During the first of three spacewalks planned during Endeavour´s 13-day mission, astronauts Robert Behnken and Nicholas Patrick prepared the 18-tonne module to be transferred from the shuttle´s cargo bay to the station.

Crewmates Kay Hire and Terry Virts then used the station´s robot arm to install the module onto the station´s Unity node, the last major assembly task for the U.S. portion of the $100 billion orbital outpost.

The station, a project of 16 nations, had been under construction 220 miles 354 km above Earth since 1998.

Endeavour lifted off from the Kennedy Space Center on Monday with the connecting hub, named Tranquility, and a seven-sided viewing port, slated to be installed later this week.

Tranquility, made in Italy, takes its name from the site where U.S. astronauts first landed on the moon in 1969.

Tranquility will be outfitted as a second habitation module for the live-aboard station crew, housing a toilet, oxygen generator, air scrubber and water recycling system.

Four more shuttle missions remain to finish outfitting the station before NASA retires its three-ship fleet at the end of the year.