The International Space Station's Mission Management Team approved a contingency space walk late Friday, an excursion that will send NASA astronauts Chris Cassidy and Tom Marshburn to the site of a sudden and elusive leak of toxic ammonia coolant from an aging thermal control system radiator on the orbiting lab's vaulting solar power system.
The two men, who teamed up on two previous spacewalks nearly four years ago, were to venture from the station's U. S. segment airlock on Saturday, shortly after 8 a.m., EST, for six to seven hours of troubleshooting.
Cassidy and Marshburn were given the green light by the NASA-led MMT to remove and replace the leading suspect as a leak source, a bulky pump and flow control system electronics box, a component on the 13-year-old P-6 solar power module, the oldest of the station's electricity generators. If the loss of toxic ammonia coolant is not stopped with the swap out, more spacewalks by future space station crew members may be necessary, Mike Suffredini, NASA's International Space Station program manager, told a news briefing shortly before the MMT deliberations.
That may force station operations to fault down from eight to seven space station solar power channels, which over time could restrict the electricity available for the scientific research and technology demonstrations that became the focus of ISS activities following the end of the station's lengthy assembly period in mid-2011. The leaky radiator is part of a distributed thermal control system that disperses the heat from the solar power generators and station avionics into space.
Without active cooling, temperatures rise to levels that will curtail power generation.
On Thursday, a small but regularly monitored ammonia leak rate in the P-6 solar power module suddenly jumped from five pounds of coolant annually to five pounds a day. Canadian astronaut Chris Hadfield, the station's commander, noticed frozen flakes of ammonia escaping from the solar module on the far port side of the station and alerted NASA's Mission Control.
"Today, the operations team has been in a full court press," said Norm Knight, NASA's chief flight director and lead for the ground based planning responsible for planning the spacewalk.
Though the leak rate was great enough to prompt a shutdown of the P-6 thermal control system's 2B channel late Friday, flight controllers were optimistic the system would retain enough internal pressure to assist in the search for the tiny leak source. The tell tale sign would be the emergence of starchy looking flakes of frozen ammonia pushed out by nitrogen gas in the thermal control plumbing.
"The hard part will be finding the location," stressed Suffredini. "We are talking about a very small hole."
Cassidy and Marshurn logged three spacewalks during a July 2009 space shuttle station assembly mission. They joined forces on two of the spacewalks that took them to P-6 to replace power storage batteries.
Previous experience and the prospect of enough residual pressure in the thermal control system to dispatch tell tale ammonia flakes contributed to the urgency of the outing, said Suffredini.
The leak surge surfaced just four days before three of the station's six U. S., Russian and Canadian crew members are scheduled to depart for Earth.
Cosmonaut Roman Romanenko, Hadfield and Marshburn are to depart the station on Monday shortly after 7 p.m., EDT, in their Soyuz TMA-07M capsule for a descent under parachute into Kazakhstan, ending a 146 day expedition.
Their departure remained scheduled, and the crew's morale seemed unaffected by the sudden leak.
"It's been a fun busy day on board," Hadfield assured NASA's Mission Control.
On Nov. 1, previous U. S. and Japanese space station crew members Sunita Williams and Akihiko Hoshide ventured to the same site, 150 feet from the U. S. airlock, on a spacewalk to install jumper cabling and re-deploy an inactive radiator to bypass a leak that flight control teams began to monitor last June.
Until Thursday's ammonia spurt, flight control teams were pleased with the outcome of the November repair.
If the ammonia leak escapes repair on Saturday, new U. S., European and Russian crew members will inherit the trouble shooting. Karen Nyberg, Luca Parmitano and Fyodor Yurchikhin are scheduled to lift off from Kazakhstan and dock with the station in their Soyuz TMA-09M capsule on May 28.
They will join hold over ISS crew members Pavel Vinogradov, the new commander, Cassidy and cosmonaut Alexander Misurkin.