Endeavour mission specialist Chris Cassidy works outside of the International Space Station to replace a set of old solar array batteries during the third spacewalk of STS-127 on July 22, 2009.
Credit: NASA TV
When an unexpected glitch pops up in an astronaut?s spacesuit during a spacewalk, NASA takes no chances. If it looks like a problem, it?s time to call it quits.
Such was the case Wednesday, when flight controllers at NASA?s Mission Control in Houston ordered two astronauts working outside the International Space Station to cut their spacewalk short after detecting rising levels of carbon dioxide in one of their spacesuits. The move was just a precaution - at no point was the astronaut in danger - but it provided a glimpse into how NASA treats spacesuit malfunctions in the middle of a spacewalk.
?A spacesuit is a very small spacecraft and there?s just really not much margin for error,? NASA?s lead space station flight director Holly Ridings told reporters late Wednesday.
The glitch occurred inside astronaut Chris Cassidy?s spacesuit while he and fellow spacewalker Dave Wolf were replacing old solar array batteries on the International Space Station. The canister used to scrub carbon dioxide from the atmosphere inside Cassidy?s suit apparently wasn?t doing its job right, NASA officials said.
?There is a team meeting to discuss exactly what the signature was that caused the carbon dioxide to trend upward near the end of the spacewalk,? said Kieth Johnson, NASA?s lead spacewalk officer for the space shuttle Endeavour?s current mission to the station.
NASA measures the amount of carbon dioxide in a spacecraft using millimeters of mercury. For example, the acceptable limit for space shuttle and the space station is about 5 millimeters of mercury, Johnson said. The typical range for a spacewalking astronaut is between 0.3 and 0.5 millimeters of mercury, he added.
At its worst, the carbon dioxide level in Cassidy peaked at about 3 millimeters of mercury, well below the accepted threshold for the shuttle and nowhere near the 8 millimeter mark that would have set off an alarm in his spacesuit warning of an impending problem, mission managers said.
It is only when levels reach 15 millimeters of mercury that astronauts would begin to feel the effects of carbon dioxide poisoning, known as hypercapnia, Johnson said.
?It?s an uncomfortable situation. A crewmember would start feeling warm and a bit of air hunger,? Johnson said. ?In part of our training, we allow the crewmember to experience that elevated [carbon dioxide] to know just exactly that it is a condition that they?re experiencing, and how to react to it.?
If that ever occurred, spacewalking astronauts could refer to an emergency procedure in a checklist attached to their spacesuit gloves, Johnson said. It lists exactly what to do in order to return to the airlock in the event of spiking carbon dioxide levels, he added.
But NASA?s nominal plan is to always return astronauts to the safety of the airlock before rising carbon dioxide levels can become a problem. That?s why Mission Control ordered Cassidy and Wolf back inside the space station a half hour earlier than planned.
Cassidy later told his crewmates and a flight surgeon in Mission Control that he felt fine and never experienced any symptoms related to carbon dioxide poisoning, mission managers said.
The slightly elevated levels detected in Cassidy?s spacesuit were on an upward trend, possibly because the lithium hydroxide canister used to scrub carbon dioxide from the suit?s atmosphere dried out early in the spacewalk or stopped letting air flow through it properly, Johnson said.
A new canister will be loaded into Cassidy?s spacesuit for his next spacewalk on Friday. Endeavour?s seven-astronaut crew is in the middle of a 16-day mission to deliver a new crewmember, spare parts and a Japanese experiment porch to the space station.
Wednesday?s spacewalk was the third of five planned for the mission. Endeavour launched toward the space station last week and is due to return to Earth July 31.
The problematic canister behind Wednesday?s shortened spacewalk will be returned to Earth for analysis when Endeavour lands.
?The team is evaluating what happened and how it might affect the next couple of [spacewalks] that we?re doing, and we?ll just go from there,? Johnson said.
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SPACE.com is providing continuous coverage of STS-127 with reporter Clara Moskowitz and senior editor Tariq Malik in New York. Click here for mission updates and SPACE.com's live NASA TV video feed.