Scaling the EVA Wall
By Irene Brown
Of all the complexities and nuances of building a live-aboard
science outpost in orbit, none is more staggering than what NASA
stoically refers to as the EVA wall. The obstacle is not a tangible
hurdle, but it might as well be.
To complete assembly of the International Space Station, NASA
is planning to stage about 165 spacewalks — or EVAs, an acronym from
the technical term extravehicular activity — spanning 1,073 hours.
Sounds reasonable, but consider this: As the shuttle prepares to make
its 100th flight, astronauts have spent just 634 hours spacewalking
outside the shuttle. Even adding outings by Gemini, Apollo and Skylab
crews to NASA's cumulative spacewalk experience falls short of what
will be expected from space station assembly and maintenance crews in
the future.
"We have a chart that shows the buildup of the time over the
next five years. It shows what we did starting back in Gemini and
then Apollo walking on the moon. Those are little bumps down at the
bottom and then you get up to space station assembly and there's this
huge spike over the next five years. It is really a big concern and
it's something that requires constant vigilance," said Greg Harbaugh,
an astronaut who heads NASA's EVA office at the Johnson Space Center
in Houston.
Shuttle-based crews visiting the budding space station
already have completed six spacewalks lasting a total of about 21
hours, but the real grunt work begins during the next station
assembly flight, scheduled for launch in October. Two teams of
spacewalkers will be heading into orbit to whittle away another 26
hours of exterior chores.
"We've laid the foundation. Now we're ready to begin the real
construction," said Daryl Schuck, who is overseeing the four
spacewalks planned during shuttle Discovery's weeklong stay at the
International Space Station.
The two-man teams will work on alternate days hooking up the
station's first exterior support segment, which is crammed with
communications, positioning, coolant, electrical and other gear vital
to the station's future growth.
The cube-shaped Z1 truss is probably not the prettiest
segment of the International Space Station, said Discovery rookie
pilot Pamela Melroy. "When we just saw the bare bones (of the truss)
it looked like one of those old-fashioned milk crates on steroids.
It's quite large, but it was this boxy-shaped thing with just
structural pieces. Now there's a lot of equipment on it and (thermal)
blankets, so it looks like a large white cube."
The cube is the core of the station's structural spine, which
will span more than the length of a football field when completed.
The station's power-producing solar arrays will be attached to the
truss, along with its communications antennas, gyroscopes, electrical
converters, coolant lines, power cables and other equipment.
"The Z1 is like a backbone for the rest of the station," said
Robert Cabana, a shuttle astronaut who commanded NASA's first station
assembly mission and now manages the program's international
operations.
"It's like the utility closet where all your transformers and power controls are
located."
Unlike your home utility closet, however, the station's power
cannot be completely shut down when it's time to make the electrical
connections.
"It would be nice to just go hit a master circuit breaker and
turn everything off. But that would shut down the entire space
station, so we can't do that," said astronaut William McArthur, who
will be paired with Leroy Chiao for the first and third spacewalks
during the upcoming Discovery mission.
"We've had to pay a lot of attention to how the sequence of
tasks is choreographed so that we have the right plugs turned off
when we stick the connectors in. It's quite an interesting task to
get this all scheduled and choreographed," said McArthur.
Few aspects of shuttle Discovery's assembly tasks are easy.
For example, the robot arm operator Koichi Wakata will have to pick
up and attach large pieces of equipment without a direct line of
sight out the shuttle's window. Instead, Wakata will rely on an
artificial space vision system developed by the Canadian Space Agency
and the directions of spacewalkers positioned outside the shuttle.
The margin for error is slim. If the Discovery crew fails to
attach the Z1 truss for some reason, the station assembly sequence
grinds to a halt.
"Every mission has to work because the follow-on missions
depend on you getting your things done," said Discovery commander
Brian Duffy. "That fact hasn't been lost on my crew nor on all of the
other crews that are assigned and training right now."
