This story was updated at 12:22 p.m. EDT.
After more than a decade of construction, the International Space Station is spreading its last pair of solar wings Friday as astronauts and Mission Control watch with bated breath.
The new U.S.-built solar arrays are the final piece of the $100 billion space station's power grid. Astronauts attached them to the starboard side of the orbiting laboratory in a Thursday spacewalk, but the real test comes later today, when they push the button inside that will remotely unfurl the power-generating solar wings.
The last time astronauts attempted to unfold similar arrays, one of the wings snagged on a guide wire and ripped, sending engineers on Earth scrambling for a fix. Spacewalking astronauts ultimately stitched the tear back together and the wing has been working ever since.
NASA, and the shuttle Discovery astronauts who delivered the new $298 million solar arrays, are hoping for less drama today.
"We have made solar array deploys exciting in the past," said Dan Hartmann, head of NASA's space station mission management team, late Thursday, adding that his team will be on edge. "Pins and needles? Yes, a little bit — it;s kind of the nature of the game."
The 115-foot (35-meter) long solar arrays are vital since they will complete the station's power grid, boosting the current system by 25 percent, Hartmann said. Their installation Thursday completed the station's 335-foot (102-meter) backbone, which supports the outpost's railcar, robotic arm base and three other sets of expansive solar arrays. In all, the station is designed use all four sets of solar wings to produce enough electricity to power 42 houses on Earth, NASA has said.
Astronauts and scientists are counting on that power supply so they can ramp up science operations and double the station's crew size up to six people later this year. This last set of solar wings should generate about 36 kilowatts total, 15 kilowatts of which is reserved for science. It would double the current science power supply, mission managers said.
And when the new solar arrays are deployed, the station will finally look like the nearly complete orbital outpost depicted in artist illustrations throughout its10-year construction history.
"We're looking forward to it," Hartmann said. "There will be all kinds of emotions and, hopefully, jubilation at the end."
Flight controllers on Earth extended each of the new solar arrays just slightly in the predawn hours early Friday. Astronauts aboard the docked Discovery and station began the deployment process in full at 11:06 a.m. EDT (1506 GMT).
Solar wing challenges
The space station's new solar wings are mostly folded up, much like an accordion or an oversized map, inside a set of storage boxes on the starboard edge of the outpost's backbone-like main truss. They've been locked away in those boxes for years — one for longer than any of the station's seven other wings — and astronauts fully expect their folds to stick together.
NASA has seen the phenomenon — which it calls "stiction" — before, in 2000. That glitch, too, required a spacewalk fix. Now, NASA deploys solar arrays in stages, giving each wing time to warm up so they are less conducive to sticking.
"We are a little concerned because these arrays have been in the box longer than others," said Discovery astronaut John Phillips before flight. Phillips will be the one with his finger on the button, in charge of unfurling the solar arrays or hitting the emergency stop if things go awry.
But he is not the lone solar wing sentinel, space station flight director Kwatsi Alibaruho told reporters late Thursday.
All 10 shuttle and station astronauts will be watching, either through windows, cameras or on computer screens. A host of engineers will be backing them up on Earth, staring at the same views on consoles in Mission Control at NASA's Johnson Space Center in Houston.
The goal, Alibaruho said, is to be ready to spot any rip, tear or glitch from the past, or something new and unexpected, as early as possible.
There is a time limit too: about five hours.
In order to orient the new solar arrays so they warm up - to reduce stiction - the station has to fly in a less optimal orientation. The position can lead to communications drop outs and expose some parts of the station to extreme cold, while risking overheating in others. Mission Control wants to maintain constant communications while the arrays are being extended.
"Right now, the biggest threat that I see, barring some problem with the mechanism, are gaps in communications," Alibaruho said. "We may simply not get through the deployment of both solar arrays before we're required by our mission rules to maneuver back to equilibrium."
If Discovery astronauts are unable to completely extend the arrays today, there is some extra time on Sunday, which is when they were initially due to be unfurled. Mission managers moved the deployment to today to fill in time originally reserved for an extra shuttle heat shield inspection that was no longer needed.
While the solar arrays are extended outside the station, astronauts plan to make repairs to the outpost's urine recycling system inside the orbital lab. The spaceflyers will install a new centrifuge to distill urine back into drinking water. The system is part of a larger water recycler that converts urine, astronaut sweat and condensation back into pure water for drinking, food preparation and bathing.
The fix is in
Even of Discovery's crew hits a snag in today's solar wing deployment, mission managers have a stockpile of repair methods and tools cobbled together from previous missions.
"All those tricks are in our hip pocket, all those tools are in our bag," Discovery's lead spacewalk officer Glenda Laws-Brown said Thursday night. "We have trained the crews on all those prior techniques."
Discovery launched toward the station late Sunday on a 13-day mission to swap out a member of the outpost's crew and deliver the new solar arrays truss. Two of the mission's three spacewalks remain. Four were originally planned, but NASA trimmed the flight due to launch delays in order to complete the construction work and depart the station before the launch of a previously scheduled Russian Soyuz spacecraft next week.
Laws-Brown said she was confident the solar arrays would be unfurled successfully. Watching their installation during Thursday's spacewalk felt like being at the start of a rollercoaster, she added.
"It's exciting and you're thinking 'Oh my, gosh it's too late to get off now, I better hold on for a great ride,'" Laws-Brown said. "I told my team to pay attention and don't miss a minute. This is the good stuff."
SPACE.com is providing continuous coverage of STS-119 with reporter Clara Moskowitz and senior editor Tariq Malik in New York. Click here for mission updates and SPACE.com's live NASA TV video feed.