Mission Endeavour: Astronauts to Repair ISS Gyroscope in Spacewalk

HOUSTON -- Two spacewalking astronauts will step outside the International Space Station (ISS) Monday to replace a broken U.S. gyroscope while shuttle mission managers discuss a deep gouge in the Endeavour orbiter's underbelly.

Endeavour shuttle astronauts Rick Mastracchio and Dave Williams are due to begin their 6.5-hour orbital repair job at about 11:31 a.m. EDT (1531 GMT) from the station's Quest airlock.

The entire spacewalk is dedicated to replacing a failed control moment gyroscope, one of four used in the space station's U.S. attitude control system that orients the orbital laboratory without firing rocket thrusters.

"The gyro, essentially, is a spinning disk that conserves momentum," Williams, a Canadian Space Agency astronaut, said before flight. "It's used to stabilize the station."

The space station trades off between propellant-less U.S. gyroscopes and Russian thrusters to orient itself in Earth orbit. ISS flight controllers shut down one of the station's four gyroscopes in October 2006 when it began to vibrate excessively.

While the station can control its position in space with only two functioning gyroscopes, all four are preferred to be operational as new truss segments and modules are added to the ISS during its construction.

Monday's spacewalk will mark the second for both Mastracchio and Williams, who successfully helped install a new starboard truss segment to the ISS during an Aug. 11 excursion.

While Mastracchio and Williams toil in space, engineers on Earth are expected to perform a detailed thermal analysis on a 3 1/2-inch (nine-centimeter) long divot across two of the heat-resistant tiles on the shuttle Endeavour's belly-mounted heat shield. A baseball-sized piece of fuel tank foam carved the gouge during Endeavour's Aug. 8 launch, penetrating through the entire 1.12-inch (2.5-centimeter) thick tile and prompting concerns that a spacewalk repair may be needed to address the damage.

"We hope the analysis will turn out that we don't have to go to a repair," said John Shannon, chairman of Endeavour's mission management team, adding that since the 2003 Columbia accident NASA has developed new heat shield modeling and repair methods to meet such tile damage challenges. "We have really prepared for exactly this case since Columbia."

Commanded by veteran shuttle flyer Scott Kelly, Endeavour's seven-astronaut crew is hauling about 5,000 pounds (2,267 kilograms) of cargo to the ISS, along with a newly installed starboard side truss segment and a still undelivered spare parts platform. The crew also includes former Idaho schoolteacher Barbara Morgan, who first joined NASA's as the backup to Teacher in Space Christa McAuliffe before the 1986 Challenger shuttle accident.

Gyroscope swap

Monday's spacewalk will mark the second of four planned spacewalks for Endeavour's STS-118 spaceflight. Mission managers decided Sunday to extend the shuttle flight three extra days based on the successful performance of a new power transfer system that allows Endeavour to draw on the space station's solar power grid for electricity rather than its own fuel cell resources.

During their spacewalk repair, Mastracchio and Williams will first remove the faulty gyroscope from its mount in the station's Zenith 1 (Z1) truss and set it aside, and then retrieve its replacement from Endeavour's payload bay. To do that, Williams will ride down to the shuttle on the station's robotic arm.

"When I'm on the end of the arm going down to the payload bay, I'll have nothing in my hands," Williams told SPACE.com before flight, adding that the moment is one he's been looking forward to most on the mission. "I will have this majestic view of the planet Earth?it's just going to be an incredible, incredible experience."

The spacewalkers will temporarily stow the new gyroscope on an older spare parts platform to prepare it for installation, with Williams then due to haul the 600-pound (272-kilogram) flywheel to the Z1 truss at the tip of the space station's Canadian-built robotic arm to lock in place.

The older, defunct gyroscope will be stowed on one of the station's spare parts platforms for later return to Earth, leaving the ISS with a fully functional U.S. attitude control system.

"Obviously we want the fourth one in case another one fails or has problems," Mastracchio said of the new gyroscope. "So it's basically for redundancy."

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