HOUSTON --
Two spacewalkers replaced a broken gyroscope outside the International Space
Station (ISS) Monday as engineers on Earth discussed whether Endeavour shuttle astronauts
will have perform a risky heat shield repair later this week.
Spacewalkers
Rick Mastracchio and Dave Williams replaced the
broken gyroscope, one of four used to stabilize the ISS properly without
using Russian rocket thrusters, with a pristine replacement during a six-hour,
28-minute spacewalk. The new gyroscope was powering up by the excursion's end.
"That's
great news," Mastracchio said.
Meanwhile,
NASA image analysts and engineers conducted a detailed
thermal analysis of a 3 1/2-inch (nine centimeter) long gouge in heat-resistant
tiles on Endeavour's underbelly. A baseball-sized piece of fuel tank debris
scuffed several tiles and carved
a gouge through a 1.12-inch (2.8 -centimeter) thick tile during Endeavour's
Aug. 8 launch.
John
Shannon, chair of NASA's STS-118 mission management team, said Sunday that he
would not send a spacewalker out under Endeavour's belly to repair the fragile
tiles unless absolutely necessary. Such a repair could place a spacewalker at
the tip of Endeavour's 100-foot (30-meter) robotic arm and inspection boom, and
runs the risk of causing more tile damage.
"If
it's required, we'll do it. If it's not required, we won't do it," Shannon
said in a Sunday briefing. "As to which way we'll go, well see through the
analysis over the next 24 to 48 hours."
Monday's
spacewalk began at 11:32 a.m. EDT (1532 GMT) as the ISS and the docked shuttle
Endeavour passed 214 miles (344 kilometers) over China. Teacher-turned-astronaut
Barbara Morgan, who served as NASA's backup for Teacher in Space Christa
McAuliffe before the 1986 Challenger accident, positioned cameras to view the
spacewalk and oversaw cargo transfer between Endeavour and the ISS.
Gyroscope
shell game
The sole
spacewalking task for Mastracchio and Williams was the replacement of a broken 600-pound (272-kilogram)
gyroscope in the space station's U.S. attitude control system.
Known as
Control Moment Gyroscope-3 (CMG-3), the gyroscope contains a large, flat
stainless-steel flywheel that spins about 6,600 times per minute to help
provide the angular momentum required to move the massive space station.
CMG-3 failed on
Oct. 10, 2006 when it began vibrating excessively enough to prompt flight
controllers to shut it down. The space station only requires two functioning
gyroscopes to orient itself without the aid of fuel-consuming Russian thrusters,
but flight controllers would like all four operational to maintain redundancy.
"It is
nice to have four," ISS flight director Heather Rarick said of the gyroscope
replacement early Monday. "We're looking forward to getting this one back."
Replacing
the failed gyroscope required spacewalkers to play an orbital shell game of
sorts to first removed the broken unit, retrieve its replacement from
Endeavour's payload bay, then stow the older one on an ISS spare parts platform
for return to Earth on a later space shuttle mission. Mastracchio and Williams
did encounter some stubborn bolts, requiring a bit of elbow grease and a manual
ratchet, to loosen the new gyroscope for installation.
STS-118
mission specialist Tracy Caldwell choreographed Monday's spacewalk from inside
Endeavour, while shuttle pilot Charlie Hobaugh and ISS flight engineer Clayton
Anderson wielded the station's robotic arm to move spacewalkers and the
gyroscopes into position.
"I'd
say this is the most amazing ride I've ever had," Williams said from the
tip of the Canadian-built robotic arm.
While
spacewalkers worked outside the ISS, the station's Expedition 15 commander
Fyodor Yurchikhin and Oleg Kotov completed the installation of a new command
processor, known as a BOK-3, as part of planned five-day repair of Russian
command and navigation computer systems. The two cosmonauts completed the
repair two days earlier than planned, with only cleanup work remaining.
The BOK-3
installation followed work by Yurchikhin and Kotov earlier this week to replace
corroded cables that Russian engineers identified as the culprit behind a
massive computer crash aboard the ISS in June.
Monday's marked
the 90th spacewalk dedicated to ISS assembly or maintenance and the 13th staged
from the orbital laboratory this year. It was the second career spacewalk for
both Mastracchio and Williams.
NASA plans
two more spacewalks, on Wednesday and Friday, for Endeavour's 14-day
construction mission to the ISS.
NASA is
broadcasting Endeavour's STS-118 mission live on NASA TV. Click here for mission updates and
SPACE.com's NASA TV feed.
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