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.
Spacewalk
safety
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.
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.
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