A planned spacewalk outside the International Space Station was called off early Tuesday after NASA received an overnight warning about possibly threatening space debris. The delay came two weeks after a Russian anti-satellite test created a cloud of debris in low-Earth orbit.
The station's seven-member crew was not told to take shelter in their Earth-return spacecraft and no other actions were required. But the spacewalk was delayed, NASA said in a blog post, "due to the lack of opportunity to properly assess the risk (the debris) could pose to the astronauts."
The post did not say whether the debris in question was generated in the Russianthat generated some 1,700 trackable fragments and an unknown number of smaller pieces. The test was blasted by the U.S. as "reckless," and NASA in their Crew Dragon spacecraft as a precaution soon after the satellite was destroyed.
The astronauts took the news of the spacewalk delay in stride on Tuesday.
"Sorry for the news," mission control radioed the station crew overnight. "We're probably almost as disappointed as the crew members today, but I know it's a little bit harder for you guys waking up to this news."
"It's just real life, this is how things work out sometimes," astronaut Mark Vande Hei replied. "I'm really glad you fellows are looking out for our safety."
By Tuesday afternoon, NASA had completed a more detailed risk assessment, telling spacewalkers Thomas Marshburn and Kayla Barron to plan on carrying out the delayed excursion Thursday.
"The ISS program met this morning and deemed us go for Thursday, so we'll continue the planning .. for Wednesday and going EVA on Thursday," ground control radioed.
"Great, we've got some rejoicing on the station," an astronaut replied.
Marshburn, a veteran spacewalker, and Barron, a Navy submariner making her first spaceflight, had planned to carry out a six-and-a-half-hour excursion Tuesday to replace a failed S-band radio antenna on the left side of the lab's power truss.
The station has multiple antennas in a highly redundant communications system, but managers want to replace the failed unit before the lab's orbit carries it into extended periods of sunlight and higher temperatures.
The spacewalk will be the first since the Russian military destroyed a retired electronic intelligence satellite two weeks ago that created a cloud of debris along the spysat's orbital track.
Given that the space station and everything else in low-Earth orbit is moving at nearly five miles per second, debris impacts pose a potentially catastrophic hazard, regardless of the source.
Thousands of objects the size of a softball and larger are tracked by U.S. Space Command radars on the ground. For trackable objects that pose a collision risk, the station's Russian thrusters can be fired to move out of the way. But objects too small to track can hit without warning.
Debris formed in the wake of the ASAT test is slowly spreading out, and NASA officials said Monday the risk to Marshburn and Barron was only about 7 percent higher than the normal 1-in-2,700 odds of a spacesuit penetration.
SpaceX founder Elon Musk weighed in on the topic Tuesday, tweeting the company "had to shift some Starlink satellite orbits to reduce probability of collision."
The space station and SpaceX Dragon capsules "have micrometeorite shields (ultra high velocity impact absorption), but EVA suits do not, hence higher risk for spacewalk," Musk tweeted.
During a pre-spacewalk news briefing Monday, Dana Weigel, deputy manager of the space station program at the Johnson Space Center in Houston, said debris from the Russian ASAT test initial was "very concentrated."
"Over time it has dispersed, but initially it was very concentrated and as ISS passed through the orbit of the debris, we had a heightened concern for about 24 hours after the event," she said, adding the background debris environment is still "slightly elevated, it's about two times what it had been prior to the event" for the station overall.
The change in the risk to spacewalkers was "on the order of about 7 percent," she said. "So, this particular EVA, its risk falls within the family of what we've had for EVAs over the last few years."
The shuttle-era spacesuits, or extra-vehicular mobility units -- EMUs in NASA-ese -- are multi-layer pressure suits with backpacks housing batteries, air tanks, carbon dioxide removal chemicals and pumps that circulate water through small tubes sewn into the astronauts' undergarments. The water can be heated or cooled as needed to keep spacewalkers comfortable in sunlight or darkness.
The suit also feature a 30-minute emergency oxygen supply to enable an astronaut with a puncture or tear to make it back to the safety of an airlock. In the hundreds of station and non-station spacewalks carried out to date, no such emergency has ever occurred.
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