Heat shield safety concerns raise expectations for Nasa’s Artemis II Moon mission

Astronauts Reid Wiseman, Victor Glover, Christina Koch and Jeremy Hansen are preparing to launch into space on a journey that will make them the first humans to visit the Moon in more than half a century.

Their 10-day mission, known as Artemis II, has orbited the Moon but will not land. It will see them travel 4,700 kilometers (7,600 miles) beyond the far side of the moon on NASA’s Orion spacecraft. Thus, four astronauts will travel farther from Earth than any human before them.

Artemis II’s quarter-millionth journey has been enjoyable, but it’s the last five minutes of the mission that may be the cause of greatest concern for the astronauts’ safety.

The failed Orion spacecraft test in 2022 began by highlighting problems with the heat shield. This is the part of Orion that carries the weight of the thermal capsule during re-entry into Earth’s atmosphere.

When engineers inspected the Orion heat shield from the Artemis I mission in 2022, they found large parts of the material were missing. The concern was that, if this happened again during the Artemis II crew mission, it could expose the interior of the capsule to dangerously high temperatures.

Since the early days of human flight, engineers have protected capsules from the extreme heat of reentry with so-called “ablative” heat shields, made of materials designed to burn evenly as the capsule burns through space.

To meet the requirements of a reusable spaceship, NASA developed an amazing heat shield made of ultra-light tiles of silica fibers covered with glass. Although this heat shield had extraordinary thermal properties, it was also exceptionally fragile, and required full maintenance after each shuttle mission.

It was the destruction of this weak and transparent protection system that caused the tragic loss of the space shuttle Columbia in 2003. For the Artemis program, NASA returned to the concept of an ablative heat shield.

The heat shield for the Orion capsule is made of a material called Avcoat, which is based on material developed for the Apollo program. Although NASA considered other new materials for the Orion heat shield, they ultimately decided on the equipment that had been flown by the Apollo missions.

However, the design of Orion’s heat shield is different from that used during Apollo. Apollo’s heat shield was made of a single honeycomb of about 320,000 individual hexagonally filled sections. To make the heat shield for Orion efficient and reproducible, NASA chose to have about 180 blocks.

This heat shield was tested for the first time in 2014 when the undeveloped Orion capsule was launched to an apogee of 3,600 kilometers by a Delta IV rocket. The plate ignited in the atmosphere on its return at a temperature of about 2,200°C (4,000°F), but the heat shield proved to be able to withstand such heat.

The next test of the Orion capsule was the Artemis I mission in 2022. This was the first flight of the powerful Space Launch System rocket, and an inactive demonstration of the planned mission for Artemis II. Entering the Earth’s atmosphere farther than the first test, the spacecraft reached temperatures of about 2,800°C (5,000°F). This is where the first doubts about the Avcoat heat shield were raised.

Instead of burning evenly on the surface, Artemis I’s heat shield parts were suddenly lost in uneven sections. This unstable reduction makes the re-entry model unpredictable, and raises the possibility that the Orion capsule could be exposed to dangerous heating conditions.

Upon investigation, the cause of this uneven flow was found to be the abnormal release of gases trapped in the heat shield, which is compounded by the “re-entry process” adopted by the project.

In jump mode, Orion first grazes the edge of the spacecraft to slow down. It then uses strong winds to lift the capsule back out of space, before re-entering for a descent to Earth. The skip profile is so named because it resembles a stone crossing a pond.

NASA researchers have found that, as temperatures decrease during reentry, thermal energy accumulates in the Avcoat material. This led to the build-up of gases and, in turn, internal pressure – causing cracks and uneven discharge of the material.

Based on lessons learned from Artemis I, NASA took several steps to protect the crew of Artemis II. For the first mission of the program’s crew, NASA kept Avcoat’s thermal protection properties, but revised the design of the blocks to help gases escape during re-entry.

Additionally, instead of a skip profile, NASA has chosen a direct re-entry method for the Orion capsule. This reduces the instability of the heating profile and means less peak temperature time for trapped gases to damage the heat shield, but also means that workers will be slowed down when re-entering.

Safety first

In the dramatic setting of the Apollo 13 film, flight director Gene Kranz cheerfully informs the mission control team that “failure is not an option”.

Although the line was actually a product of the film’s writers, it became not only the second line quoted in the film, but also almost a mantra for NASA itself.

Nowhere is this more true than with the Artemis II heat shield. In the final phase of Artemis II’s mission, there is no backup, no emergency, and no escape. The four astronauts on board will rely on resin-coated silk to protect themselves from the heat approaching half of the Sun’s surface.

Human spaceflight has always brought calculated risks, but it has also provided a unique human perspective on our place in space. The Artemis II mission will make its crew the first people in more than half a century to see the blue marble of planet Earth in its entirety with their own eyes.

Employees will carry with them the hopes and aspirations of a new generation of researchers. They will rely on the careful work of thousands of scientists and engineers for their safe return, bringing with them a renewed perspective for humans not just on the Moon, but on the planet we all call home.