This story was updated at 10:52 a.m. ET.
Two astronauts floated outside the International Space Station early Friday to begin the tricky task of replacing a large, old ammonia tank on the orbiting laboratory?s hull.
Discovery shuttle astronauts Rick Mastracchio and Clayton Anderson had to use a pry bar to pull the new ammonia tank off its mooring in Discovery?s payload bay after it got stuck. But after that, its move to a temporary storage point on the station went smoothly.
?Clay, you did a great job holding that tank steady," the astronauts said.
The spacewalkers spent 6 1/2 hours working outside the station to disconnect an empty ammonia tank on the station and prepare its replacement for installation on a later spacewalk. It was the first of three spacewalks planned for their mission.
?Great job,? Mastracchio said of the orbital work.
The 1,700-pound (771-kg) ammonia tanks are large boxy units filed with toxic coolant for the space station?s thermal control system. The old tank the spacewalkers are replacing is empty, and will be returned to Earth on Discovery to be refurbished and refilled.
Anderson is especially qualified to man-handle the space station?s ammonia tanks.
After all, he tossed a different one off the station in 2007 to discard it while on a six-month mission on the orbiting laboratory. The tank stayed in orbit as space junk for more than a year before burning up in Earth?s atmosphere.
Other spacewalk chores
Mastracchio and Anderson also retrieved a Japanese experiment from the station?s hull and replaced a faulty rate gyroscope sensor.
They had planned to work on the space station?s old port-side solar array batteries after the ammonia tank work, but Mission Control scrapped that chore due to the possibility of a shock risk to the spacewalkers. NASA learned of shock hazard just before Discovery?s flight, mission managers said.
It crops up only when astronauts are working at the very edges of the station, where its outer solar wings are, and when the station is flying over high latitudes of the Earth. At least two spacewalkers worked on the port solar arrays last year before NASA learned of the shock risk, mission managers said.
?The risk was still out there at that time, but the particular environment may not have been present at that time,? lead station flight director Ron Spencer told reporters of the earlier spacewalk. Future spacewalkers will have better-insulated tool kits to avoid the risk, he added.
So instead of battery work, Mastracchio and Anderson tackled some unplanned chores to work on cables and clamps.
?Here?s where our EVA goes different,? said astronaut Dorothy ?Dottie? Metcalf-Lindenburger ? a former teacher - using NASA?s term for spacewalks as she choreographed the work from inside Discovery. ?So we just wanted to say, ?Slow down.??
But the work went swiftly, with the astronauts accomplishing all of their chores. It was the fourth spacewalk for both Mastracchio and Anderson.
Mastracchio said the space station is much larger since his last trip to the outpost in 2007. It took a lot more work to move around, simply because there was a lot more space station.
?It?s like a marathon with your arms,? Metcalf-Lindenburger said.
The astronauts plan to venture back outside the station on their second spacewalk on Sunday. Metcalf-Lindenburger, NASA?s last educator-astronaut expected to fly on a shuttle, will oversee that spacewalk as well.
?She?s a former teacher, so we have to do whatever she says,? Anderson joked with Mission Control after the spacewalk. ?Otherwise she?ll rap our hands with a ruler.?
Inside space work
While the spacewalkers worked outside, it was also a busy day inside the space station as well.
The rest of the 13 astronauts aboard the linked shuttle and station worked to gather to start unloading huge racks of equipment ? each the size of a double-wide refrigerator ? from inside the Leonardo Multi-Purpose Logistics Module (MPLM), a cargo pod delivered to the station on Thursday.
?Outside the station, the first spacewalk is going on. Inside the station, we moved four racks from MPLM to ISS. Big moving day!? station astronaut Soichi Noguchi, who represents Japan, wrote on his Twitter page, where he?s been chronicling his mission as Astro_Soichi.
Discovery launched into orbit Monday and is due to stay docked at the space station until next Friday after completing three spacewalks at the orbiting lab. But the shuttle astronauts will likely get an extra day at the station due to an antenna failure on Discovery that has forced the crew to rely on the space station to beam large data files back to Earth.
The extra day would be used to inspect parts of Discovery?s heat shield late in the mission, a standard check, and send the video and images to Earth for analysis to make sure the shuttle is safe to re-enter Earth?s atmosphere. It would extend Discovery?s space mission to a 14-day flight.
Mission Control radioed the shuttle astronauts late Thursday night to inform them that no extra focused inspection of Discovery?s heat shield will be required while they?re docked at the space station.
Discovery?s mission is one of NASA?s last few shuttle missions before the space agency retires its aging three-orbiter fleet in the fall. After this flight, only three more shuttle flights remain.
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SPACE.com is providing complete coverage of Discovery's STS-131 mission to the International Space Station with Managing Editor Tariq Malik and Staff Writer Clara Moskowitz based in New York. Click here for shuttle mission updates and a link to NASA TV