Two astronauts will float outside the International Space Station late Thursday to attach a new room and long-awaited observation deck to the orbiting laboratory.

Endeavour shuttle astronauts Robert Behnken and Nick Patrick will venture outside the space station at 9:09 p.m. EST to begin more than six hours of construction work to install the new connecting node, called Tranquility, and its window-lined observation portal.

The astronauts had to do some last-minute changes for their spacewalk today when a power harness on Behnken's spacesuit failed. They had to cannibalize a spare from a spacesuit on the space station.

"So I'm basically going out with a different suit that doesn't have that power harness in it," Behnken said late Wednesday in a televised interview. "So it shouldn't give us any more trouble."

The spacesuits are modular so they can be tailored for an astronaut's size and comfort. So Behnken will still be able use his fitted leg, arm and glove attachments from his old suit.

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Behnken is the spacewalking chief of Endeavour's mission. Patrick is making his first spacewalks on the 13-day spaceflight. Both are veteran spaceflyers.

Thursday night's spacewalk is the first of three planned excursions by Behnken and Patrick to install and activate the new Tranquility room and observation deck, which is known as the Cupola.

The new additions are NASA's last big pieces for the space station. They launched aboard Endeavour on Monday and arrived two days later when the shuttle docked at the orbiting lab. The space station will be 98 percent complete once Tranquility and the Cupola are installed.
About the size of a small bus, Tranquility is a cylindrical module about 24 feet (7.3 meters) long and nearly 15 feet (4.5 meters) wide. It was named after NASA's historic Apollo 11 moon base and weighs almost 40,000 pounds (18,143 kg).

The $382 million module will be the home for the station's life support, exercise and robotic arm control systems. It will be attached to the left side of the station's central Unity module.

The Cupola is a seven-window observation portal with a central round pane that is the largest space window ever launched into orbit. The $27.2 million portal launched on the end the Tranquility module and will be moved to an Earth-facing side of the room later in the mission.

All 11 astronauts aboard the linked station and Endeavour are looking forward to the new views of Earth and space promised by the new window unit.

Both Tranquility and the Cupola were built in Italy for NASA by the European Space Agency. The $100 billion space station is the product of 16 different countries and has been under construction since 1998.

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