On April 10, the New York Times reported that after successfully completing a flight around the moon, the astronauts on the Artemis 2 mission now had to rely on a defective heat shield to protect them when they re-entered the Earth's atmosphere. This is probably the most dangerous phase of the entire mission.

The spacecraft will withstand high temperatures

Even NASA admitted that the Artemis 2 mission's heat shield was defective.

The heat shield is a key protective layer at the bottom of the spacecraft. It is used to withstand extremely high temperatures when returning to the Earth's atmosphere, protecting the spacecraft itself and the astronauts inside. If a heat shield fails, the underlying metal structure can melt, crack, and disintegrate.

Moreover, it had no backup plan and the astronauts had no means of escape.

However, NASA officials still said that despite the known heat shield defect, the four astronauts on the Artemis 2 mission will be able to arrive safely and in good condition when they return to Earth at nearly 24,000 miles per hour on Friday night to complete their 10-day mission around the moon and back.

NASA Administrator Jared Isaacman said in an interview in January that extensive analysis and testing of the heat shield materials "give us confidence that we have adequate safety margins to carry out this mission."

5% probability of disaster?

However, former NASA astronaut and heat shield expert Charlie Camarda said that NASA should not launch the Artemis 2 mission at all.

He pointed out that NASA did not have enough understanding of the probability that the heat shield might fail. Although the mission is currently going well, it could still end in the death of the astronaut.

"I'm going to pray that nothing happens," he said in an interview days before the launch of Artemis 2.

Camarda judged that the probability of the astronauts returning safely is about 95%, but this also means that the probability of disaster is 1/20. By comparison, the International Air Transport Association calculates the probability of fatality in a commercial aviation accident to be about 1 in 9 million.

Technicians inspect the heat shield of the Artemis 2 mission

The core of his disagreement with NASA is how much certainty is needed when a definite, perfect answer is not available.

During NASA's "Artemis 1" unmanned mission around the moon in 2022, a spacecraft named "Orion" survived re-entry into the atmosphere. If there were astronauts in the cabin at the time, they would not have noticed anything unusual.

But when the spacecraft was lifted from the ocean, the heat shield (the same design as on Artemis 2) was riddled with holes and had larger chunks of material missing.

Subsequently, NASA conducted an investigation that lasted several years. NASA officials said their analysis assessed what might happen in a worst-case scenario. They said the results of these analyses, combined with adjustments to the reentry path of the Artemis 2 astronauts as they return to Earth, provide an adequate safety margin for the mission.

The crew of the Artemis 2 mission knew the risks of the flight and how NASA was dealing with them. Artemis 2 commander Reid Wiseman said in September last year: "We were involved in virtually every aspect of the spacecraft's construction and were on site from beginning to end."

However, Dr. Camarda countered that NASA still failed to understand the basic physics of what happened during the Artemis 1 mission and therefore could not truly say what the worst-case scenario might be.

NASA officials have been downplaying concerns about the heat shield. At a press conference in January 2024, current NASA Deputy Administrator Amit Kshatriya said that Artemis 1’s heat shield had experienced “unexpected phenomena that we need to make sure we fully understand.” But he also said that from a thermal protection perspective, its performance was "very good."

Cause of defect

Photos of Artemis 1's heat shield were withheld from the public until May 2024, when they emerged in a report from NASA's inspector general, an independent watchdog agency.

The heat shield is made of a material called Avcoat, similar to the material used during the Apollo program more than 50 years ago. It's designed to gradually carbonize and burn away as it absorbs the heat of re-entry, preventing heat from being transferred to the rest of the spacecraft.

But while investigating the Artemis 1 heat shield, engineers concluded that gas built up inside certain parts of the shield, allowing pressure to create cracks that caused chunks of Avcoat material to suddenly break away, rather than ablate smoothly and slowly as the design intended.

Artemis 1's heat shield damaged

For future missions, NASA has modified Avcoat's formula to make it more porous so that gases trapped inside can escape.

However, the "Artemis 2" mission still uses the original formula, because the heat shield using the original formula has been manufactured and installed on the "Orion" spacecraft. Replacing the heat shield or the entire spacecraft will further delay the launch plan.

compromise

Therefore, NASA engineers came up with a solution: adopting a steeper and shorter re-entry trajectory will minimize the time the spacecraft is exposed to high temperatures and help ensure the safety of astronauts.

Dan Rasky, a heat shield engineer who retired from NASA in December, joined Dr. Camarda in opposing NASA's decision.

"Let me use an analogy," he said, "if you're driving on the highway and your tires start coming off piece by piece, do you keep driving and hope it's okay? Or do you pull over and change the tire because you're worried about a puncture?"

Artemis 1 heat shield material falls off

He said that the decision to directly carry out the "Artemis 2" mission without any replacement of the heat shield was "not wise" and "in fact, it was reckless."

If Artemis 2's heat shield performs as well as the one used on Artemis 1, the astronauts will splash down in the Pacific Ocean without a hitch.

Lessons from the past

But the situation is disturbingly reminiscent of the worst two days in NASA's history: On January 28, 1986, the space shuttle Challenger disintegrated 73 seconds after liftoff. On February 1, 2003, the space shuttle Columbia crashed on its way back into the atmosphere from orbit.

In these two accidents, the space shuttle actually had warning signals during previous flights, but NASA management mistakenly regarded "previous missions successfully completed" as a safety basis, which created a false sense of security and did not solve the problem with enough urgency, which ultimately led to the death of the astronauts.

Now, the key question facing Artemis 2 and its flawed heat shield is: Will cracks form and grow at a catastrophic rate?

However, it is extremely difficult to accurately calculate this possibility. Even the fastest computers would struggle to simulate the hypersonic flow of air molecules around the bottom of a space capsule. For heat shields, other complex phenomena must also be taken into account: the heat flow generated by the compression of air molecules, and the unpredictable process of how cracks form and propagate in the Avcoat material, sometimes suddenly.

"If it were me, I would choose to suspend the mission," Dr. Camarda said. "I would put together a team and build real analytical capabilities that incorporate all the fundamental physics principles."

Danny Olivas, another former NASA astronaut and scientist with expertise in materials properties, said this is not something NASA has done yet.

"Camarda is 100 percent correct," said Dr. Olivas. "We don't yet have a model based on physics to explain this problem. Because of the behavior of this material, this is unrealistic and almost impossible to do."

But Dr. Olivas said he initially had doubts about launching Artemis 2 with known flaws, but was ultimately reassured by NASA's analysis.

Unlike Camarda, Dr. Olivas had been commissioned by NASA to conduct an independent technical review of its investigation and recommended the formation of a larger panel of outside experts, which NASA adopted.

Dr. Olivas said that NASA's simulation hypothesis is that if the temperature inside an Avcoat unit block rises to a certain level, it will crack; and if it cracks, a layer of Avcoat material will fall off the entire unit block. This creates a cavity in which heating accelerates and another layer of material breaks away.

Dr. Olivas said that even under this "unoptimistic" assumption, repeated simulations of various re-entry scenarios found that enough parts of the heat shield would remain intact and the spacecraft would survive.

Another analysis looked at what would happen if a whole piece of Avcoat material fell off. The analysis concluded that a structure made of carbon fiber and titanium beneath the heat shield would keep the crew module intact during re-entry.

Dr. Olivas pointed out that NASA engineers were very cooperative. "What I can tell you is that every time I talk to someone, Columbia is always on their mind," he said. "They're grateful that I pushed them, they're grateful that I challenged them, and they're grateful that I forced them to essentially prove this."

He also said he has tried to provide a channel for dissenters who may be too afraid to raise their concerns for fear of NASA leadership to contact him, but he has not heard back from anyone.

Dr. Olivas said it was a far cry from the cultural climate he encountered after the Columbia disaster.

In January of this year, NASA Administrator Isaacman invited Dr. Olivas and Dr. Camarda to participate in a one-day technology demonstration. At the meeting, NASA engineers explained their rationale for using the defective heat shield.

But Dr. Camarda was not convinced. "NASA has no data at all to prove that this is safe," he said. "I realized that they were still using the same flawed thinking and crude analytical tools as they did during the Columbia and Challenger eras."

For Dr. Olivas, however, the meeting dispelled his last few remaining concerns. He sent a text message to Wiseman, commander of Artemis 2, expressing his belief that NASA had done a good job in reducing the risks to the Artemis 2 crew.

"If I didn't really think so, I wouldn't tell them that out of respect for them or their families," Dr. Olivas said. "I would never give a hasty release for the benefit of NASA."