Spacewalking astronauts have made urgent repairs to the International Space Station, three days after a critical relay box abruptly failed.
The 250-mile-high replacement job fell to commander Peggy Whitson, the world's most experienced female astronaut, as she equalled the record for most spacewalks by an American at 10.
Even though a second relay box managed the data load after Saturday's breakdown, Nasa scrambled to put together a spacewalk to restore back-up capability.
The system is vital for operating the station's solar panels, radiators and robotic equipment.
The failed data-relay unit - recently refurbished with upgraded software - was installed in March.
Hauling out a spare, Ms Whitson photographed the faulty device to help engineers figure out what went wrong, then she quickly removed it and bolted down the spare, an identical 50lb box measuring 14in by 8in by 13in - officially known as an MDM or multiplexer-demultiplexer.
Ms Whitson and colleague Jack Fischer carried out a previous spacewalk on May 12, but that excursion was cut short by leaking station equipment, leaving two antenna installations undone. Mr Fischer completed the task on Tuesday.
The latest spacewalk was expected to last no more than a few hours - exceedingly short by Nasa standards.
"Here we go again," French astronaut Thomas Pesquet said on Twitter as his US colleagues suited up for the spacewalk. As they ventured outside, he cautioned: "You guys be safe."
It was only the second spacewalk for Mr Fischer, a rookie astronaut who arrived at the orbiting lab last month.
"What's more awesome than being on @Space_Station? Getting a call from mission control 4 another spacewalk! Dancing w/ the cosmos," he said in a tweet.
After he had installed the antennas, he radioed, "Oh my gosh, it's so beautiful," as the station sailed out over the tip of South America and over the South Atlantic.
Ms Whitson is more than halfway through a nine-month mission. On her third spaceflight, she has spent more time off the planet than any other American and, at 57, is the oldest woman to fly in space.
The latest excursion put her in third place on the all-time spacewalking list .