CAPE CANAVERAL, Fla. - NASA completed its part in the construction of the International Space Station on Friday, with the final spacewalkers in the 30-year shuttle program attaching an extension boom.

"Twelve years of building and 15 countries and now it's the Parthenon in the sky and hopefully the doorstep to our future," spacewalker Gregory Chamitoff said before heading back inside. "So congratulations, everybody, on assembly complete."

Chamitoff said it was fitting for the shuttle Endeavour to be at the space station for the end of construction since it was there for the first assembly mission in December 1998.

It was the fourth space walk in a week for the Endeavour astronauts, who are headed back to Earth in just a few days to end NASA's next-to-last shuttle flight. One last flight in July will bring up supplies.

NASA managers on Friday afternoon gave Endeavour the all-clear for an early Wednesday landing. A final review of 3-D images showed the shuttle's delicate heat shield was not damaged.

No other significant U.S. components are due to fly to the space station, which still has a decade of life ahead. That was the job of the space shuttles - to haul up the big building blocks as well as loads of smaller items - and now they're retiring. The last room - a storage closet - was attached earlier this year.

While NASA's role in space station construction is over, the Russian Space Agency plans to add at least one more chamber in another year or two, a job that will require multiple space walks. The Russians also will continue to provide rides to and from the orbiting lab for U.S. astronauts until private companies in America are able to take over the job.

Chamitoff and his spacewalking partner, Mike Fincke, teamed up with robot-arm operator Gregory Johnson to add one last finishing touch.

"Assembly complete. Amazing," Chamitoff said once the 50-foot boom was latched securely in place. "Boy, this is a big space station," he marveled several minutes later. With its solar panels, the station spans a football field.

This was the 164th space walk by shuttle astronauts; the first was performed back in 1983. The bulk - 110 - were for the space station, and 23 involved the Hubble Space Telescope.

Another milestone was achieved Friday: 1,000 hours of spacewalking at the orbiting outpost.