Artemis II's last test: Will its heat shield work?
Excess charring on Artemis I's heat shield had NASA racing to understand what went wrong—and to ensure the technology was still safe for the current crew's high-speed reentry.
When Artemis II comes blazing back through the Earth’s atmosphere, it will be traveling at around 25,000 miles per hour and reach temperatures up to 5,000 degrees Fahrenheit. The only thing protecting the crew from the fires of re-entry? Orion’s heat shield.
NASA builds redundancy into everything. There’s almost always a backup system or a secondary option in case something goes wrong. But there is no redundancy for the heat shield. It simply has to work.
But will it? NASA is confident that the answer is a resounding “yes.” Yet it’s taken a long time to reach that conclusion, including months of testing after performance issues arose on Artemis I—and still at least one prominent critic is left unconvinced.
Artemis I came back charred
On December 11, 2022, the Orion capsule splashed down in the Pacific Ocean, marking the end of Artemis I. Over 25 days, the uncrewed mission successfully tested the Space Launch System (SLS) rocket and the capsule’s performance, executed two lunar flybys, and traveled around 270,000 miles away from Earth.
When examinations of the Orion capsule began after splashdown, engineers noticed something odd. The heat shield for Artemis I didn’t ablate (i.e., melt) in the way they’d expected.
Orion’s heat shield measures 16.5 feet in diameter and is made up of Avcoat, a material comprised of silica fibers, which was originally developed for the Apollo command module. On Orion, the heat shield covers the bottom of the capsule and is applied in blocks. That blunt end faces the Earth during re-entry, and the shield blackens and gently chars away on the outside, protecting the capsule and its crew.
At least, that’s what is supposed to happen.
But NASA discovered, worryingly, that there were chunks missing from the heat shield and that it was cracking in places. It is “normal behavior” for the heat shield to scorch upon re-entry, said NASA associate administrator Amit Kshatriya at a press conference in December 2024. But it is not normal for there to be large, chunked losses of that charred material.
It’s important to note that even in this degraded state, the shield still would have served its purpose. “There would not have been any impact to the crew’s safety” if astronauts had been aboard, Kshatriya said at a press conference the following month. Sensors on the spacecraft detected no abnormal heating of the interior or exterior.
However, it was clear there was a disconnect between the actual performance of the heat shield and NASA’s expectations. When entire pieces of the heat shield are missing, it raises concerns about the risk for loss of the vehicle during re-entry. They needed to understand what had happened to the shield before launching Artemis II, with humans flying in Orion for the first time.
NASA's heat-shield dilemma
NASA undertook an extensive months-long investigation on the root cause of the excess charring. While the agency didn’t test another Orion capsule during an actual reentry, they did try to replicate its extremely hot conditions here on Earth at NASA’s Ames Arc Jet Complex and the Laser Hardened Materials Evaluation Laboratory at Wright‐Patterson Air Force Base in Ohio. In addition to those tests, NASA put the heat shield through wind tunnel tests, permeability tests, and, importantly, also sent samples of the recovered Artemis I heat shield to experts at the Marshall Spaceflight Center in Alabama for up-close analysis.
In December 2024, NASA finally revealed the root cause of the heat shield anomaly: the heat shield wasn’t porous enough, and gases became trapped within its decomposition layer, which led to the cracking and loss of charred material. Underscoring their conclusions, the team determined that the more permeable parts of the Artemis I heat shield did not suffer the same char loss as the less permeable part.
Ultimately, they determined a large contributor to this pressure buildup was the specific re-entry trajectory of the first Artemis flight: skip entry.
Skip entry was tested on Artemis I as a way to bleed off speed while also increasing the precision of landing in an exact location in the ocean. During the skip entry procedure, Orion re-entered the atmosphere, burned off speed, and then briefly lifted back up. It then re-entered and proceeded to splash down 2.1 nautical miles from the recovery ship, well within its 5-mile target radius. Skip entry, then, was phenomenally successful both in terms of reducing Orion’s speed and making a precision landing.
But during that lift and re-entry process, the Avcoat heated up enough for gases to build pressure, eventually cracking it in over 100 places, according to a report from NASA’s Office of the Inspector General.
NASA then faced a difficult decision: Should they redesign the heat shield for Artemis II or fly as is? The Artemis II heat shield is even less permeable than the one flown on Artemis I, a decision NASA made in order to make it easier to test the strength of the bonds between Orion and its heat shield. But replacing the heat shield would add significant delays to a program that was already behind schedule.
After extensive testing and a focus on worst-case scenarios, NASA decided that it would be safe to fly Artemis II as is. But they chose to change the re-entry trajectory to minimize the thermal strain on the shield.
Artemis II will still perform a skip, according to Rick Henfling, entry flight director. It just will be a shorter skip that won’t expose the Avcoat to as much heat as Artemis I. “We don't need to do such a pronounced skip entry,” Henfling told National Geographic. “We can do a little bit more of a gradual entry profile.” This is called “lofted entry.”
Simply put: With this new re-entry profile, Henfling expects Orion will not experience the levels of heat that led to Artemis I’s charring.
Even with lofted entry, there is still potential for some abnormal heat shield behavior. “You will potentially see some char loss, so it’s not zero,” explained Howard Hu, Orion program manager. “But not to the magnitude we saw in Artemis I because we changed the trajectory.”
A heat shield expert has lingering doubts
But not everyone is convinced by NASA’s conclusions. Charles Camarda is a former astronaut and heat shield expert, who flew on the first mission after the Columbia disaster in part to inspect the integrity of the space shuttle’s heat shield. And recently, he has been vocal in his criticism of NASA’s decision to fly Artemis II in its current configuration.
After being invited to review NASA’s investigation in January, Camarda wrote an open letter to NASA Administrator Jared Isaacman decrying the agency’s decision. He believes that the same NASA culture that produced the shuttle disasters, and the more recent Starliner mishap, "led to the wrong launch decision,” he told National Geographic in an email.
Camarda thinks that NASA does not truly understand the root cause of the heat shield char loss because, in his opinion, the agency did not use “a validated, integrated multi-physics analysis,” saying in his open letter he would have like to have seen a review with more outside researchers, like those at the U.S. Department of Energy or in academia.
NASA stands by their decision to fly the heat shield as is. When asked for comment, National Geographic was directed to a statement put out by Isaacman in January. “Human spaceflight will always involve uncertainty,” Isaacman said. “NASA’s standard engineering process is to identify it early, bound the risk through rigorous analysis and testing, and apply operational mitigations that preserve margin and protect the crew. … With this disciplined approach in place every step of the way, we are moving steadily—and confidently—toward sending astronauts farther into space than ever before.”
Camarda wasn’t the only one to express doubts about the heat shield. John “Danny” Olivas, another former astronaut and engineer who completed an on-orbit repair of the space shuttle’s thermal protection system, had similar concerns, and conveyed them personally to Isaacman. Olivas reviewed NASA’s investigation along with Camarda.
But unlike Camarda, he was convinced by NASA’s conclusions. While Olivas agrees Camarda is “100 percent accurate” in pointing out that NASA doesn’t have a complete physics picture of what happened on Artemis I—as they cannot completely recreate the conditions of reentry on Earth—he was won over by the agency’s worst-case scenario simulation.
If an entire block of Avcoat popped off the spacecraft, heat would penetrate deeper into the craft, and closer to the crew compartment, tempting catastrophe. But even if that damage occurred, Orion’s backshell, made of carbon fiber, would not fail, according to NASA’s analysis. The crew would return safely, even in that worst-case scenario. During his review, Olivas also felt assured by NASA’s transparency. “I was basically given carte blanche to go anywhere I wanted to, and see anything I wanted to, and actually have independent tests done on my behalf. And I exercised all of those,” he said.
Spaceflight is inherently risky. But NASA works to determine what an acceptable level of risk is. Here, they have determined that it was worth it to fly Artemis II.
For their part, the crew does feel confident in the readiness of Orion for all aspects of Artemis II. “This is the first time we're going to try this,” Artemis II commander Reid Wiseman said at a press conference before the launch. “This is the first time we're loading humans on board. And I will tell you, the four of us, we are ready to go. The team is ready to go, and the vehicle is ready to go.”
Follow along with National Geographic’s continued coverage of Artemis II here.
