Can Artemis II’s heat shield withstand the force of reentry?

Jacqueline McCleary, an assistant professor of physics, said the odds of the Orion capsule’s heat shield failing are largely uncertain.

NASA astronaut and Artemis II Commander Reid Wiseman peers out of one of the Orion spacecraft’s main cabin windows, looking back at Earth, as the crew travels towards the Moon. Photo by NASA

The crew of Artemis II is in the final hours of a 10-day voyage to the far side of the moon and back, and is scheduled to splash down into the Pacific Ocean at around 8 p.m. EST.

But to do so, the four-person crew will have to survive what might be the most dangerous part of the mission: reentry into the Earth’s atmosphere at a mind-bending 25,000 miles per hour under the protection of a slightly modified, yet to be tested heat shield, designed to absorb and shed extreme heat during reentry. As the capsule slams through the atmosphere, temperatures outside can soar to nearly 5,000 degrees Fahrenheit — hot enough to melt steel.

Questions about the Orion space capsule’s 16.5-foot-wide heat shield arose during an unpiloted mission, Artemis I, in 2022, when engineers observed that its ablative outer material, which is meant to burn up and erode, was not able to relieve pressure inside the capsule and carry heat away from it as expected. The pressure buildup contributed to cracking in the heat shield’s outer layer, with portions of the charred material breaking away in several locations, according to officials with the National Aeronautics and Space Administration, or NASA.

NASA has since refined how the material, a heat-absorbing coating called Avcoat, is applied to account for the uneven erosion observed during that earlier mission. But Artemis II’s reentry will be the first real test of those changes.

Can the heat shield withstand the violence of reentry? One former NASA astronaut, Charles Camarda, estimated a 1 in 20 chance of disaster, or 95% success rate.

But Jacqueline McCleary, an assistant professor of physics, said that 95% number is “purely vibes-based,” adding that scientists don’t actually know what the odds of failure are.

Portrait of Jacqueline McCleary, a woman with pale skin and long hair wearing a striped button down shirt. An image of the galaxy is projected onto her and in the dark background. — Jacqueline McCleary, associate professor in the College of Science, said humans are bad at estimating tail risks or “black swan” events. Photo by Matthew Modoono/Northeastern University

“Humans are bad at estimating tail risks or ‘black swan’ events,” she told Northeastern Global News.

She said the challenge mirrors a familiar problem in astrophysics, which is that even the most advanced simulations have to zoom in on the tiniest details to understand the bigger picture. She pointed to simulations of how galaxies form and evolve. Even the most powerful supercomputers can’t capture every star and process in detail, so scientists have to simplify smaller, complex pieces in order to extrapolate and model the vast structure of the universe.

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McCleary said the same limitations apply to modeling the Orion heat shield, where engineers must approximate how materials behave under extreme conditions that are difficult to fully simulate.

Air molecules are about a millionth the size of the cracks and 100 millionth the size of the whole capsule, McCleary explained. “Trying to resolve the relevant physics at these vastly different scales is intractable,” she said. “It doesn’t surprise me to learn that these heat shield models face similar difficulties.”

Despite the concerns, McCleary said she would personally be confident riding in the capsule, adding that she is “somewhat reassured by the fact that this heat shielding on a capsule is a fairly primitive system.” As systems accumulate moving parts, the probability that any single one fails increases, she said.

“There are only so many things that can go wrong,” she said. And things have gone wrong. In 2003, foam from the external tank of the Space Shuttle Columbia damaged the orbiter’s wing and led to its breakup during reentry, killing all seven members on board. McCleary’s not so worried this time.

NASA officials have also emphasized that the risks with Artemis II are understood and accounted for.

“Our early Artemis flights are a test campaign, and the Artemis I test flight gave us an opportunity to check out our systems in the deep space environment before adding crew on future missions,” Amit Kshatriya, a senior NASA official overseeing the Moon to Mars program said in a press release. “The heat shield investigation helped ensure we fully understand the cause and nature of the issue, as well as the risk we are asking our crews to take when they venture to the Moon.”

Ahead of Friday’s splashdown, Kshatriya reiterated that NASA has “high confidence in the system, in the heat shield and the parachutes and the recovery systems we put together.”