A spacewalk planned for Tuesday (Nov 30) to repair a faulty antenna on the International Space Station was postponed indefinitely, NASA said, citing a "debris notification" it received for the orbiting research laboratory.
Two US astronauts had been scheduled to venture outside the space station at 7.10am Eastern time (1210 GMT) to begin their work, facing what NASA officials had called a slightly elevated risk posed by debris from a Russian anti-satellite missile test this month.
But about five hours before the outing was to have commenced, NASA said on Twitter that the spacewalk had been called off for the time being.
"NASA received a debris notification for the space station. Due to the lack of opportunity to properly assess the risk it could pose to the astronauts, teams have decided to delay the Nov 30 spacewalk until more information is available," the space agency tweeted.
It was not made clear how close debris had come to the space station, orbiting about 402km above the Earth, or whether it was related to the Russian missile test.
NASA TV had planned to provide live coverage of the six-and-a-half hour "extravehicular activity", or EVA, operation by astronauts Thomas Marshburn and Kayla Brown.
The outing would be the fifth spacewalk for Marshburn, 61, a medical doctor and former flight surgeon with two previous trips to orbit, and the first for Barron, 34, a US Navy submarine officer and nuclear engineer on her debut spaceflight for NASA.
The objective is to remove a faulty S-band radio communications antenna assembly, now more than 20 years old, and replace it with a new spare stowed outside the space station.
According to plans, Marshburn was to have worked with Barron while positioned at the end of a robotic arm operated from inside the station by German astronaut Matthias Maurer of the European Space Agency, with help from NASA crewmate Raja Chari.
The four arrived at the space station Nov 11 in a SpaceX Crew Dragon capsule launched from the Kennedy Space Center in Cape Canaveral, Florida, joining two Russian cosmonauts and a NASA astronaut already aboard the orbiting outpost.
Four days later, an anti-satellite missile test conducted without warning by Russia generated a debris field in low-Earth orbit, and all seven crew members took shelter in their docked spaceships to allow for a quick getaway until the immediate danger passed, according to NASA.
The residual debris cloud from the blasted satellite has dispersed since then, according to Dana Weigel, NASA deputy manager of the International Space Station (ISS) programme.
But NASA calculates that remaining fragments continued to pose a "slightly elevated" background risk to the space station as a whole, and a 7 per cent higher risk of spacewalkers' suits being punctured, as compared to before Russia's missile test, Weigel told reporters on Monday.
Although NASA has yet to fully quantify additional hazards posed by more than 1,700 larger fragments it is tracking around the station's orbit, the 7 per cent higher risk to spacewalkers falls "well within" fluctuations previously seen in "the natural environment", Weigel said.
Still, mission managers cancelled several smaller maintenance tasks under consideration for Tuesday's spacewalk, Weigel added.