INTERNATIONAL: Two Expedition 66 crewmates Tom Marshburn and Kayla Barron have exited an airlock of the orbiting research lab, some 400 kilometres above Earth. The two have completed the 6-hour, 32-minute extravehicular activity (EVA) on Thursday, having installed a spare S-band Antenna Subassembly (SASA) on the station's backbone truss in place of a degraded unit, which was stowed on an exterior pallet.

Marshburn and Barron began the spacewalk at 6:15AM switching their extravehicular mobility unit (EMU) spacesuits to internal battery power shortly before exiting the station's U.S. Quest airlock. After gathering up their respective tools, the astronauts then headed to their first planned workstations, facing what NASA called a minimally heightened risk posed by orbital debris left from a Russian missile test weeks ago.

The start of the "extra-vehicular activity" (EVA) followed a 48-hour delay prompted by an orbital debris alert - believed to be the first such postponement in more than two decades of space station history - which NASA later deemed inconsequential.

The origin of the debris in question was left unclear by NASA, though a spokesperson said there was no indication it came from fragments of the defunct satellite that Russia blew to pieces with a missile test earlier this month.

Thursday's outing is the fifth spacewalk for Marshburn, 61, a medical doctor and former flight surgeon with two previous trips to orbit, and a first for Barron, 34, a U.S. Navy submarine officer and nuclear engineer on her debut spaceflight for NASA.

Their objective is to remove a defective S-band radio communications antenna assembly, now more than 20 years old, and replace it with a spare stowed outside the space station.The space station is equipped with other antennae that can perform the same functions, but installing a replacement system ensures an ideal level of communications redundancy, NASA said.

Marshburn will work with Barron while positioned at the end of a robotic arm maneuvered from inside by German astronaut Matthias Maurer of the European Space Agency, with help from NASA crewmate Raja Chari.

The four have arrived at the space station on 11 November in a SpaceX Crew Dragon capsule launched from the Kennedy Space Center in Florida, joining two Russian cosmonauts and a NASA astronaut, Mark Vande Hei, already aboard the orbiting outpost.

The operation was postponed after NASA received notice from U.S. military space trackers warning of a newly detected debris-collision threat. NASA concluded later there was no risk to spacewalkers or the station after all, and the antenna replacement was rescheduled for Thursday morning.

Thursday's exercise marks the 245th spacewalk in support of assembly and upkeep of the space