This story was updated at 3:50 p.m. ET.
HOUSTON - Two shuttle astronauts ventured outside the International Space Station (ISS) early Tuesday to complete work to install a massive new coolant tank during the third and last spacewalk of their mission.
Discovery astronauts Rick Mastracchio and Clayton Anderson floated outside the station?s Quest airlock at 2:14 a.m. (0614 GMT) for their last 6 1/2-hour service call on the space station.
Their first task: Hooking up four hoses to funnel liquid ammonia coolant and nitrogen for the space station?s onboard systems.
Mastracchio, 50, tackled the job in short order, even pausing at times while Anderson snapped photos.
?Oh baby, you?re going to want to take this one to the grandkids,? said Anderson, 51, after taking one particularly picturesque shot.
It was quick work for the spacewalkers, but Mission Control hit a snag while activating the new ammonia tank. A nitrogen valve appeared to be stuck, NASA officials said.
Space station flight director Ron Spencer said the stuck nitrogen valve is more than an annoyance. If Mission Control cannot free it remotely, space station astronauts will likely have to replace the nitrogen component of the hulking, ammonia tank, which is the size of a large refrigerator.
The space station uses nitrogen to pressurize its tanks of liquid ammonia coolant. It has two spare nitrogen system parts on storage shelves attached to the orbiting laboratory's backbone-like main truss.
Spencer said that replacing the nitrogen assembly with a spacewalk is a last resort since it could be a serious interruption to current shuttle mission, the space station crew and potentially NASA's next shuttle flight, which is slated to launch May 14. But if NASA does not fix the problem ? the valve itself can't be reached by spacewalkers, only its nitrogen system housing ? half of the space station's systems will have to be switched off.
"We do have a couple of tricks up our sleeves," Spencer said.
While Mission Control wrangled with the stuck valve during the spacewalk, Mastracchio and Anderson tackled their own obstacle in space. A stubborn bolt held up their work to install an empty ammonia tank (the one they replaced) in Discovery?s payload bay to be returned to Earth.
Bolts have been familiar foes for the astronauts, who have had to use pry bars and brute strength to defeat them on two earlier spacewalks. This bolt was misaligned, and it took both spacewalkers extra time to loosen another bolt, fix the misalignment and retighten everything back in place.
?Great job you guys,? said teacher-astronaut Dorothy ?Dottie? Metcalf-Lindenburger, who choreographed the spacewalk from inside Discovery. ?Now I can finally say good job, we have the ammonia tank in the payload bay.?
Mastracchio and Anderson also had some trouble with the space station?s airlock hatch at the start of their orbital work. But after checking the hatch?s handle (to make sure it was in place) they told Mission Control that all was well.
Discovery?s last spacewalk
The spacewalk was the sixth career spacewalk for both Mastracchio and Anderson, and the 143rd spacewalk dedicated to space station assembly since construction began in 1998.
Both men have about 38 1/2 hours of spacewalking work under their belts. In all, they spent 20 hours 16 minutes working outside Discovery and the space station on this flight.
Tuesday?s spacewalk was also the last ever planned for the shuttle Discovery. When the shuttle next flies to the space station (a delivery mission slated for September), there are no spacewalks planned. That September flight is expected to be the last flight for Discovery and NASA?s final space shuttle mission.?
A couple of hours into the spacewalk, Anderson noticed some wear on one of his spacesuit gloves, but Mission Control said it was no problem unless the bottom layers on the glove were cut.
?I?ve got 51-year-old eyes but it doesn?t look like it?s cut,? Anderson said.
astronauts retrieved some unneeded aluminum debris shields from the space
station?s exterior, chores left over from the astronaut duo?s earlier spacewalk
Anderson was unable to hitch a ride on the station?s Canadian-built robotic arm to the European Space Agency?s Columbus laboratory to retrieve an experiment because time was short. Instead, he prepared some tools that would be needed on NASA?s next shuttle mission in May.
Mastracchio prepared an antenna for installation during a future shuttle mission, before checking on a beam that was installed during an earlier spacewalk that flight controllers have noticed wobbling. The astronauts were spacewalking on the 40th anniversary of the oxygen tank explosion that crippled the Apollo 13 mission in April 1970.
The two spaceflyers made sure to take time to enjoy their work in space, since it could be the last time they see a shuttle in space from the outside. The views of Earth from space were astounding, they said.
?Wow, what a view,? said Anderson.
?It?s incredible, isn?t it?? Mastracchio replied.
?The Earth is a beautiful place,? Anderson said. ?It?s too bad more people can?t have that view. Maybe, one day.?
Meanwhile, inside the space station the spacewalkers? crewmates have been hard at work transferring some of the 17,000 pounds (7,711 kg) of cargo Discovery hauled to the space station inside its Leonardo cargo pod.
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.
The mission is one of NASA?s last few shuttle missions before the fleet is retired in September. Only three more shuttle flights are scheduled after this one.
NASA?s space shuttle fleet is the only spacecraft currently flying that is large enough to haul parts like the ammonia tanks, which are the size of large refrigerators, to the International Space Station. Once the fleet is retired later this year, NASA will be dependent on Russia to ferry astronauts to and from the station until American commercial spacecraft become available.
President Barack Obama is expected to discuss the future of NASA?s space exploration plan Thursday during a summit in Florida.
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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