CAPE CANAVERAL, Fla. ? Spacewalking astronauts wound up 12 years of International Space Station assembly this past week, putting the finishing touches on a legitimate contender for the greatest engineering achievement of all time.
But many think the $100 billion space station —a project involving 100,000 people in 15 nations on three continents — deserves consideration, too.
After all, the construction site is 220 miles high and requires a rocket-powered commute that's significantly more dangerous than traveling in airplanes, trains, ships or automobiles.
Construction workers labor in a vacuum environment, where one small rip in protective spacesuits could cause instant blood-boiling death.
And the difficulty of the job, some say, far exceeds anything previously attempted by human beings.
"I think the missions we are executing now, in complexity, are the most difficult missions that not just NASA, but any nation, has ever flown in space," NASA Shuttle Program Manager John Shannon said.
"And I would include Apollo in that discussion. I think the missions we do right now are more complicated than what we were doing even during the moon landings."
Many inside and outside NASA once feared that the space station project was simply unrealistic. Undoable.
There were serious concerns about the agency's ability to conduct the staggering number of highly hazardous spacewalks required to raise the outpost.
During the decade that encompassed the Gemini, Apollo and Skylab programs, NASA performed 38 extravehicular activities, or EVAs, average of about four spacewalks per year.
In the first 18 years of shuttle operations, NASA's annual average was two.
Consequently, the projections for International Space Station assembly were downright frightening: 20 to 30 annual outings in the deadly vacuum of space would be required.
PowerPoint bar charts graphically showed the spacewalk spike, which widely became known as "The Wall Of EVA."
The sheer number of spacewalks dwarfed NASA's existing experience base.
"It was just this unbelievable step-up in terms of the amount of work that we had to do and the number of EVAs," said NASA Space Station Flight Director Derek Hassmann.
"It was like nothing we had ever tried before."
LeRoy Cain saw the wall looming during a post-Columbia visit to a Kennedy Space Center high bay filled with grounded space station modules, truss segments and solar wings awaiting the shuttle fleet's return to flight.
NASA's deputy shuttle program manager strolled across an elevated catwalk that gave him a good vantage point of the jam-packed floor.
"It really gave me pause it really made me stop and think about the task we had in front of us," Cain said. "Every one of those pieces of hardware involved one or more spacewalks to assemble" in orbit.
NASA tackled the spacewalks one at a time. Astronauts spent hundreds of hours practicing in a giant pool near Johnson Space Center in Houston — the closest Earthly proxy for the weightless environment in low Earth orbit.
Five to 10 end-to-end practice runs typically were performed for every single station assembly spacewalk, and piece by piece, the modular outpost kept getting bigger and bigger.
"We worked through one spacewalk at a time, choreographing all these things and every potential contingency in a level of detail that makes your eyes water," Cain said.
Once an annual standard, three to four spacewalks became the norm for most space station construction crews.
Add it all up and there were 159 spacewalks done during the assembly of the station, the first two building blocks of which were linked in low Earth orbit in late 1998.
About 70 spacewalking astronauts and cosmonauts from the U.S., Russia, Japan, Canada, Germany, France and Sweden tallied 1,002 hours and 37 minutes building the outpost. The final ever spacewalk by a shuttle crew wrapped up early Friday.
Now, the station weighs 1 million pounds. It has a metallic backbone that stretches 335 feet — the length of an football field. Its massive solar panels stretch 240 feet from tip to tip — longer than the wingspan of a Boeing 777 jumbo jet.
With the same habitable volume as a five-bedroom house, the station has its own wastewater treatment plant and an electrical power system that could light up a neighborhood with 55 homes. It sleeps six people on long stays; up to 12 or 13 for shorter visits.
Rotating crews have staffed the station round-the-clock since the first tenants opened the outpost in November 2000.
At the same time, all those spacewalking excursions largely came off without a hitch.
"Pretty amazing overall, when you look at what we have on orbit today," Cain said.
"We've said it over and over again: You kind of pinch yourself, and it's something for the (assembly) team to be really, really proud of. And certainly the station is a huge part of the shuttles' legacy."
So how does it rate among the greatest engineering achievements of all time? How does it size up with the Channel Tunnel, the Transcontinental Railroad, Boston's Big Dig or the Hoover Dam?
"For me, I think the Apollo moon landing will always be the pinnacle — for whatever reason. Just my personal perspective," Hassmann said.
"But I would certainly put the space station right up there with anything else we've accomplished."