Spacewalkers Set to Begin ISS Construction Work

Two spacewalking astronauts will venture outside the International Space Station (ISS) today to attach a new piece of the orbital laboratory's metallic backbone.

Clad in their bulky NASA spacesuits, Endeavour shuttle astronauts Rick Mastracchio and Dave Williams are due to step outside the ISS at about 12:31 p.m. EDT (1631 GMT) for their planned 6.5-hour construction job in space.

"It's going to be a really exciting spacewalk for us," Williams, a veteran Canadian Space Agency astronaut, said in a preflight NASA interview. "We're both highly trained spacewalkers, and we've both been to space, but neither one of us has actually done a spacewalk before."

The two astronauts, working alongside their crewmates inside Endeavour and the ISS, will help install the Starboard 5 (S5) truss to the station's starboard-most edge. The new addition will prime the space station for the delivery of new solar arrays next year.

Mastracchio will lead today's spacewalk with Williams while their STS-118 crewmate Tracy Caldwell choreographs the excursion from inside Endeavour. Shuttle pilot Charlie Hobaugh will wield the station's robotic arm during the extravehicular activity.

Their crewmates, meanwhile, will prepare for a Sunday inspection of a small gouge in the heat-resistant tiles lining Endeavour's underbelly.

Space station's 'Stubby' piece

Built by Boeing, the S5 truss is a spacer segment about the length of a small compact car. It is designed to serve as a bridge between the station's current starboard solar array segment, delivered last June, and a second solar wing still awaiting launch.

"The S5 truss, actually we call it 'Stubby,' is a little structural piece," said schoolteacher-turned-astronaut Barbara Morgan, an STS-118 mission specialist who waited 22 years for her spaceflight since first joining NASA in 1985 as its backup Teacher in Space.

Weighing in at 4,010 pounds (1,818 kilograms), the $11 million S5 truss is about 11 feet (3.3 meters) long, 14 feet (4.5 meters) wide and stands 13 feet (4.2 meters) tall. Mastracchio and Williams help guide the new truss into position, with only inches of clearance at some points, then remove a series of launch locks and restraints before securing it in place with drill-driven bolts.

The task is a mirror image of the installation of Port 5 (P5) truss - or "Puny" - to the station's port side in December 2006 during NASA's STS-116 mission.

"We watched it intently, of course, to make sure that we understood any challenges they had," Hobaugh said of the STS-116 crew's Port 5 truss installation.

In addition to the S5 installation, Mastracchio and Williams will retract a radiator extending from the space station's mast-like Port 6 truss, preparing the segment for its relocation to the port-most side of the orbital laboratory later this year.

Today's spacewalk is the first of three planned excursions for the STS-118 crew. The successful activation of a new space station-to-shuttle power transfer system Friday gave shuttle mission managers confidence that they will be able to extend Endeavour's flight by three days and add a fourth spacewalk to the STS-118 crew's docket, lead shuttle flight director Matt Abbott said.

Commanded by veteran spaceflyer Scott Kelly, the shuttle astronauts are delivering about 5,000 (2,267 kilograms) of cargo and a new spare parts platform to the ISS in addition to the S5 truss.

Mastracchio has said he expects today's spacewalk and others planned for the STS-118 mission will be the high point of his spaceflight.

"[T]here's going to be nothing but space below me and Earth, and it's going to be a fantastic view," Mastracchio said in a NASA interview. "I'm really looking forward to that."

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: First Spacewalk
• VIDEO: Teaching the Future: Teacher-Astronaut Barbara Morgan
• Complete Space Shuttle Mission Coverage