Unprecedented ISS Spacewalk Set for Saturday Morning

In a spacewalk characterized as unprecedented for the International Space Station (ISS), two U.S. astronauts will venture outside their home in space Saturday morning to see if they can find and fix a vexing ammonia leak in the ISS electrical power system.

Tom Marshburn has been preparing for his return to Earth on Monday after nearly 5 months in space.   NASA officials stressed today that there is no change to the plan for Marshburn and two other ISS crewmembers to come home on Monday, but first he gets another chance to do a spacewalk.

Marshburn and Chris Cassidy, who is part of a different set of ISS crewmembers that is remaining onboard the station, have already done two spacewalks together (on STS-127 in 2009) and worked in the area where they need to go tomorrow.  Their experience helped NASA officials decide that it was OK to go ahead with this spacewalk with less than 48 hours notice.  NASA chief flight director Norm Knight said that performing a spacewalk with so little advance planning is “precedent setting” for ISS missions (called “increments”), though perhaps not for space shuttle flights. 

ISS crewmembers observed “snowflakes” coming off one of the ISS solar array trusses yesterday that was quickly determined to be an ammonia leak in one of the eight power channels that provide electricity.  There is one power channel for each solar array.  Ammonia is used as a coolant.

This leak is in the vicinity of a previous leak that NASA was never able to identify so it is not known if something happened to increase that leak or if this is something unrelated.   ISS program manager Mike Suffredini stressed the difficulty of finding leaks, which may come from very tiny holes, perhaps caused by a Micrometeoroid Orbital Debris (MMOD) hit.  Or the leak may be from a seal in the pump.  They simply don’t know.  Marshburn and Cassidy will do a visual inspection and replace the pump.

The decision to do a spacewalk quickly was driven largely by the desire to observe the leak when a lot of ammonia is being released precisely so that the source can be identified.   The ammonia in the system is expected to be depleted in a day or so. 

The opportunity to discover the source of the leak coupled with the experience of these two ISS crew members were major factors in the decision to go ahead with the spacewalk tomorrow, Suffredini said.   It is not a matter of an emergency situation aboard the station.  The crew is in no danger from the leak and the ISS can operate with minimal impact using the other seven channels.   If the astronauts cannot identify the source of the leak and replacing the pump does not remedy the situation, the ISS can continue operating almost normally at least in the short term.   For the long term, operating with only seven instead of eight electrical channels could reduce the amount of research that can be conducted.   This is “not critical from a safety standpoint,” Suffredini said, but “if we have to live with this channel down for a long period of time” it will have an impact on research.   The main purpose of the ISS is to serve as a scientific research laboratory for experiments that need to be conducted in microgravity.

Marshburn and Cassidy are scheduled to open the hatch to exit the ISS at 8:15 am Eastern Daylight Time  (EDT) tomorrow morning (7:15 am Central Daylight Time).   During the 6 hour 15 minute spacewalk, they will inspect the area of the leak and replace the pump.   They then will inspect each other’s spacesuits for signs of ammonia contamination since NASA knows there is a lot of leaked ammonia in the area.   A 30-45 minute “bake out” period will ensue as a precaution to allow any unnoticed ammonia to evaporate.  They will then reenter the airlock and pressurize it to 5 pounds per square inch (psi) where another test will be conducted to ensure they are not bringing any ammonia into the station before full repressurization.

Marshburn, Russian cosmonaut Roman Romanenko and Canadian astronaut Chris Hadfield remain on schedule to return to Earth on Monday, May 13, in their Soyuz TMA-07M spacecraft.  Undocking is scheduled for 7:08 pm EDT, with landing at 10:31 pm EDT (8:31 am May 14 local time at the landing site in Kazakhstan).  They were launched on December 19, 2012.

NASA TV will cover tomorrow’s spacewalk beginning at 7:00 am EDT (6:00 am CDT).   It also will cover the landing on Monday, as detailed in NASA’s press release.