As this illustration of the Apollo VIII command module returning to Earth shows, heat shields protect the crew capsule from high temperatures. Credit: NASA Johnson Space Center
Off the coast of Baja California in December 2022, the sun sparkled on the choppy sea as waves crashed around the dock ship USS Portland. The Navy officers on the bridge scanned the sky for a signal. The glow suddenly appeared.
At first it was a tiny dot, but it gradually transformed into a round circle that fell at great speed from the edge of space. This was NASA’s Orion capsule, which would soon conclude its 25-day Artemis I mission around and beyond the Moon with a violent splashdown in the ocean.
Orion’s reentry followed a sharp-angled trajectory, during which the capsule fell at incredible speed before opening three red and white parachutes. When the mission finished its journey of more than 270,000 miles (435,000 kilometers), it appeared to those on the deck of the USS Portland as if the capsule had returned home in one piece.
As the recovery crew lifted Orion onto the carrier’s deck, shock waves rippled across the surface of the capsule. That’s when crew members began to spot large cracks on Orion’s undersurface, where the exterior of the capsule bonds to the heat shield.
But why shouldn’t a shield that has withstood temperatures of around 5,000 degrees Fahrenheit (2,760 degrees Celsius) suffer damage? Seems natural, right?
This mission, Artemis I, was unmanned. But NASA’s ultimate goal is to send humans to the Moon in 2026. So, NASA needed to make sure that any damage to the capsule – even to its heat shield, which is expected to suffer some damage – wouldn’t put future lives at risk. . crew.
On December 11, 2022 – the time of Artemis I’s reentry – this shield suffered severe damage, which delayed the next two Artemis missions. While engineers are now working to prevent the same problems from happening again, the new launch date is scheduled for April 2026 and is fast approaching.
As a professor of aerospace technology, I enjoy researching how objects interact with the atmosphere. Artemis I offers a particularly interesting case – and an argument for why having a functional heat shield is critical to a space exploration mission.
Taking the heat
To understand what exactly happened to Orion, let’s rewind history. As the capsule re-entered Earth’s atmosphere, it began skimming its uppermost layers, which act a bit like a trampoline and absorb some of the kinetic energy of the approaching spacecraft. This maneuver was carefully designed to gradually decrease Orion’s speed and reduce thermal stress on the shield’s internal layers.
After the first dive, Orion bounced off into space in a calculated maneuver, losing some of its energy before submerging again. This second dive would take it into lower layers with denser air as it got closer to the ocean, further decreasing its speed.
During the fall, the drag exerted by the force of air particles against the capsule helped reduce its speed from about 27,000 miles per hour (43,000 kilometers per hour) down to about 20 mph (32 km/h). But this slowdown came at a price: The air friction was so great that the temperature on the lower surface of the capsule facing the airflow reached 5,000 degrees Fahrenheit (2,760 degrees Celsius).
At these scorching temperatures, air molecules began to split and a hot mixture of charged particles, called plasma, was formed. This plasma radiated energy, which you could see as red and yellow burning air surrounding the front of the vehicle, curling it backwards in the shape of a candle.
No material on Earth can withstand this hellish environment without being seriously damaged. So, the engineers behind these capsules designed a layer of material called a heat shield to be sacrificed through fusion and evaporation, thus saving the compartment that would eventually house the astronauts.
Protecting anyone who might one day find themselves inside the capsule, the heat shield is a critical component.
In the form of a shell, it is this shield that encapsulates the wide end of the spacecraft, facing the incoming airflow – the hottest part of the vehicle. It is made of a material designed to evaporate and absorb the energy produced by the friction of the air against the vehicle.
The case of Orion
But what really happened with Orion’s heat shield during that 2022 descent?
In Orion’s case, the heat shield material is a composite of a resin called Novolac — a relative of the Bakelite that some firearms are made of — absorbed into a honeycomb structure of fiberglass strands.
When the surface is exposed to heat and airflow, the resin melts and shrinks, exposing the fiberglass. The fiberglass reacts with the surrounding hot air, producing a black structure called char. This carbon then acts as a second thermal barrier.
NASA used the same heat shield design for Orion as the Apollo capsule. But during the Apollo missions, the charred structure did not break down like on Orion.
After nearly two years spent analyzing samples of the charred material, NASA concluded that the Orion project team had overestimated the heat flow as the craft skimmed the atmosphere upon reentry.
As Orion approached the upper atmosphere, the shield began to melt and produced gases that may have escaped through the pores of the material. Then, as the capsule regained altitude, the outer layers of resin froze, trapping the heat from the first dive inside. This heat vaporized the resin.
As the capsule dipped into the atmosphere for the second time, the gas expanded before finding its way out as it warmed up again – a bit like a frozen lake melts from the bottom up – and its escape produced cracks in the surface of the capsule where the charred structure became damaged. These were the cracks the recovery crew saw on the capsule after splashdown.
In a press conference on December 5, 2024, NASA officials announced that the Artemis II mission will be designed with a modified reentry trajectory to avoid heat buildup.
For Artemis III, scheduled to launch in 2027, NASA plans to use new manufacturing methods for the shield, making it more permeable. The exterior of the capsule will still become very hot during reentry and the heat shield will continue to evaporate. But these new methods will help keep astronauts comfortable in the capsule throughout the journey.
Chonglin Zhang, assistant professor of mechanical engineering at the University of North Dakota, co-researched this article.
Marcos Fernandez Tous is an assistant professor of space studies at the University of North Dakota. He does not work for, consult with, own shares in, or receive funding from any company or organization that would benefit from this article, and has not disclosed any relevant affiliations beyond his academic appointment.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
