During maintenance on an external cooling system, the astronauts had to take extra safety precautions after possibly getting toxic ammonia on their suits.
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Victor Glover and Mike Hopkins had no trouble removing and venting a couple of old jumper cables to remove any ammonia still lingering in the lines. But so much ammonia spewed out of the first hose that Mission Control worried some of the frozen white flakes might have gotten on their suits.
Hopkins was surprised at the amount of ammonia unleashed into the vacuum of space. "Oh yeah, look at that go. Did you see that?" he asked flight controllers. "There's more than I thought."
Even though the stream of ammonia was directed away from the astronauts and the space station, Hopkins said some icy crystals may have contacted his helmet. As a result, Mission Control said it was going to "be conservative" and require inspections.
The astronauts' first suit check found nothing amiss. "Looks clean," Hopkins called down.
NASA did not want any ammonia getting inside the space station and contaminating the cabin atmosphere. The astronauts used long tools to vent the hoses and stayed clear of the nozzles, to reduce the risk of ammonia contact. If necessary, the spacewalkers could spend extra time basking in the sunlight before heading back inside, to bake off any residue.
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Once the hoses were emptied, the astronauts moved one of them to a more central location near the NASA hatch, in case it's needed on the opposite end of the station.
The ammonia jumper cables were added years ago following a cooling system leak.
The hose work should have been completed during a spacewalk a week ago, but was put off along with other odd jobs when power upgrades took longer than expected.
Saturday's other chores included: replacing an antenna for helmet cameras, rerouting ethernet cables, tightening connections on a European experiment platform, and installing a metal ring on the hatch thermal cover.
Eager to get these station improvements done before the astronauts head home this spring, Mission Control ordered up the bonus spacewalk for Glover and Hopkins, who launched last November on SpaceX. They teamed up for back-to-back spacewalks 1 1/2 months ago and were happy to chalk up another.
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Saturday's spacewalk got started almost an hour late. Before going out, the astronauts had to replace the communication caps beneath their helmets in order to hear properly. "I've got you loud and clear," Hopkins said once the new cap was on his head.
It was the fifth spacewalk - and, barring an emergency, the last - for this U.S.-Russian-Japanese crew of seven.
Glover and Hopkins are expected to spend around six and a half hours on a spacewalk to work on the space station's cooling system and communications equipment.
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The Associated Press contributed to this report.