Astronauts put the last big addition on the International Space Station early Friday, attaching a new room with an enormous bay window that promises to provide unprecedented panoramic views of Earth.

The room, named Tranquility, was hoisted into place by the station's Canadarm 2 robotic arm as a pair of spacewalking astronauts floated nearby, excitedly watching everything take shape.

"We've got a whole bunch of camera views … and every one of them looks spectacular," astronaut Stephen Robinson radioed from inside the shuttle-station complex.

"It looks spectacular from here, too," replied spacewalker Nicholas Patrick.

Patrick and Robert Behnken carried out the first spacewalk of the mission with amazing speed. They were an hour ahead at one point and picked up some extra chores 350 kilometres up.

Behnken pushed out so hard and fast that Mission Control urged him to slow down as he moved over to Endeavour's payload bay and got the new compartments ready for installation on the space station.

The Tranquility hookup job is so big and complicated it will require three spacewalks. Thursday night's excursion — which stretched into the wee hours of Friday — was the first.

Robinson noted it was a "noble start" to this spacewalk extravaganza.

Station 98% complete

Tranquility and the domed lookout represent $400 million in home improvements. The lookout, with its seven windows, including the largest ever sent into space, already has astronauts salivating over the anticipated views of the home planet, as well as their orbital home.

Behnken and Patrick had to wait for the seven metre long Tranquility to be anchored onto the space station before they could hook up power and data cables. The heavy lifting fell to the astronauts inside who operated the Canadarm.

The plumbing will be tackled during a second spacewalk Saturday night.

The dome — which resembles a cupola 1.5 metres deep and nearly 3 metres in diameter — will be moved to its final location on Tranquility next week. Only then will the window shutters be unlocked and raised.

NASA readily acknowledges the observation deck and its 360-degree views will improve the quality of life aboard the orbital outpost, where astronauts spend up to six months at a stretch.

The addition of the Italian-designed Tranquility and dome leaves the space station 98 per cent complete.

Four more shuttle visits remain before the fleet is retired, primarily to stockpile spare station parts and supplies. NASA hopes to wrap everything up by the end of September.