracchio and Mike Hopkins will conduct a series of spacewalks beginning Saturday to replace a faulty pump module outside the space station. Credit: NASA

Just in case, Rick Mastracchio and Mike Hopkins, scheduled to begin the first of up to three spacewalks Saturday morning, will have water-absorbing pads positioned in their helmets just behind their heads and homemade snorkel-like tubes that would allow them to breathe on the way back to the station's airlock if any flooding occurs.

"We think that's an extremely clean suit and it's ready to go," space station Flight Director Dina Contella said of the suit that leaked in July. "We have high confidence that it's essentially just like a unit that was refurbished on the ground."

She said she asked flight controllers Wednesday if anyone had any concerns about pressing ahead and "there was not much to be said. I think we're ready to go out the door Saturday and our team has no lingering, outstanding, show-stopping issues that we're working right now."

Mastracchio, call sign EV-1, will be wearing a spacesuit with red stripes while Hopkins, EV-2, will be wearing an unmarked suit. They plan to switch their suits to battery power at 7:10 a.m. EST (GMT-5) Saturday to officially kick off a planned 6.5-hour spacewalk.

A second spacewalk is planned for Monday with a third on tap, if necessary, Christmas day.

This will be the seventh spacewalk for Mastracchio, a shuttle veteran, and the first for Hopkins, who is making his first space flight. He will be wearing spacesuit No. 3011, the same suit worn by European Space Agency astronaut Luca Parmitano during a frightening excursion in July.

During that outing, the suit's internal plumbing developed a leak, flooding Parmitano's helmet with a steadily growing, floating blob of water that wrapped around his head, blocking his ears, obscuring his vision and threatening to cover his nose and mouth.

Parmitano safely made it back to the station's airlock, but the leak prompted a major engineering investigation to find -- and fix -- whatever had gone wrong.

After extensive troubleshooting, engineers concluded the leak most likely was caused by contamination that blocked a filter and allowed water to back up in the system, moving into a line that feeds air to the helmet.

The source of the contamination is not yet known, but critical components in Parmitano's suit -- the one Hopkins will wear during the upcoming spacewalks -- were replaced and Contella said engineers are confident the suit's system's are pristine.

Mastracchio's suit also was inspected and engineers believe its internal filters are not saturated with contaminants that could cause similar leakage.

Even so, lead spacewalk officer Allison Bolinger said both men would use a helmet absorption pad, or HAP, and the homemade snorkel. Just in case.

Spacewalk officer Allison Bolinger holds a snorkel like the ones to be used by Mastracchio and Hopkins. Credit: NASA TV/Spaceflight Now

"It's a modified absorbent pad that's designed to be installed in back of the crew member's helmet," she said of the HAP. "This is our first line of defense, that if water were to enter the helmet again ... this is designed to hold anywhere from between 600 and 800 milliliters of water."

The crew will periodically tilt their heads back to determine whether the pads are "squishy."

"That's the technical term we're using to define whether we think water has entered the helmet," Bolinger said. Based on testing on the ground, astronauts can first detect "squishiness" around the 200-milliliter mark.

"As soon as the crew member experiences squishiness in his HAP, that's the sign there is a problem in the (spacesuit) and it's time to come inside," she said.

The snorkel was fashioned from plastic lines that were cut to fit and wrapped in Velcro. The lines will extend from the front of the helmet, within easy reach of the astronaut's mouth, down into the chest area.

If water floods the helmet and exceeds the capacity of the absorbent pad, the astronaut as a last resort could breathe fresh air from lower in the suit while making his way back to the airlock.

The goal of the spacewalks is to replace a refrigerator-size ammonia pump module on the right side of the station's power truss. Last week, a valve inside the 780-pound module malfunctioned, preventing the station's computer from properly regulating the coolant's temperature.

The partial loss of coolant loop A forced flight controllers to power down a variety of non-essential systems and left the station one failure away from a much more serious problem if some other malfunction disabled coolant loop B.

As it turns out, this is the second problem with a coolant loop A pump module in the past three years. But space station Program Manager Mike Suffredini said different components in the module malfunctioned and "this is not an ISS-aging-vehicle issue."

"It's an unfortunate anomaly ... with a relatively young pump," he said. "But when you do mean time between failures, it's all about the averages, and so these are the kinds of things that can happen. We're prepared to deal with it."

The space station relies on a complex cooling system to dissipate the heat generated by the lab's myriad electronic systems. Components inside the station's habitable modules are cooled by water circulating through "cold plates" to absorb the heat produced by components bolted to them.

That warmed water then flows through interface heat exchangers where the "thermal load" is transferred to ammonia coolant from the station's external thermal control system. Toxic ammonia is not allowed inside the habitable modules and the coolant flowing through the heat exchangers is physically isolated from the internal water loops.

The station is equipped with two independent external ammonia coolant circuits. Both of them pick up heat from the station's water-cooled interior components and both directly cool major electrical components mounted in the station's solar power truss, including circuit breakers, voltage converters and other equipment associated with the lab's solar power grid.

The two external coolant loops include separate ammonia pump modules that push the coolant through complex plumbing and out to huge radiators where the heat is radiated away into space. The cooled ammonia then flows back through the heat exchangers and other equipment to begin a new cooling cycle.

The station can operate with just one coolant loop, but only essential life support equipment, communications gear, computers and other critical systems can remain powered. Non-essential systems must be powered down to reduce the thermal "load" on the system.

In August 2010, coolant loop A went into a sudden shutdown when its ammonia pump shorted out, triggering an emergency power down. In that case, the coolant loop no longer worked at all, and flight controllers had to shut down a variety of critical systems, including a communications channel, two of four stabilizing gyroscopes, a GPS navigation sensor and several computer control boxes and heaters.

The pump module was replaced during three spacewalks, using one of four spares that were on board at the time. The failed unit was shipped back to Earth on the final shuttle mission in 2011 for refurbishment and eventual relaunch.

In this case, the loop A pump is still operational and continues to provide cooling to components mounted in the station's solar power truss. While non-essential systems have been shut down, many more systems remain up and running thanks to the partial cooling provided to the truss components.

Even so, research gear and other components in the station's three forward modules -- NASA's Harmony, or Node 2, module, the European Space Agency's Columbus laboratory and Japan's Kibo research module -- have been shut down, interrupting scientific experiments.

Flight controllers initially hoped to correct the problem in coolant loop A by implementing a software patch that would enable another valve in the system to control the ammonia's temperature. While the patch appeared to work during preliminary tests, mission managers decided they could not afford to rely on it and ordered the spacewalks instead.

The loop A pump module is mounted on the forward face of the station's main power truss, just above and to the right of the Destiny laboratory module, in the starboard 1, or S1, truss segment. Its loop B counterpart is mounted to the left of Destiny in the port 1 truss segment.

The Boeing-built pump modules are roughly the size of refrigerators, measuring 5-and-a-half feet long, 4 feet wide and 3 feet high.

During the first spacewalk Saturday, Mastracchio and Hopkins plan to set up tools, disconnect four fluid lines and hook up two others to a "jumper" box that will prevent the loop A coolant system from over pressurizing during temperature swings in orbit.

During the second spacewalk Monday, the astronauts plan to remove the faulty pump module and temporarily mount it on a storage fixture. The replacement pump then can be installed and electrical connections restored. During the third spacewalk on Christmas day, the fluid lines will be connected to the replacement pump module and the old unit will be moved to a more secure storage platform.

Bolinger said the astronauts possibly could complete the pump module swap out in two spacewalks if everything goes as planned and no major problems develop. But that is far from certain.

During the 2010 pump replacement, two spacewalkers ran into problems disconnecting one of the ammonia lines during their first spacewalk and were unable to remove the failed pump module before time ran out. They successfully replaced the unit during a second spacewalk, but a third excursion had to be added to make up for the lost time.

NASA managers are hopeful lessons learned will help Mastracchio and Hopkins complete the S1 pump module replacement this time around in two spacewalks. If not, the astronauts are prepared for a third spacewalk on Christmas day.

NASA has conducted six previous spacewalks during the Christmas holiday season and if a third spacewalk is required for Mastracchio and Hopkins, it will come on the 40th anniversary of a Christmas Day outing during the Skylab space station program in 1973.