Flight controllers are closely monitoring carbon dioxide levels on the International Space Station and the space shuttle Endeavour after a problem with the device in the U.S. portion of the ISS that scrubs the gas from its atmosphere.
Brian Smith, the first-shift ISS lead flight director, says there is no danger to the 13-member combined crews of the two spacecraft from a circuit breaker on the carbon dioxide removal assembly (CDRA) that tripped at midday Saturday. Controllers were able to get the unit functioning again by operating it manually -- a task that requires a pair of extra controllers for each shift to handle the 50 or so commands required every two-and-a-half to three hours.
By tonight mission managers hope to have some new software sent to the station that will ease that workload at Mission Control Center - Houston. But the other two scrubbers -- one in the orbiter and one in the Russian end of the station -- aren't adequate to handle the carbon dioxide load generated by the record-setting crowd on board if the U.S. CDRA shuts down again.
That leaves the station in a slightly uneasy situation. There are lithium hydroxide canisters on board that can take up the slack, but the supply is limited and managers don't want to use them unless absolutely necessary. Once Endeavour departs, there are enough canisters on hand to support the six-person ISS crew for about three weeks with only the Russian unit operating, Smith said during today's mission status briefing.
So engineers will work hard to write the new software, Smith says, and to find the root cause of the heating that caused the circuit breaker to trip in the first place. Meanwhile, carbon dioxide levels in the station and orbiter are safe.
"The CDRA is operating fine now," Smith said.
The problem won't effect the fifth and final spacewalk of the STS-127 mission tomorrow, because spacewalkers Chris Cassidy and Tom Marshburn will be using rechargeable carbon dioxide scrubbers in their spacesuits. That extravehicular activity (EVA) will rewire the power source for two of the control moment gyros that help maintain station attitude control without the need to use precious fuel for thruster firings.
Currently the two units draw their power from the same circuit, and station managers want to split it up so a single failure won't take down the two gyros.
The spacewalkers also will reposition some insulation on Canada's big special purpose dexterous manipulator, a robotic "hand" for the station robotic arm that will be commissioned this fall to ease the need for EVAs in the future. They will set up two video cameras on Japan's Kibo laboratory module, and they will deploy a mechanism that will allow a future mission to stow spare parts on the station truss.
Cassidy had some carbon dioxide problems of his own during his first two EVAs, including a harrowing incident on July 22 when rising levels of the gas in his spacesuit forced an early halt that day to work replacing the oldest set of batteries on the station. Cassidy and Marshburn finished that work on July 24, and Smith said all six new batteries have accepted a charge from the solar array on the P6 truss element and are back in the station grid.
The crew started off the day today by using the station and shuttle robotic arms to pull an experiment carrier off the new porch on Kibo, and stow it in Endeavour's payload bay. Here the station arm moves in on the carrier at the beginning of the robotic ballet, which went off without a hitch.