Two NASA astronauts finalized their preparations today, ahead of Thursday’s spacewalk, to complete battery swaps on the outside of the International Space Station. NASA Flight Engineer Bob Behnken and Expedition 63 Commander Chris Cassidy organized their tools and readied the Quest airlock for the spacewalk set to begin tomorrow at 7:35 a.m.

NASA astronaut Doug Hurley joined the duo Wednesday afternoon for a spacewalk review and conference with specialists on the ground. Hurley will assist the astronauts in and out of their U.S. spacesuits and monitor their spacewalk activities.

The trio will stay in readiness mode for a second spacewalk scheduled to begin at the same time on Tuesday, July 21. They will finish swapping the aging nickel-hydrogen batteries with new lithium-ion batteries on the station’s truss structure that began 3.5 years ago. The veteran spacewalkers will then set up the Tranquility module for the upcoming installation of a NanoRacks airlock. The new commercial airlock will support public and private experiments exposed to the space environment.

All three astronauts started the day with standard health checks ahead of their spacewalk. Hurley took on the crew medical officer role and briefly examined his crewmates similar to a doctor conducting a checkup on Earth.

Meanwhile, cosmonauts Anatoly Ivanishin and Ivan Vagner were back on human research duty this morning. The Russian duo collected and stowed blood and saliva samples for a pair of studies looking into bone loss and the immune system. The pair then split up as Ivanishin checked out communications gear and Vagner worked on Progress 75 resupply ship cargo transfers.