The International Space Station (ISS) hit a milestone for human spaceflight Wednesday, marking five years of continuous human habitation in Earth orbit.

On Nov. 2, 2000, the first three long-duration astronauts took charge of the ISS, beginning an unbroken chain of missions that stretches across 12 expeditions and has weathered one NASA disaster, a crew cutback and a series of construction delays.

Today, ISS Expedition 12 commander Bill McArthur and flight engineer Valery Tokarev are approaching the end of their first month in orbit. The two astronauts are serving a six-month term aboard the ISS almost seven years after the first piece of the station - the Russian-built Zarya control module - launched into orbit on Nov. 20, 1998.

"We are proud to celebrate an important accomplishment in space," Tokarev said during a recorded statement by the Expedition 12 crew for the station's fifth anniversary. "This would not have been possible without the cooperation of the 16 partner nations."

Still unfinished, the ISS is a cooperative effort between NASA, Russia's Federal Space Agency and the multi-national European Space Agency (ESA), as well as Canada, Brazil and the Japan Aerospace Exploration Agency (JAXA).

Image Gallery: Building the International Space Station

"The international nature of the thing is totally essential," said Alan Thirkettle, head of development for the ESA's Directorate of Human Spaceflight, in a telephone interview. "We wouldn't have been able to build a space station ourselves."

It's taken about 17 shuttle flights, a series of unmanned Russian supply ships and 62 spacewalks to build and maintain the ISS so far. Expedition 12's McArthur and Tokarev plan to make their first spacewalk on Nov. 7.

NASA spokesperson Melissa Matthews told SPACE.com Tuesday that, to date, the ISS has cost the U.S. space agency about $23.5 billion - research costs excluded - though ESA projections state the station's total cost could exceed $100 billion spread across the participating nations. NASA hopes to launch 18 more shuttle flights to the ISS and complete construction by 2010, when the agency plans to retire the orbiters, as well as fly one Hubble servicing mission.

The unfinished outpost

Despite an interior living space about the size of a three-bedroom home, the ISS remains far from complete.

A series of truss segments and JAXA's Kibo experiment module currently wait to launch toward the ISS aboard a future shuttle flight at the agency's Kennedy Space Center in Florida. The Columbus laboratory module, built for the ESA by EADS Space Transportation, sits all-but completed in its Bremen, Germany plant, ESA officials said.

Station construction has been waylaid first by NASA's tragic loss of the space shuttle Columbia and its crew in 2003, then by delays caused by multiple hurricanes and ongoing work to limit the amount of potentially harmful debris shed by orbiter fuel tanks during launch - a problem that doomed Columbia and appeared during the launch of Discovery's STS-114 mission on July 26. Discovery's flight marked the first shuttle to visit the ISS since the Columbia accident.

Prior to the Columbia disaster, station planners were targeting 2004 for the completion of the U.S. contribution to the project, NASA officials told SPACE.com.

NASA's three remaining orbiters - Discovery, Atlantis and Endeavour - are vital to the space station's construction since only they can haul many of the heavy modules and supply containers to the orbital research platform. But the 2010 retirement date has limited the number of flights - once slated to require about 28 missions - that may be available.

"We've talked to the partners about the results of the shuttle [schedule] and station configuration options, and that is one of the key elements," explained NASA spokesperson Debra Rahn, of the agency's Washington D.C. headquarters. "We're still working with the international partners on the order of the flights."

Rahn said at least two large components - a centrifuge module built for NASA by the Japanese Aerospace Exploration Agency (JAXA) and Russia's Solar Power Platform - have been cut from the shuttle's manifest. Any more changes could be discussed during a planned meeting of the "Heads of Agencies" meeting in early 2006, she added.

Other items, like the space station's Cupola window, that sit at the end of the shuttle's launch manifest could also be at risk of not flying should delays prompt further flight reductions, Matthews said.

International plans

Some of NASA's international partners are looking not only toward the resumption of ISS construction, but also space station landmarks of their own.

ESA officials are eager to see NASA's STS-121 mission launch in May 2006 since it will carry the space agency's first long-duration astronaut, Thomas Reiter of Germany, to the orbital outpost. Reiter's planned flight will not only extend European space science missions beyond their eight-day ISS stays, it will also allow ESA scientists a chance to perform some science originally slated for the Columbus module.

"We're desperately anxious to get Columbus up," Thirkettle said. "Having 10 tons of hardware sitting around in Bremen, or even in Florida, doesn't do a lot for you."

Thirkettle concedes that there has been some benefit from the ISS construction delays. The added time has allowed engineers to add new, up-to-date equipment to the module and address issues with the interior finishing of the Columbus module at its factory.

"It gave us the opportunity of completely cleaning that up and replacing it rather than doing a botch up and keeping our fingers crossed," Thirkettle said, adding that science payloads are once again being reinstalled into Columbus.

ESA engineers are also recertified the space agency's European Robotic Arm - an 36-foot (11-meter), seven-jointed robot arm - to launch aboard a Proton rocket with the Russia's Multipurpose Laboratory Module in November 2007. The 1,388-pound (630-kilogram) arm was slated to ride a shuttle into space as part of the Russian power platform, but was reassigned to the MLM after the module's redesign, Thirkettle said.

"It's like the station's arm, capable of walking from base point to base point," Thirkettle said, adding that it will be able to pluck payloads from an airlock aboard the new Russian module's and place them on the ISS exterior.

Meanwhile, the ESA has experienced some delays of its own. The agency's first Automated Transfer Vehicle (ATV), an unmanned cargo ship designed to resupply the ISS, will not launch toward the space station until at least May 2007 after inspections turned up multiple hardware and software problems.

Japan is also developing its own H-2 Transfer Vehicle (HTV) to deliver additional cargo to the ISS. Both the ATV and HTV will complement Russia's unmanned Progress vehicles that steadily resupply the space station today.

Staging ground

Last week, NASA astronauts lauded the space station as a testing ground for the space agency's goal of returning humans to the Moon and pushing out toward Mars.

During the two-year gap in ISS-bound shuttle flights, while NASA worked to recover from the Columbia accident, station crews were reduced to two astronauts, which led to the first spacewalks ever to leave the orbital platform empty of humans, additional ground control operations of the ISS, and a series of repairs that would have previously been performed back on Earth only after the faulty hardware had been shipped off the outpost. In 2004, for example, Expedition 9 flight engineer Michael Fincke demonstrated that astronauts could make unplanned, meticulous repairs of the U.S.-built spacesuits in orbit.

"One of the things we're really learning from the space station is how things break," said Expedition 7 flight engineer Ed Lu during the Oct. 27 event, adding that much of the equipment aboard the ISS is being flown for the first time. "The only way to get to the reliability levels we need is to have these things fail, and then iron out the bugs. I think we're doing that."

Lu served aboard the station's first two-astronaut crew - down from three astronauts - which has subsequently been followed since by five others. The loss of an extra person has limited the amount of scientific research astronauts are able to perform, since much of their time is devoted to maintenance and other activities. Expedition 11 flight engineer John Phillips, who returned to Earth with mission commander Sergei Krikalev on Oct. 10, told SPACE.com that he was unable to perform all of the experiments he'd hoped due to time pressure.

"I think that with a two-person crew here, I was optimistic," he said in an Oct. 6 interview.

But the astronauts agreed that the experience of assembling massive structures in space, learning how the human body copes with long duration spaceflight and the daunting task of many nations working together toward a single orbital goal will pave the way for future space explorers to reach beyond Earth orbit.

"We're doing something really special that's bigger than any one of us," Fincke said last week. "Here's to five more years and beyond."

• NASA Tackles External Tank Tests, Aims for May '06
• Shuttle Flight Delays Complicate European ISS Plans
• Shuttle's Retirement May Affect ISS Construction

SPACE.com's archive of stories from previous ISS expeditions:

Exp. 12 | Exp. 11 | Exp. 10 | Exp. 9 | Exp. 8 | Exp. 7 | Exp. 6 | Exp. 5 | Exp. 4 | Exp. 3 | Exp. 2 | Exp. 1