Friday, July 24, 2009

Spacewalkers to "take it easy" on long day

NASA managers instructed Chris Cassidy and Tom Marshburn to take it easy starting today's spacewalk on the International Space Station, the fourth of five planned during Endeavour's 16-day mission.

The two rookie mission specialists hope to remove and replace four old batteries connected to the set of solar array wings on the far left side of the station's central truss.

Only two batteries were replaced Wednesday, half as many as planned, because Cassidy's spacesuit registered too-high levels of carbon dioxide gas, which could be poisonous if it exceeded allowable thresholds.

Managers suspect the 39-year-old former Navy SEAL may have been a little too pumped up to start his first-ever spacewalk.

He may have climbed so quickly from the Quest airlock to the battery work site that it threw off the performance of a carbon dioxide-scrubbing lithium hydroxide canister in his suit, which has been replaced.

"If you go out and you have a very high metabolic rate at the very beginning, then the canister doesn't work as well for the duration," said Holly Ridings, lead space station flight director. "There was not anything specifically wrong with the (canister), it's a feature of the way that it works."

Cassidy was told to slow down and took the advice with good humor, Ridings said.

"The (spacewalk) guys were kind of doing the words to the song 'Take it easy' (Thursday), just for a little bit of light heartedness to make sure that they did understand," she said.

A little extra time was allotted for Cassidy and Marshburn, a 48-year-old former emergency physician, to get to the work site on the Port 6 truss.

Today's is the second career spacewalk for both men, who worked previously with lead spacewalker Dave Wolf.

Thursday evening, Cassidy and Marshburn began a standard pre-spacwalk "campout" procedure in the Quest airlock, where the air pressure was reduced from 14.7 pounds per square inch to 10.2 psi.

The campout reduces the amount of time they'll need to breathe pure oxygen before beginning their excursion, a process that purges nitrogen from their bloodstreams and reduces the chance of experiencing decompression sickness.

Ridings said she hopes all four nickel-hydrogen batteries can be replaced today, but if not, the work would continue Monday during the mission's fifth and final planned spacewalk.

She hopes a minimum of two batteries are replaced. The old ones, in operation since late 2000, will be returned home by Endeavour.

The spacewalk is set to start at 9:58 a.m. EDT and end about seven hours and 30 minutes later.

It will be the 129th spacewalk in support of station assembly and maintenance since the outpost's first components were launched in 1998.

The action follows Thursday's successful transfer of two science experiments and a communications system to the new "porch" on Japan's Kibo science lab, putting the lab's two robotic arms to operational use for the first time.

Mission managers are also monitoring one of three shuttle fuel cells whose heater stayed on for longer than expected. The fuel cell was activated in response, causing it to use more liquid hydrogen and liquid oxygen, but Ridings said the amount consumed would not require a shorter mission.

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