Orbital Repair: Spacewalkers Replace Broken Space Station Gyroscope

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.

• VIDEO: STS-118 Mission Profile: Second Spacewalk
• VIDEO: Teaching the Future: Teacher-Astronaut Barbara Morgan
• Complete Space Shuttle Mission Coverage