An STS-119 spacewalker works outside the International Space Station at the start of the mission's second spacewalk.
Credit: NASA TV
This story was updated at 9:45 p.m. EDT.
Two spacewalking astronauts ventured outside the International Space Station Saturday to set the stage for future construction and prime the outpost for Japan?s first space freighter.
Discovery shuttle astronauts Steven Swanson and Joseph Acaba pushed through a packed list of maintenance chores to ease the workload for future spacewalkers. They started with a trek to the very edge of the station?s port side to loosen bolts the outpost?s oldest solar array batteries.
An alarm rang out as the spacewalkers completed the battery work, but flight controllers said it meant the orbiting lab?s attitude-controlling gyroscopes were overwhelmed by the work on the end of the station?s backbone-like truss, which is longer than a football field. Mission Control turned control of the station over to the shuttle Discovery?s thrusters.
?Nothing to worry about,? Mission Control told the spacewalkers.
Later, an incorrectly stowed metal pin blocked the spacewalkers efforts to set up a spare parts platform on the station?s main truss. The swing-out cargo carrier was one of two the astronauts hoped to prepare during the spacewalk, but Mission Control told the spacewalkers to abandon both tasks and tie down the half-deployed platform for the time being as time ran short.
?Great job today,? said astronaut Richard Arnold II, who choreographed the spacewalk from inside Discovery. ?Sorry for all the changes we threw at you.?
Space station flight director Kwatsi Alibaruho told reporters late Saturday that, despite the platform glitch, the spacewalkers accomplished the critical chores on their excursion.
Saturday?s 6 1/2-hour spacewalk began at 12:51 p.m. EDT (1651 GMT) as the linked space station and shuttle Discovery flew 220 miles (354 km) above the South Pacific. The excursion came one day after the astronauts unfurled the space station?s last pair of gleaming solar wings, with Mission Control lauding the work in the Discovery crew?s morning mail.
?[The station] now looks like the artist renderings that we've been seeing for years,? Mission Control wrote. ?A day to celebrate!?
It was also a big day for Acaba, a former Florida schoolteacher, who made his spacewalking debut.
?Joe, great seeing you outside,? said Arnold, who is also a fellow teacher-turned-astronaut.
?Thank you,? Acaba called back.
Upgrading space station
During their orbital work, Swanson and Acaba installed a new global positioning antenna on the outer hull of the station?s Japanese-built Kibo laboratory. The antenna is one of two that will aid the docking of Japan?s first space freighter when it arrives at the station later this year.
They also performed the bulk of their planned maintenance on a cable panel, as well as an infrared camera survey of the space station?s radiators, one of which has been damaged. But they ran out time to finish some of those tasks.
While working on the station?s portside solar array batteries, the spacewalkers routinely checked to make sure no metal from their spacesuits was exposed as a safeguard against the remote chance of electrical shocks. Mission managers said the risk was small and within safety limits, but the extra measure were just a precaution.
The spacewalk was the second of three planned during Discovery?s mission, with some of the chores pulled from a planned fourth spacewalk that was later cut from the flight due to launch delays.
?We sure appreciate the work you did for our beautiful space station,? station skipper Michael Fincke told the spacewalkers. ?You guys proved flexibility is definitely key.?
Mission managers are now working to replan the third spacewalk on Monday and determine whether the reattempt the spare parts platform set up work.
NASA trimmed the spacewalk and a day from Discovery?s now 13-day flight in order to complete the shuttle flight before the arrival of a Russian Soyuz spacecraft next week carrying new station crewmembers and an American space tourist.
Space urine recycler fix
While the spacewalkers worked outside, astronauts inside the station tested repairs to the outpost?s urine processer. Astronauts replaced a broken distillation assembly, which spins like a centrifuge to begin filtering urine back into drinking water, on Friday.
"You can see the same good spinning," Fincke told Mission Control during a dry run of the device. "We could barely hear any change in noise, which is much different than the last time we did this."
The urine processor is part of a larger recycling system designed to convert astronaut urine, sweat and condensation into pure drinking water that can be used for drinking, food preparation, bathing and oxygen generation.
Saturday?s orbital work marked the 122nd spacewalk dedicated to space station construction and maintenance. It was the fourth career spacewalk for Swanson, who now has 26 hours and 22 minutes of spacewalking time, and the first for Acaba. Swanson and Arnold will perform the mission?s third spacewalk on for Monday
Discovery is due to leave the space station on Wednesday after eight days docked at the space station. The shuttle is due to land on March 28.
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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