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A decade ago, the United States docked its first module, Destiny, to an embryonic International Space Station. Tonight, the space shuttle Discovery is set to launch an American segment that could be called “Finally!”

The last major US-built component – a 15.5-ton truss bearing the station’s last set of solar panels – is nestled snuggly in Discovery’s cargo bay, awaiting the orbiter’s launch, now scheduled for Thursday night after Wednesday’s planned launch was scrubbed due to a hydrogen gas leak.

Installing the final solar array is a critical step in preparing the station to hold its full complement of six crew members and run all the experiments the international partners have planned.

At least seven more shuttle missions remain to put the finishing touches on the international project before the shuttles are retired by the end of 2010. Still, this mission has a milestonelike feel to it, says Paul Dye, the shuttle mission’s lead flight controller.

“When we got started on this, it was a dream,” he says. Now, after watching it grow for more than 10 years, the prospect of seeing a virtually finished product “is just amazing,” he adds.

Thursday’s launch – which should be visible to people living along the US East Coast for about eight minutes after liftoff – was originally set for Feb. 12. But mission managers delayed the launch to deal with a problem that cropped up during the shuttle Endeavor’s mission to the space station in November.

During Endeavour’s launch Nov. 14, controllers noticed that the shuttle’s hydrogen fuel tank was returning odd pressure readings. After Endeavour and its crew returned, technicians found that a small piece of an important valve had broken off. The valve, about the size of a pop-up lawn sprinkler, is one of three that control the flow of hydrogen gas siphoned from the shuttle’s main engines and returned to the fuel tank to keep it pressurized.

Engineers traced the problem to metal fatigue. The concern: In certain scenarios, a broken piece of valve could lead either to the main engines shutting down prematurely or to the main fuel tank exploding. So managers opted to replace similar, heavily used flow-control valves on Discovery with newer ones.

The hydrogen gas leak discovered Wednesday was traced to the fuel lines outside the shuttle and is unrelated to valves, NASA officials said.

Discovery’s seven-member crew faces a full schedule during the 15-day mission, with a half-day break on the mission’s 10th day. The activities include four spacewalks. Besides installing the final set of solar panels – which carried a price tag of $289 million – spacwalking astronauts are slated to perform a variety of maintenance tasks. Inside, astronauts will replace a balky waste-water distillation unit. The device is designed to convert wastewater into drinking water.

Once it is fixed, the unit will eliminate the need to bring water from Earth, increasing the amount of other supplies cargo craft can carry. Astronaut-plumbers also will try to flush the station’s drinking-water dispenser, which has been offering water with bacteria counts that are too high for NASA’s liking.

Troubleshooting aside, Discovery’s mission also has an “old home week” feel for two members of the orbiter’s crew.
Mission specialist John Phillips, for instance, has spent time on Russia’s space station, Mir. He also served as flight engineer on the International Space Station for 179 days in 2005. In all, this will mark his third trip to the International Space Station.

“This is going to be different for me,” he acknowledges. “It’s going to be a mixture of a little nostalgia to see the little compartment that I lived in for six months. The little kitchen in the Russian module hasn’t changed. But it’s also going to be really cool to see the bright shiny new modules. I’m looking forward to seeing the whole thing.”

For Koichi Wakata, a Japan Aerospace Exploration Agency astronaut who will remain as a member of the station’s crew when Discovery departs, the contrast between his first mission to the station in 2000 and this mission is even more stark.

“We delivered the first truss to the International Space Station,” he recalls. At the time, no one was living aboard the station. “It was so small. It was a little bigger than my apartment in Tokyo.”

Now, the 305-ton orbiting outpost boasts a solar-panel wingspan of a Boeing 747 airliner and some 13,000 cubic feet of shirt-sleeve living and working space.

“It will be very interesting to see this expanded International Space Station,” he says.

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