Heading home: The most dangerous phase of the Artemis II moon mission is yet to come

The Artemis II astronauts have faced down numerous dangers on their historic moon mission — including white-knuckling through liftoff on April 1 as their rocket burned through millions of gallons of fuel and braving perilous fields of radiation en route to the moon.

But perhaps the most daunting milestone lies ahead: reentry.

During this phase of flight, the astronauts’ spacecraft comes roaring toward Earth and dips back into the thick inner band of our planet’s atmosphere while still traveling more than 30 times the speed of sound. The process causes a violent compression of air molecules that can heat the capsule’s exterior to more than 5,000 degrees Fahrenheit (2,760 degrees Celsius).

“I’ll be honest and say, I’ve actually been thinking about entry since April 3, 2023, when we got assigned to this mission,” Artemis II astronaut Victor Glover said of reentry during an event with media Wednesday. “One of the first press conferences, we were asked, what are we looking forward to? And I said, splashdown. And it’s kind of humorous, but it’s literal as well — that we have to get back. There’s so much data that you’ve seen already, but all the good stuff is coming back with us. There’s so many more pictures, so many more stories.”

Reentry is considered one of the most — if not the most — precarious steps of any flight to space. And Artemis II will be going through it with a known issue mission controllers are tracking.

The problem came to light after the uncrewed Artemis I test flight around the moon in 2022, after which mission teams found that the capsule’s heat shield had returned with concerning pockmarks and cracking. A heat shield is a crucial piece of hardware designed to protect a spacecraft and its astronauts from extreme temperatures as they’re descending back to Earth.

The Artemis I Orion spacecraft still returned home safely and in one piece, but the damage raised questions about how well engineers understood the material used to create this hardware, called Avcoat, and how it behaves during the dangerous and dynamic final phase of flight.

If the heat shield becomes damaged or cracks in a particular way, it could lead to catastrophic failure. And there is no escape mechanism that could save the astronauts during this point in the journey. If the heat shield fails, the mission and crew would be lost.

The Artemis II Orion spacecraft has a heat shield that’s nearly identical to the one that flew on Artemis I. And NASA officials have acknowledged that it is less than ideal. But the agency maintains that it can bring the astronauts — NASA’s Reid Wiseman, Victor Glover and Christina Koch and Canadian Space Agency astronaut Jeremy Hansen — home safely, due to some changes made to the mission’s reentry strategy.

Mission managers say they are confident they have done their homework and understand the heat shield’s limitations and how to protect the crew, said Amit Kshatriya, NASA’s associate administrator, during a Thursday news briefing. And “the crew is going to put their lives behind that confidence,” he said.

But he acknowledged the stakes are high.

“The Orion spacecraft will enter the Earth’s atmosphere at approximately 25,000 miles per hour. That heat shield … will bear the full force of that reentry,” he said. “Every system we’ve demonstrated over the past nine days — life support, navigation, propulsion, communications — all of it depends on the final minutes of flight.”

The issues seen on Artemis I prompted more than a year of investigations, analysis and ground tests as NASA tried to understand the heat shield’s unexpected behavior.

Crucially, however, by the time Artemis I came back, the heat shield was already installed on the Artemis II capsule. That meant it was too late to alter the structure or design of the heat shield for this astronaut flight.

To address the problem, NASA has opted to put the Artemis II capsule and astronauts on a different trajectory than Artemis I took for its return home.

While the 2022 test flight used a “skip” reentry in which the capsule briefly plunged into the atmosphere before raising its altitude again and making a second plunge — this trip will attempt more of a “loft,” according to NASA Flight Director Rick Henfling.

The altered path is meant to create more favorable heating conditions, in the hopes that it will limit — but not eliminate — cracking on the heat shield.

The investigation process has given experts across NASA confidence that, even if the heat shield does not perform optimally, the astronauts will get home safe.

Howard Hu, NASA’s Orion program manager, repeated that sentiment in a prelaunch interview in late March. He also confirmed that the space agency will begin evaluating the Artemis II heat shield’s performance immediately upon return.

Following Orion’s expected splashdown off the coast of San Diego, as the astronauts are airlifted to a recovery vessel, a diver will plunge into the ocean to photograph the heat shield from below — providing mission managers some of the first evidence of how it performed.

“This is a deviant heat shield,” Dr. Danny Olivas, a former NASA astronaut who served on a space agency-appointed independent review team that investigated the incident, told CNN in January. “There’s no doubt about it: This is not the heat shield that NASA would want to give its astronauts.”