Now in the home stretch of a complex, multi-year upgrade, two space station astronauts floated outside the lab complex Thursday and completed the replacement of aging batteries in one of the lab's four sets of solar arrays.With the completion of Thursday's six-hour spacewalk, multiple astronauts participating in 11 extra-vehicular activities, or EVAs, have now replaced 46 of 48 aging nickel-hydrogen batteries with 23 more powerful lithium-ion units.
A replacement for a lithium-ion battery that was damaged in 2019 when a battery charger shorted out has not yet been installed. But a unit flown to the station in January will be installed during a spacewalk later this year, taking the place of the final two nickel-hydrogen units.
That swap out will finally complete an upgrade that began in January 2017. The new batteries, along with additional planned upgrades, are expected to keep the station functioning through the end of the decade if not beyond.
Thursday's spacewalk began at 7:10 a.m. EDT when station commanderand astronaut , floating in the lab's Quest airlock, switched their spacesuits to battery power, officially kicking off the 230th EVA since ISS assembly began in 1998.
After checking safety tethers and collecting tools, the astronauts headed for the far right end of the lab's power truss to continue work started during spacewalksand to replace 12 older nickel-hydrogen batteries at the base of the outboard set of solar arrays with six lithium-ion power packs.
The space station is equipped with four huge solar wings, two at each end of the power truss, that feed electricity into eight power distribution channels. Twelve nickel-hydrogen batteries at the base of each wing, six per power channel, keep the station functioning when it's in orbital darkness.
Starting in 2017, astronauts began replacing the old batteries with lithium-ion units. Because they are more efficient, only six lithium-ion batteries are needed at the base of each solar wing, along with circuit completing adapter plates to take the place of batteries that were removed but not replaced.
During spacewalks in 2017 and 2019, spacewalking astronauts replaced all 24 nickel-hydrogen batteries used by the left and right inboard arrays. But one of the replacement batteries blew a fuse when the charger it was connected to shorted out. That lithium-ion battery was removed and two older units were installed in its place pending launch of a replacement.
The left-side outboard solar wing, meanwhile, was upgraded during spacewalks in 2019 and earlier this year, leaving just the right-side outboard set — 12 batteries feeding two power channels — for Cassidy and Behnken.
They completed the battery work for one power channel during their two earlier spacewalks.
During Thursday's outing, they removed the six remaining nickel-hydrogen batteries and installed all three of the remaining lithium-ion units, along with a final three adapter plates. Cassidy also installed a high-definition camera boom on an inboard power truss.
NASA planners originally thought the battery work would take two spacewalks per power channel, but Cassidy and Behnken ran well ahead of schedule during their first two EVAs and again on Thursday.
They plan to carry out one more spacewalk next Tuesday to make preparations for installation of a commercial research airlock; to install a tool storage box; and to remove two of six no-longer-needed ground-handling fixtures at the base of the solar wings. That will clear the way for future power system upgrades.
Assuming Tuesday's spacewalk runs exactly six-and-a-half-hours as planned, Behnken will move up to third on the list of most experienced spacewalkers with 62 hours and 11 minutes of EVA time over 10 outings. Cassidy's 10-spacewalk mark will stand at 55 hours and 52 minutes, moving him up to eighth in the world.
Cosmonaut Anatoly Solovyev holds the all-time spacewalk record with 78 hours and 21 minutes over 16 EVAs. Retired astronaut Michael Lopez-Alegria is second with 67 hours and 40 minutes over 10 excursions.