This story was updated at 11:49 a.m. ET.
HOUSTON ? Two astronauts battled with a stubborn bolt early Sunday while hooking up a massive new ammonia coolant tank outside the International Space Station during the second of three spacewalks dedicated to the in-space maintenance chore.
Discovery shuttle astronauts Rick Mastracchio and Clayton Anderson ventured outside the station?s Quest airlock at 1:30 a.m. EDT (0530 GMT) filled with confidence for what turned out to be a 7 1/2-hour service call on the orbiting lab.
?I think we're very well prepared,? said Anderson, 51, commenting from orbit about the one day?s rest they had between spacewalks. ?We're in pretty good shape for old men and I think we'll be ready to rock and roll.? It?s the fifth career spacewalk for both Anderson and Mastracchio, 51.
But the stuck bolt and snags slowed the new 1,700-pound (771-kg) ammonia tank?s installation, as well as the removal of an empty tank that will be returned to Discovery?s payload bay during the astronauts? third spacewalk on Tuesday.
?Crud, you silly beast. Jiminy Christmas!? said Anderson, as he got hung up on a lanyard and pin.
?Just go slow. It?s fine,? replied teacher-astronaut Dorothy ?Dottie? Metcalf from inside Discovery, where she choreographed the work. ?There's no rush."
The spacewalkers ran out of time before they could hook up the fluid lines that transport liquid ammonia coolant from the tank to the station. They also had to drop some other minor tasks.
The spacewalk drama unfolded on the 40th anniversary of the launch of NASA?s Apollo 13 mission to the moon. That mission took an unexpected turn when an oxygen tank exploded en route to the moon, crippling the spacecraft and leaving its three-man crew in peril. Engineers worked furiously on the ground to rescue the astronauts, successfully returning them to Earth in one of NASA?s most memorable missions.
Mastracchio and Anderson said the biggest challenge of today?s work was the choreography between man and machine to coordinate when spacewalkers would work on the tanks, and when refrigerator-sized units were grappled by the station?s robotic arm.
?The biggest challenge is just the integration of all the robotics,? Mastracchio said in an interview Friday. ?It's going to require a lot of teamwork to get that to work out smoothly.?
Discovery mission specialist Stephanie Wilson and pilot Jim Dutton controlled the station?s arm during the spacewalk.
Because of the location of the old ammonia tank assembly, the station's robotic arm could not reach it from the same location that it had to be in to remove the new tank from space shuttle Discovery's payload bay on Friday. That meant that the spacewalkers had to unpack and store the new assembly, then come back inside the station while the Canadian-built robotic arm was repositioned.
"You guys are doing great,? said shuttle commander Alan Poindexter. ?It's really awesome to see you working."
After removing the old ammonia tank assembly, Mastracchio and Anderson temporarily stored it on a tool cart. During the third spacewalk, Mastracchio and Anderson will pack the old assembly into shuttle?s cargo bay for its return to Earth.
The spacewalkers were relieved as soon as the old ammonia tank was grappled by the station?s robotic arm for its Tuesday move.
?I?m glad to get that out of my hands,? said Mastracchio.
?Oh, you got that right,? Anderson replied.
After two straight spacewalks with ammonia tank difficulties, Metcalf-Lindenburger said it was time for the old, empty tank to behave on the third spacewalk.
?We?ll see,? both spacewalkers said.
Sunday?s spacewalk was the 142nd dedicated to space station assembly since construction on the nearly complete $100 billion orbiting lab began in 1998. One more spacewalk is planned for Discovery?s crew on Tuesday.
While Mastracchio and Anderson focused on the spacewalk activities, Japanese astronaut and ?loadmaster? Naoko Yamazaki will continue overseeing the transfer of supplies, equipment and experiments from the Leonardo logistics module that Discovery launched and temporarily attached to the station.
So far, the astronauts are just about halfway through delivering the nearly 17,000 pounds (7,711 kg) of science equipment, supplies and other gear to the space station. The astronauts also successfully repaired the space station?s water processing assembly, restoring the orbiting lab?s water recycling system, mission managers said.
A spare part launched on Discovery was used to make the water system repair.
Discovery blasted off last Monday and is in the midst of a 14-day flight to the International Space Station and due to land in Florida on April 19.
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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