Story Continues BelowThe spacewalk began slightly behind schedule at 9:20 p.m. EST last night, but the two spacewalkers quickly made up for lost time and stayed ahead of schedule throughout the EVA.

The goal of the spacewalk was to connect Tranquility to the station's the ammonia cooling system. This involved routing four coolant lines between ISS and the new module and wrapping them in special thermal protection sleeves.

Two independent, redundant loops connect Tranquility and ISS, each with two lines for supply and return. During the EVA, both loops were connected but only the primary A loop will be activated while the orbiting astronauts and ground controllers continue the process of powering up and outfitting Tranquility.

"The major activities, of course, are installing the ammonia lines and wrapping them in protective insulation," said ISS Flight Director Bob Dempsey. "This is going to be the major portion, the first half of the EVA.

"All four ammonia jumper lines will be installed, but only two of them, what we call the A loop, will be integrated into the external thermal control system. At that point, the ground will have already prepared the node 3 by activating some of the basic computer and power systems. Once we have that thermal control system activated, we will then complete the activation of the module. At that point, at the end of the EVA, we'll have one leg of our redundant system fully operational.

The B loop will be pressurized and activated during the mission's third spacewalk, scheduled for Tuesday.

Originally, Tranquility was to have been attached to Unity's Earth-facing nadir port, with the cupola on the end. Three years ago, station planners decided to berth it to the node's port-side common berthing mechanism instead so as to provide better visibility while crews are operating the station's robot arm and to improve clearance for Soyuz spacecraft when the dock to the outpost.

However, the late change in location required NASA to come up with custom-designed 16 foot long extension hoses to connect Tranquility to the station's ammonia cooling system. Problems with the new hoses, which experienced failures during acceptance pressure testing, forced engineers to devise a new system of splicing shorter, flight-qualified, lines together to form the required length.

By far the most difficult task of the spacewalk was setting up the thermal blankets, formed from multi-layer insulation (MLI), that will insulate the ammonia lines from space. Each blanket is 20 feet long and shaped like a giant "Y", making them cumbersome to lay unroll and lay out correctly before stringing the cooling hoses over them.

"We put that MLI insulation behind some tethers that hold it in the right place and then, one at a time, we take the ammonia jumpers out of their bag and connect them to the lab and the new node 3 and open the valves," said Patrick.

A portion of the spacewalk consisted of tasks to install handrails to the exterior of Tranquility and setting up so-called gap-spanners that will make it easier for future spacewalking astronauts to maneuver between the module and the Unity node it is connected to without interfering with the cooling lines.

The final task was the removal of launch locks around the berthing mechanism which the seven-windowed cupola will be attached to Sunday night. The locks need to be removed so that four protective petals over the port's seal can be retracted, allowing for mating of the cupola.

The only hitch in the spacewalk occurred during the initial work to connect the new ammonia lines when a small amount of the liquid leaked from a connector and sprayed in the direction of Patrick's suit and helmet.

"I have a small spray of ammonia coming out from under the cap," Patrick said. "I've backed away, ammonia did come in my direction. No more is coming out. I would estimate the total amount to be on the order of 1 cc (cubic centimeter)."

"I saw some small pieces that just looked like snow, no continuous stream, just flakes is what I saw," Behnken said.

"Yep, I do not see any that's adhered to me," Patrick said. "It came out in the general direction of my right glove and my visor, but I see none adhering. I believe no more is coming out."

"And again, nothing visible currently on your helmet?" Robinson asked a few moments later.

"Nothing at all. It was about the kind of quantity of stuff that you would expect if you didn't empty the straw at the end of your drink bag."

"If you were drinking ammonia," Robinson said.

"If you were drinking ammonia," Patrick laughed.

Ammonia is a highly noxious chemical and even small amounts getting into the space station's air supply could disable the crew or, in the least, make them ill.

Engineers put in place a special protocol for dealing with ammonia leaks during spacewalks that involved baking out the fluid under the sunlight and vacuum of space and then extended monitoring inside the airlock before an astronaut is cleared to re-enter the station's internal environment.

Since the leak occurred early in the EVA, Patrick went about his scheduled tasks and let the ammonia bake out during the course of the spacewalk. The two spacewalkers concluded the EVA slightly early, which wasn't a problem since they were ahead of the timeline and completed all tasks in the flight plan, and returned to the airlock at 3:14 a.m. EST after a 5 hour and 54 minute spacewalk.

In the end, the efforts of Behnken and Patrick paid off when mission control informed the astronauts onboard Endeavour and ISS that the cooling system was performing as expected.

Today's spacewalk brings the total EVA time devoted to the assembly and maintenance of the International Space Station to 8667 hours and 28 minutes. There have been a total of 139 spacewalks in the life of ISS.

(The Spacearium / SpaceflightNews.net)