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Two Nasa astronauts ventured out on a spacewalk on Thursday to replace a faulty antenna on the International Space Station, facing what Nasa called a minimally heightened risk posed by orbital debris left from a Russian missile test weeks ago.

The planned 6.5-hour spacewalk began at 7.10am Eastern time (1210 GMT) as astronauts Thomas Marshburn and Kayla Barron left an airlock of the orbiting research lab about 400km above Earth to begin their work.

“Sun’s coming up and you will be over South America in a couple of minutes,” mission control told the astronauts as they exited the airlock.

The start of the “extravehicular 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, though a Nasa spokesperson said there was no indication it came from fragments of the defunct satellite that Russia blew to pieces with a missile test earlier in November.

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 US 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. It 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 manoeuvred from inside by German astronaut Matthias Maurer of the European Space Agency, with help from Nasa crewmate Raja Chari.

The four arrived at the space station on November 11 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.

Four days later, an antisatellite missile test conducted without warning by Russia generated a debris field in low-Earth orbit, forcing the seven ISS crew members to take shelter in their docked spaceships to allow for a quick getaway until the immediate danger passed, Nasa said.

The residual cloud of debris from the blasted satellite has since dispersed, according to Dana Weigel, Nasa deputy manager of the ISS programme. Nasa has calculated that the remaining fragments continue to pose a “slightly elevated” background risk to the space station as a whole, and a 7% higher risk of puncturing spacewalkers’ suits, as compared to before Russia’s missile test, she told reporters on Monday.

Nasa has determined that those risk levels fall within an acceptable range and moved ahead with preparations for a spacewalk on Tuesday as originally planned, only for mission control to delay the EVA hours before it was to start.

The operation was postponed after Nasa received notice from US 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 station, and the first on record delayed due to a debris alert, Nasa spokesperson Gary Jordan said.



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