NASA astronauts attached a Russian docking and research module onto the International Space Station this week, bringing it to near completion.
The compartment, known as Rassvet - Russian for "dawn" - was delivered aboard shuttle Atlantis, which is making the third-to-last planned flight of NASA's shuttle program.
With Rassvet, the station now has 13 rooms, including two core Russian modules, Zarya and Zvezda, three major laboratories - the US's Destiny, Europe's Columbus and Japan's Kibo complex - two airlocks, two combination docking compartments/mini research labs, three connecting hubs, and an observatory, known as the Cupola.
There also is an outside porch for science experiments and a storage room on the Kibo lab.
Atlantis and six astronauts arrived at the station on Sunday for an eight-day stay.
During the first of three spacewalks on Monday, astronauts Garrett Reisman and Steve Bowen installed a spare communications antenna and outfitted the station's Canadian robotic crane with a work platform.
Spacewalks on Wednesday and Friday are devoted to replacing six huge batteries for the station's solar power system.
The shuttle, which blasted off on Friday, is due back at the Kennedy Space Center in Florida on May 26.
NASA says Atlantis would be processed on return as normal just in case she was needed for a "rescue mission" in the event of an emergency with the two remaining shuttle flights.
The agency has two planned shuttle missions remaining. Discovery is due to fly in September with a final load of spare parts and a storage pod that will be left at the station.
Endeavour, on NASA's 134th shuttle flight, will mark NASA's program finale with the delivery of the $2 billion Alpha Magnetic Spectrometer particle detector. The physics experiment will be mounted outside the station.
US space agency officials have not entirely ruled out the possibility of Atlantis taking flight one last time on a comprehensive mission to the ISS next year - provided President Barack Obama gives the go-ahead.
Russia plans to launch its prime research laboratory in 2012, which will complete the $100 billion orbital outpost, a project of 16 nations that has been under construction 350 kilometres above Earth since 1998.