NASA flight control teams began powering down systems aboard the International Space Station late Thursday, after the six man crew spotted a growing ammonia coolant leak from the orbiting science lab's oldest U. S. segment solar power system segment.
ISS with thermal control system radiators extended down. Photo Credit: NASA
The seepage from the near-13-year-old thermal cooling loops of the P-6 solar array segment were spotted by ISS commander Chris Hadfield earlier in the day, and confirmed as increasing through telemetry monitored by NASA's Mission Control and video imagery transmitted over external station video cameras.
Unabated the leak was growing fast enough to trigger an automatic shutdown of the cooling loop on Friday. Port and starboard side external radiators circulate ammonia to remove heat generated by the solar power system and electronics within the station's life support, research labs and other systems. The cooling requirements ease as the power draw is decreased by shutting down equipment that is not essential. Thermal control can be assigned to a second cooling apparatus as well.
The leak surfaced just four days before Hadfield, U. S. astronaut Tom Marshburn and cosmonaut Roman Romanenko are scheduled to depart the ISS in their Soyuz TMA-07M capsule and descend into southern Kazakhstan, ending a 146 day mission. As the station crew went to bed late Thursday, flight control teams were working on a leak response plan that did not rule out a postponement of the departure and possible spacewalks.
A long-running low-rate leak in the P-6 thermal radiator prompted a Nov. 1 spacewalk by former ISS crew members Sunita Williams and Akihiko Hoshide, U.S. and Japanese astronauts. During a near-seven hour excursion, they installed jumper cables to circumvent the seepage while station experts monitored the cooling apparatus for further trouble.
Launched in late 2000, P-6 and its outstretched solar panels began supplying electricity during the early years of the station's staffing and assembly. In late 2007, as construction was nearing completion, shuttle astronauts moved the 35,000 pound P-6 to a permanent place on the station's far port side.
Hadfield, Marshburn, Romanenko, NASA's Chris Cassidy and cosmonauts Pavel Vinogradov and Alexander Misurkin were in no danger, NASA stressed in a status report.
The station crew was tipped off by telltale flakes of frozen ammonia coming from the P-6 truss early Thursday.
"All of us agreed they were coming up mainly and repeatedly enough that it looked like it was a point source they are coming from," Hadfield said in an exchange with ISS communications officer Doug Wheelock in Mission Control.
"We have folks in the room agreeing with that," Wheelock responded.
As the day unfolded, the leak rate doubled and predictions of an automatic shutdown of the P-6 cooling apparatus within 48 hours were cut to 24 hours. The seepage could not be controlled with a valve activated by commands from Mission Control, Wheelock said, raising the significance of the video downlink to indentify the source.
As the station crew went to bed, Hadfield asked the flight control team how the situation might impact Monday's undocking plans.
"It may be too soon to ask. But does it look like once we have done a shutdown we will be in a good, stable long term-configuration so we can undock as planned?" the commander asked Mission Control.
"We don't have a long range plan for a full power down," responded Wheelock. "But, of course, we have a team in place that is currently working on the undock thinking and timeline. We don't see anything technically that we cannot overcome with the undocking. But we are still getting our arms fully around that issue. You can be sure the team down here is working on all the options. Of course, that is one option. And we would like to stay with that, if we can and feel like we are in a good configuration."
If the undocking, unfolds as planned, Cassidy would remain as the only U.S. segment crew member with Vinogradov and Misurkin
The departing crew is to be replaced on May 28 with the arrival of Russia's Soyuz TMA-09M with NASA's Karen Nyberg, European Space Agency astronaut Luca Parmitano and cosmonaut Fyodor Yurchikhin.
Typically U.S. segment spacewalks are carried out by NASA, ESA and Japanese astronauts.