Shuttle Astronauts Gear Up for Third ISS Spacewalk

HOUSTON -- A pair of spacewalking astronauts will venture outside the International Space Station (ISS) Wednesday to prime the orbital laboratory for future construction as NASA works to determine whether a heat shield repair is required for the shuttle Endeavour.

Endeavour shuttle astronaut Rick Mastracchio and ISS flight engineer Clayton Anderson are set to begin their planned 6.5-hour spacewalk at 11:01 a.m. EDT (1501 GMT) to prepare the space station's Port 6 (P6) truss for a major move later this year.

"The tasks on this spacewalk are all geared towards the STS-120 mission preparations," said Matt Abbott, NASA's lead STS-118 shuttle flight director, in a Tuesday briefing. NASA's STS-120- mission is slated to launch in late October with a new room for the ISS.

While the two spacewalkers toil outside the ISS, NASA mission managers expect to complete tests to determine whether repair tasks will be added to another STS-118 spacewalk to fix a small cut in two of the belly-mounted heat tiles along Endeavour's underbelly. The damage was caused by an errant piece of fuel tank debris about one minute after the shuttle's Aug. 8 launch.

John Shannon, chairman of Endeavour's mission management team, said late Tuesday that a preliminary analysis has shown that the orbiter is safe to fly without a repair. But engineers wanted extra time to make sure that landing without a heat shield fix would not cause further damage that could require more extensive repairs once the spacecraft returns to Earth.

A decision on whether a spacewalk repair will be necessary is anticipated for sometime late Wednesday, NASA has said.

Priming P6

Slated to launch in late October aboard the Discovery orbiter, NASA's STS-120 mission will deliver the new Harmony connecting node and feature the relocation of the space station's mast-like P6 truss from its vertical perch to a permanent position on the orbital laboratory's port-most side.

"P6 is going to eventually be moved from the zenith part of station to the portside," Mastracchio said before flight. "So we have to kind of clean up P6."

To do that, Mastracchio and Anderson will pry an S-band antenna from P6 and reattach it to the station's horizontal Port 1 truss segment. The spacewalkers will also move a pair of large equipment pushcarts from their position on the portside of the station's railcar-like Mobile Transporter to the starboard side, which will clear a path for the robotic arm base for future ISS construction.

Mastracchio and Anderson are also due to retrieve a pair of materials exposure experiments and a P6 truss transponder, as well as install new communications gear during their excursion. The spacewalk will mark the third for Mastracchio and the STS-118 mission, as well as the second career excursion for Anderson.

Commanded by veteran shuttle flyer Scott Kelly, Endeavour's 14-day mission is delivering about 5,000 pounds (2,267 kilograms) of cargo, a new spare parts platform and a small spacer segment for the starboard side of the space station's main truss. The shuttle's STS-118 crew includes teacher-turned-astronaut Barbara Morgan, who first joined NASA in 1985 as the backup for Teacher in Space Christa McAuliffe before the Challenger tragedy a year later.

Rejoining STS-118

Today's spacewalk marks a return of sorts for Anderson to Endeavour's astronaut crew.

The Expedition 15 crewmember originally trained to launch to the space station on the STS-118 mission, but he was reassigned to NASA's earlier STS-117 flight - which launched in June - to relieve fellow American spaceflyer Sunita Williams as an ISS flight engineer. But despite arriving ahead of his former crewmates, Anderson said he looked forward to a familiar set of tasks.

"My mission with 118 is essentially unchanged from what it was before," Anderson said in a preflight NASA interview. "The only thing that?s really different is, I don?t spend the first three days, from launch to docking, with the 118 crew -- I do it with 117."

Anderson is slated to perform up to two spacewalks with the STS-118 crew. In addition to today's excursion, he is also set to perform a series of ISS maintenance tasks with STS-118 spacewalker Dave Williams, of the Canadian Space Agency, on Friday.

Prior to Endeavour's arrival, Anderson spent a good portion of his time packing up experiments and cargo to ride back to Earth with the STS-118 crew.

"Clay Anderson is still part of this crew in my mind," Mastracchio told SPACE.com before launch. "Clay just went ahead of us, to get things set up for us."

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: Third Spacewalk
• VIDEO: Endeavour Shuttle Tile Damage
• Complete Space Shuttle Mission Coverage