This story was updated at 10:11 p.m. EDT.
Two schoolteachers-turned-astronauts floated outside the International Space Station Monday to wrap up some unfinished work, but were stymied by a stuck cargo carrier on the orbiting lab.
Discovery shuttle astronauts Joseph Acaba and Richard Arnold II pushed and pulled on the jammed carrier with all their strength, then attacked it with a hammer to push aside a pin that was incorrectly installed in a Saturday spacewalk.
But their efforts were to no avail, perplexing the spacewalkers. Engineers believe the catch securing the mechanism may be too stiff for spacewalkers to budge by hand.
?I just don?t see anything in the way,? one of the astronauts said.
The spacewalkers ultimately lashed down the swing-out carrier, which is intended to hold future spare parts, using sturdy tethers to keep it in place until engineers on Earth can come up with a fix. Mission Control canceled plans for the spacewalkers to swing out a second spare parts platform because of the glitch.
Mission managers said the stuck carrier could remain tied down for years if necessary, but NASA wants to use both cargo platforms to store much-needed spare parts at the station later this fall.
?Certainly, we can't fly that mission that's bringing up these critical spares for us until we get this problem resolved,? lead space station flight director Kwatsi Alibaruho told reporters late Monday from the Johnson Space Center in Houston. Alibaruho said he wasn?t too disappointed with the stuck carrier because there is time in the months ahead to fix it and Discovery?s spacewalkers achieved all their most critical goals.
Acaba and Arnold spent 6 1/2 hours working outside the space station without concern of a piece of space junk that was predicted to buzz by the orbiting lab during their spacewalk. Their piece of mind came on Sunday, when astronauts used thrusters on Discovery to move the docked shuttle and space station clear of the 4-inch (10-cm) piece of a spent Chinese satellite rocket stage.
Monday?s spacewalk was the second for Acaba and Arnold, and the first time a pair of former schoolteachers has ventured out into space together. Both astronauts joined NASA?s spaceflying ranks in 2004 as part of its cadre of educator astronauts and are making their first spaceflight.
?Take your time, enjoy it and do good work,? station commander Michael Fincke told the astronauts as they began their work at 11:37 a.m. EDT (1537 GMT).
In addition to tackling the stubborn cargo carrier, Acaba and Arnold moved an equipment cart from the port side of the space station?s backbone-like main truss to the starboard side. They also greased up the grappling end of the space station?s robotic arm and performed other maintenance work.
Monday?s spacewalk marked the 123rd dedicated to space station construction and the third and last planned for Discovery?s 13-day mission to deliver a new crewmember and the outpost?s last pair of gleaming solar wings.
Acaba and Arnold each performed an earlier spacewalk with astronaut Steven Swanson, who choreographed Monday?s orbital work from inside Discovery. Acaba finished today's work with 12 hours and 57 minutes during his two excursions, while Arnold wrapped up with 12 hours and 34 minutes of spacewalking time.
Altogether, Discovery?s crew spent a total of 19 hours and four minutes working outside the space station.
?Thanks for the hard work,? Discovery commander Lee Archambault told the spacewalkers. ?You?ve left the space station a much better place than it was 6 1/2 hours ago.?
Discovery is due to undock Wednesday from the space station and head toward a planned Florida landing on Saturday.
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.
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