Source: The Conversation (Au and NZ) – By Ed Macaulay, Lecturer in Physics and Data Science, Queen Mary University of London
The astronauts Reid Wiseman, Victor Glover, Christina Koch and Jeremy Hansen are preparing to launch into space on a trajectory that will make them the first humans to travel to the Moon in over half a century.
Their 10-day mission, known as Artemis II, loops around the Moon but will not land. It will see them travel 4,700 miles (7,600 kilometres) beyond the lunar far side in Nasa’s Orion spacecraft. As such, the four astronauts will travel further from Earth than any humans before them.
The quarter-of-a-million mile Artemis II expedition is audacious, but it’s the last five minutes of the mission that might be the most cause for concern for the safety of the astronauts.
An uncrewed test of the Orion spacecraft in 2022 first highlighted problems with the heat shield. This is the part of Orion that bears the brunt of the searing heat the capsule experiences during re-entry through Earth’s atmosphere.
When engineers examined the Orion heat shield from 2022’s Artemis I mission, they found large chunks of material had been lost. The worry was that, should this happen again on the crewed Artemis II mission, it could expose the interior of the capsule to dangerously high temperatures.
Since the earliest days of human spaceflight, engineers have protected capsules from the extreme heat of re-entry with so-called “ablative” heat shields, made from material that’s designed to burn away evenly as the capsule scorches its way through the atmosphere.
To meet the demands of the reusable space shuttle, Nasa developed an incredible heat shield system made from ultra-light tiles of glass-coated silica fibres. While this heat shield had extraordinary thermal properties, it was also exceptionally fragile, and required exhaustive maintenance after every shuttle mission.
It was damage to this fragile and exposed protection system that caused the tragic loss of space shuttle Columbia in 2003. For the Artemis programme, Nasa has returned to the concept of an ablative heat shield.
The heat shield for the Orion capsule is composed of a material called Avcoat, based on the material originally developed for the Apollo programme. Although Nasa considered other, newer materials for the Orion heat shield, they ultimately decided on the material that had been proven in flight by the Apollo missions.
However, the structure of Orion’s heat shield differs from those used during Apollo. The Apollo heat shield comprised a singular honeycomb matrix of about 320,000 individually filled hexagonal segments. To make the heat shield for Orion more efficient and reproducible to manufacture, Nasa has opted for a configuration of around 180 individual blocks.
This heat shield was first tested in 2014 when an uncrewed Orion capsule was launched to an apogee of 3,600 miles by a Delta IV rocket. The capsule blazed through the atmosphere on its return at a temperature of about 2,200°C (4,000°F), but the heat shield proved itself capable of withstanding such an inferno.
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 uncrewed demonstration of the mission planned for Artemis II. Hurtling through Earth’s atmosphere from a far greater distance than the first test, the spacecraft reached temperatures of around 2,800°C (5,000°F). It’s here that the first concerns about the Avcoat heat shield were raised.
Instead of burning away evenly over the whole surface, parts of the Artemis I heat shield were lost unexpectedly in uneven chunks. This uneven ablation makes modelling the thermal loads of re-entry more unpredictable, and raises the possibility that the Orion capsule could be exposed to dangerous levels of heating.
On investigation, the cause of this uneven ablation was found to be irregular releases of gases trapped within the heat shield material, compounded by the “skip re-entry” profile adopted by the mission.
In the skip profile, Orion first grazes the edge of the atmosphere to slow down. It then uses the aerodynamic lift of the capsule to skip back out of the atmosphere, before re-entering for its final descent to Earth. The skip profile is so named because it somewhat resembles a stone skipping across a pond.
Nasa investigators found that, when heating rates decreased during the period between dips into the atmosphere, thermal energy accumulated inside the Avcoat material. This led to the build up of gases and, in turn, the internal pressure – causing cracks and the uneven shedding of material.
Based on the lessons from Artemis I, Nasa has adopted a number of measures to protect the crew of Artemis II. For the first crewed mission of the programme, Nasa has kept the Avcoat heat shield material, but updated the design of the blocks to help the gases to escape during re-entry.
Furthermore, instead of the skip profile, Nasa has now opted for a more direct re-entry mode for the Orion capsule. This reduces the uncertainty in the heating profile and means less time at peak temperatures for trapped gases to damage the heat shield, but also means that the crew will be subjected to increased deceleration on re-entry.
Safety first
At the height of the drama in the film Apollo 13, flight director Gene Kranz famously declares to the team at mission control that “failure is not an option”.
Although the line was in fact the product of the film’s screenwriters, it’s become not just the second-most quotable line from the film, but also somewhat of a mantra at Nasa itself.
Nowhere is this more true than with the heat shield of Artemis II. During the final phase of the Artemis II mission, there’s no backup, no contingency, and no chance of escape. The four astronauts on board will be depending on a few inches of resin-coated silica to shield themselves from temperatures approaching half that of the surface of the Sun.
Human spaceflight has always brought with it calculated risks, but it has also provided a uniquely human perspective on our place in the cosmos. The Artemis II mission will make its crew the first humans in over half a century to observe the blue marble of planet Earth in its entirety with their own eyes.
The crew will carry with them the hopes and aspirations of a whole new generation of explorers. They will be depending on the meticulous work of thousands of scientists and engineers for their safe return, bringing with them a renewed human perspective on not just the Moon, but the planet we all call home.
– ref. Heat shield safety concerns raise stakes for Nasa’s Artemis II Moon mission – https://theconversation.com/heat-shield-safety-concerns-raise-stakes-for-nasas-artemis-ii-moon-mission-275853
