Over the weekend, astronauts at the International Space Station (ISS) conducted a hasty space walk to repair a cooling system ammonia leak. The unplanned event came just before NASA's celebration of the 40th anniversary of the Skylab space station – a project that started life as a big improvised repair job to save the orbiting lab.

ISS's ammonia leak was spotted around a pump on May 9, when a steady stream of telltale white “snowflakes” was spotted. Ammonia is used as a working fluid to keep the electronics cool on the solar arrays. Engineers and crew reconfigured the cooling loops and systems to lower the leak loss while taking advantage of redundant systems, as rehearsals on the ground took place for replacing the pump with an on-orbit spare.

NASA doesn't really like hasty spacewalks, especially when it involves potential contamination of spacesuits with ammonia. Spacewalkers have to "bake" in the sun before coming back inside to make sure any stray ammonia is boiled off, along with air testing to avoid contaminating the space station's environment with toxic fumes. There are also standard hazards of cutting a glove on a sharp edge, forcing a quick return to an airlock.

After a day of planning, Expedition 35 flight engineers Chris Cassidy and Tom Marshburn started their spacewalk at 8:44 a.m. EDT on Saturday, May 11, successfully swapping in a spare 260-pound pump controller box in five hours and 30 minutes. The pump and ammonia cooling loop will be carefully monitored to make sure the problem is fixed.

The weekend's spacewalk was a relative walk in the park compared to the fixes needed by the first Skylab crew. When launched into orbit on May 14, 1973, Skylab's meteoroid/sunshield shield tore off, taking with it one of two solar panels and pinning the other into place. The space station was left without adequate power and baking from the heat, threatening film, food, and the plastic insulation if the temperature got high enough. Melting of the plastic would have released poisonous gases that would have rendered Skylab uninhabitable.

Just 11 days after launch, Charles Conrad Jr., Pall Weitz and Joseph Kerwin were sent up not to occupy a functioning space station as had been originally planned, but on a salvage and rescue mission.  Engineers had to assemble a kit to free the solar panel, including a slightly modified off-the-shelf lineman's cable cutter for cutting a metal strop, and a jury-rigged collapsible parasol constructed with aluminum rods and springs designed to fit into a 20-square-centimeter opening and expand to cover an area of seven square meters.

While the repair mission was being put together, Skylab – the world's largest satellite at the time— had to be carefully steered to put enough power on the four smaller solar arrays while at the same time trying to keep station temperatures down.

 A first try at deploying the remaining solar array failed with a subsequent docking, which required taking apart the Apollo docking probe in order to dock with the station. Once inside the station, the crew was able to deploy the collapsible parasol through a small airlock to provide a replacement sunshade.   A second spacewalk freed the solar array wing, enabling adequate power for future missions.   

The second manned mission to Skylab brought up a more elaborate replacement sunshield, but its design and installation spacewalk had the benefit of weeks of time for planning and effort with a July 28, 1973 launch.