Ahead of the launch of the Artemis-2 mission, experts have once again raised concerns about issues with the Orion spacecraft’s heat shield. It is designed to protect the astronauts as they re-enter Earth’s atmosphere upon returning from their journey around the Moon. 

Heat shield of the Orion spacecraft.
Source: phys.org

Problems with the heat shield in past missions

The Artemis-2 mission covers a distance of a quarter of a million miles, but the last five minutes of it are likely to cause the greatest concern regarding the astronauts’ safety. 

During the uncrewed test of the Orion spacecraft in 2022, problems with the heat shield were identified for the first time. This is the part of Orion that bears the brunt of the intense heat the capsule is exposed to as it enters Earth’s atmosphere. 

When engineers inspected the Orion heat shield from the 2022 Artemis-1 mission, they discovered that large pieces of material had fallen off. The concern was that if this were to happen again during the Artemis-2 crewed mission, it could expose the interior of the capsule to dangerously high temperatures. From the very beginning of manned spaceflight, engineers have protected capsules from extreme heat during atmospheric reentry using so-called “abrasive” heat shields made of a material that burns away uniformly as the capsule penetrates the atmosphere. 

To meet the requirements for a reusable spacecraft, NASA developed an incredible heat shield system made from ultra-light tiles of glass-coated silica fibers. Although this heat shield possessed exceptional thermal properties, it was also very fragile and required careful maintenance after every flight. Damage to this fragile and unprotected heat shield was the cause of the tragic loss of the space shuttle Columbia in 2003. For the Artemis program, NASA has returned to the concept of an abrasive heat shield.

New heat shield design for the Orion spacecraft

The Orion capsule’s heat shield is made of a material called Avcoat, which is based on a material originally developed for the Apollo program. Although NASA considered other, newer materials for the Orion heat shield, the material ultimately chosen was one that had already proven its reliability during the Apollo missions. 

However, the structure of the Orion heat shield differs from those used during the Apollo program. The Apollo heat shield consisted of a single honeycomb-structured matrix containing approximately 320,000 individually filled hexagonal segments. To make the Orion heat shield more effective and easier to reproduce in production, NASA chose a configuration consisting of approximately 180 individual segments.

This heat shield was first tested in 2014, when the Orion uncrewed capsule was launched by a Delta IV rocket to an apogee of 3,600 miles. On reentry, the capsule passed through the atmosphere at a temperature of about 2,200 °C (4,000 °F), but the heat shield proved capable of withstanding such extreme heat.

The next test for the Orion capsule was the Artemis-1 mission in 2022. This was the first flight of the powerful Space Launch System rocket and an uncrewed demonstration of the mission planned for Artemis II. As it flew through Earth’s atmosphere at a much greater distance than during the first test, the spacecraft reached a temperature of about 2,800 °C (5,000 °F). The first concerns regarding the Avcoat heat shield arose at this point.

Instead of burning away evenly across the entire surface, sections of the Artemis-1 heat shield unexpectedly fell off in irregular pieces. Such uneven ablation makes it difficult to predict thermal loads during atmospheric reentry and increases the probability that the Orion capsule could heat up to dangerous levels. 

Causes of uneven burning of heat shields

The investigation determined that the cause of the uneven ablation was the irregular release of gases trapped within the heat shield material, which was exacerbated by the “skip re-entry” profile adopted for this mission. 

Following the “skip” profile, Orion first lightly touches the edge of the atmosphere to slow down. It then uses the capsule’s aerodynamic lift to skip back out of the atmosphere, before re-entering it for its final descent to Earth. The “skip” profile got its name because it somewhat resembles a stone skipping across the surface of a pond.

NASA researchers found that as the heating rate decreased between atmospheric re-entries, thermal energy accumulated within the Avcoat material. This led to the accumulation of gases and, in turn, to an increase in internal pressure, which resulted in the formation of cracks and uneven delamination of the material.

Necessary improvements

Based on the experience gained during the Artemis-1 mission, NASA has taken a number of measures to protect the Artemis-2 crew. For the first crewed flight under the program, NASA retained the Avcoat heat shield material but updated the tile design to facilitate gas venting during atmospheric reentry. 

In addition, NASA has now chosen a more direct re-entry profile for the Orion capsule instead of the “skip” profile. This reduces uncertainty in the heating profile and means that the trapped gases will be exposed to peak temperatures for a shorter period of time, thereby reducing the risk of damage to the heat shield; however, it also means that the crew will experience greater deceleration upon re-entry.

Safety first

At the most dramatic moment in the film Apollo 13, Mission Control Director Gene Kranz says his famous line to the Mission Control team: “Failure is not an option.” Although this phrase was actually coined by the film’s screenwriters, it has become not only the second most popular quote from the movie, but also a kind of motto for NASA.

Nowhere is this truer than in the case of the Artemis-2 heat shield. In the final phase of the Artemis-2 mission, there is no backup plan, no contingency plan, and no chance of escape. The four astronauts on board will rely on a few centimeters of resin-coated silica to protect themselves from temperatures approaching half that of the Sun’s surface. 

Human spaceflight has always involved calculated risks, but at the same time it has given us a unique human perspective on our place in the Universe. The Artemis-2 mission will make its crew the first people in over half a century to see the blue globe of Earth in all its glory with their own eyes.

The crew will carry the hopes and aspirations of an entire new generation of explorers. Their safe return will depend on the meticulous work of thousands of scientists and engineers, and they will bring back with them a new human perspective not only on the Moon, but also on the planet we call home.