HOUSTON - Two astronauts will
venture outside the International Space Station (ISS) to catch an orbital baton
of sorts on Saturday when they help attach the shuttle Endeavour's inspection boom
to the high-flying laboratory.
Astronauts Robert Behnken and Mike
Foreman are set to begin the spacewalk at 5:23 p.m. EDT (2123 GMT) to move the sensor-tipped
boom to the ISS to aid a future shuttle mission, though the chore requires intricate
choreography between spacewalkers and the crane-like robotic arms of Endeavour
and the station.
"It might seem like a kind of
routine maneuver, but it's really not," said Endeavour shuttle pilot Gregory H.
Johnson, who will wield the station's Canadarm2 robotic arm, before flight. "This
particular task keeps me up at night a bit."
Not only are astronauts handing off
the 50-foot (15-meter) long boom between two robotic cranes, but they must also
maneuver it within inches of the space station's main truss so spacewalkers can
grab it, then attach a lifeline-like heater cable to keep its laser and camera
sensors from freezing. The entire activity has a 90-minute deadline, Johnson
said.
"We have a very tricky set of hand-offs,"
lead shuttle flight director Mike Moses said of the boom's move late Friday.
Shuttle astronauts used the
inspection boom Friday afternoon to check Endeavour's wing edges and nose cap -
which experience the hottest temperatures during reentry - for damage by
orbital debris or micrometeorites. It is being left outside the ISS for NASA's
shuttle Discovery, which cannot carry its own boom when it launches May 25
because its cargo - the space station's tour
bus-sized Japanese Kibo laboratory – takes up too much room in the payload
bay.
The fuel tank that will feed
Discovery's launch in late May departed NASA's New Orleans-based manufacturing
plant this week for the agency's Kennedy Space Center spaceport in Cape
Canaveral, Fla. Delays in constructing additional shuttle fuel tanks may
lead to a slip for NASA's planned Aug. 28 launch of the Atlantis orbiter on
the final mission to overhaul the Hubble Space Telescope.
Today's spacewalk will mark the
fifth for Endeavour's planned 16-day mission, a new record for shuttle
astronauts visiting the space station. Before today, the only shuttle flights
to feature five spacewalks were Hubble
service calls, mission managers have said.
Balky station joint beckons
After moving Endeavour's inspection
boom to the station, the spacewalkers will renew NASA's scrutiny of an ailing rotary
joint designed to turn the station's two starboard solar wings like a
paddlewheel.
The survey is the latest in a series
of hunts for the exact cause of damage that contaminated the 10-foot
(3-meter) wide joint with metallic grit, crippling its ability to maximize
power production by continuously tracking the sun. A spacewalker discovered the
grit in October.
"We still haven't found the root
cause of where this debris came from," Moses said.
Astronauts have searched beneath
most of the joint's 22 protective covers for signs of damage in previous
spacewalks, and found one potential divot that Foreman will revisit during
today's excursion. He will also check under the few remaining covers for dings.
"If it is a pockmark, that might
give us a hint toward our smoking gun," Moses said of the potential ding.
The station's starboard gear is
built to rotate outboard U.S. solar arrays sunward in order to generate as much
power as possible for the growing orbiting lab. A similar gear on the station's
portside is working fine, mission managers have said.
Engineers on Earth are hoping to
pinpoint the exact source of the contamination in order to decide how best to
repair it, though any fix would likely require multiple spacewalks to switch to
a backup gear ring. Despite the addition of new modules this year, as well as a
massive Japanese lab due to arrive in late May, the station's power supply
should be sufficient to support additional construction through the fall,
mission managers have said.
"It's a huge assembly," Moses said.
"It just takes awhile to go look at it and make sure you've covered every piece
of it."
Experiment showdown
While Foreman works on the joint,
Behnken will tackle a stubborn materials exposure experiment that refused to
latch into place outside the station's European Columbus lab during a Thursday
spacewalk because of locking pins that were too large. The spacewalkers will
try to wedge the experiment in place, or else secure it with smaller pins and
then lash it down with wire ties for good measure.
Mission Control radioed up intricate
instructions on exactly how to manhandle the experiment into submission with
the shuttle crew's morning mail Friday as NASA engineers analyzed an odd glitch
in one of the shoulder joints of the station's new Canadian-built
maintenance robot Dextre. The $209-million robot is unable to pinpoint what
position the joint is in, though engineers are drawing up a potential software
patch, ISS flight director Ginger Kerrick said early Friday.
"There's still more work to be done
on that, but [it's] no impact to the mission," she added.
Commanded by veteran shuttle flyer
Dominic Gorie, Endeavour's STS-123 mission is nearing the home stretch. To
date, shuttle astronauts have swapped out one ISS crewmember, delivered the
Dextre robot and installed a storage closet for Japan's Kibo lab.
The shuttle is due to undock from
the space station late Monday and return to Earth Wednesday evening.
NASA is broadcasting Endeavour's
STS-123 mission live on NASA TV. Click
here for SPACE.com's shuttle mission coverage and NASA TV feed.
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