Two NASA spacewalkers swapped out vital solar array batteries on the International Space Station Thursday (July 16), nearly completing the extensive replacement work needed to keep the orbiting lab powered through at least 2024.

NASA astronauts Chris Cassidy and Bob Behnken zoomed through their tasks, removing six aging nickel-hydrogen batteries on the far starboard S6 truss of the ISS. The duo also installed three lithium-ion battery replacements and placed a new high-definition camera outside the orbiting lab. (The lithium-ion batteries are twice as efficient as their predecessors, so only half as many of the new ones are needed.)

The astronauts concluded the 6-hour spacewalk, or extravehicular activity (EVA), at 1:10 p.m. EDT (1710 GMT), about 30 minutes earlier than planned. Near the spacewalk's end, Cassidy and Behnken noticed some possible oddities with the pins holding the Quest airlock hatch in place; the astronauts took pictures for mission control to analyze later. Another NASA spacewalk is planned for July 21.

Related: Spacewalk photos: International Space Station gets a power upgrade

Behnken and Cassidy, each on their ninth spacewalk, were originally supposed to remove five of the six nickel-hydrogen batteries on the S6 truss. But after 4.5 hours outside of the orbiting lab, they were an hour ahead of schedule and had time to tackle a little extra work.

Calling to the duo from NASA's Johnson Space Center (JSC) in Houston, Canadian Space Agency astronaut Josh Kutryk relayed happy news from ground teams.

"There is a plan coming together down here to consider doing the final battery," said Kutryk, who was assigned to guide the spacewalkers through their tasks.

Minutes later, the astronauts dove in, quickly removing the battery. The spacewalkers began joking about the ease of their work: "There's got to be another removal. It never ends."

With laughter ringing behind him in mission control, Kutryk let the crew continue talking for a moment before joining into the banter: "We cautiously share your optimism."

It seemed the worst problem the spacewalkers faced was dealing with the bright sun while pulling the first new lithium-ion battery off an external pallet delivered by Canadarm2, a robotic arm directed by NASA astronaut Doug Hurley from inside the ISS.

"You've got that sun," Behnken commented while watching Cassidy, who was unbolting the battery while directly facing the light.

"Worst sun you can have," Cassidy said, not moving from his perch.

"Right in your face," Behnken confirmed.

Related: The Expedition 63 mission to the International Space Station in photos

Cassidy finished the unbolting and carefully removed his boots from a portable foot restraint holding him in place. Floating free, he added, "I think there was an EVA questionnaire a couple of years ago asking, 'Do you really need this hard [spacesuit] visor?' I think the answer is yes, just like in baseball."

"Copy that," Kutryk laconically responded.

Over the past 3.5 years, multiple spacewalk teams have removed 48 old ISS batteries and replaced them with 24 new ones. One recently installed lithium-ion battery shorted out in April 2019 and will be replaced during a future spacewalk, a NASA TV broadcaster said.

The agency did not disclose a replacement date, but added that the fresh battery is waiting for installation on the space station after being launched on a SpaceX robotic resupply mission in December 2019.

Once that last battery replacement is finished, the long upgrade job will finally be complete, giving the space station ample power to continue work until its planned operational end in 2024. (Several partners, including NASA, are considering extending ISS operations to at least 2028.)

Powering the station depends on keeping the batteries in good shape. The ISS passes into orbital darkness 16 times a day as it circles our planet. When the station's solar panels are not in direct sunlight, the batteries are needed to distribute power for experiments and basic functions like cooking.

The fresh lithium-ion batteries are rated to last 10 years and will therefore need to be replaced less often than the less efficient nickel-hydrogen batteries, which only have about 6.5 years of useful life. Some of the older batteries are now on the external pallet, which will later be released to burn up naturally in Earth's atmosphere. The other old batteries will be stored permanently on the ISS.

This was the third spacewalk conducted during the orbiting lab's current Expedition 63, as Behnken and Cassidy also performed battery replacements on June 26 and July 1. All three spacewalks have worked with lithium-ion batteries shipped to space in May aboard Japan's HTV-9 robotic freighter.

Assisting the crew at JSC was flight director Allison Bolinger, Kutryk and NASA astronaut Anne McClain, who served as ISS CapCom ("capsule communicator"). In space, Hurley and Russian cosmonaut Ivan Vagner assisted with spacesuit operations.

Thursday's excursion was the 230th spacewalk overall in support of ISS assembly and operations, according to statistics provided during the NASA broadcast.

Hurley and Behnken both arrived at the ISS on May 31 aboard a SpaceX Crew Dragon capsule. Their Demo-2 test mission is SpaceX's first-ever crewed spaceflight. Cassidy, Vagner and Russian cosmonaut Anatoli Ivanishin launched to the ISS on a Russian Soyuz spacecraft April 9.

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Spacewalking astronauts replace old space station batteries as part of years-long upgrade - Space.com