As NASA’s Artemis II mission unfolds, attention is turning to one of the most critical phases of the journey: the trip home.  

With the Artemis II crew now preparing for re-entry aboard the Orion spacecraft, Contact spoke with Dr Chris James, a UQ expert in hypersonic aerothermodynamics and planetary entry. He explains what the astronauts are about to experience, why this phase is so dangerous and the innovative engineering behind Orion's return to Earth.

What will be the single most dangerous moment during Artemis II’s return to Earth, and why?

The most dangerous moment will be the high-speed, hypersonic re-entry that occurs as the craft is travelling through the upper atmosphere. This is the riskiest point for several reasons. Firstly, it’s when the heating and the forces hitting the craft itself are at their maximum, due to aerodynamic drag slowing the craft from hypersonic speeds of around 11 km per second (greater than Mach 30!).

Secondly, this is also a scary moment because the gas surrounding the craft is heated up so much that it becomes an electrically conductive plasma – which blocks communication between the spacecraft and the outside world. This means the astronauts are unable to talk to anybody back on Earth for that short period of time.

When the capsule hits Earth’s atmosphere, what are the astronauts actually experiencing physically in those first few minutes?

The astronauts will be strapped in and – to put it frankly – getting the hell shaken out of them. They will experience fairly high g-forces (multiple g). However, these are definitely forces humans can survive for short periods of time. The scariest part, in my view at least, is the communications blackout blocking radio communications, as it means they’ll be completely alone during the harshest parts of the trajectory.

Spacecraft returning from low Earth orbit have found ways to remove this limitation by shooting communications into the wake behind the vehicle where the plasma is weaker, but I doubt they will be able to do this for Artemis II due to the higher-speed re-entry.

Artemis II is returning at extraordinary speeds. How does re-entry from the Moon compare to coming back from low Earth orbit?

Spacecraft in low Earth orbit are orbiting the planet at around 8 km per second, and they need to lose all this velocity to land safely back on Earth. In terms of kinetic energy (which is one half times velocity squared, or ½mv²), return from low Earth orbit involves around 32 MJ/kg of kinetic energy.

Spacecraft returning from the Moon re-enter at about 11 km per second. While this doesn't sound like much more in terms of velocity, the kinetic energy is almost doubled (to 60 MJ/kg). Because of this, the re-entry environment becomes much harsher, and the temperature behind the shockwave which envelops the vehicle during re-entry is much higher.

This results in much harsher heating to the surface of the craft from the hot gases flowing over it, as well as making the gas so hot that it begins to radiate, burning the surface of the spacecraft without even touching it. The effect of this radiation is understood less than the traditional heat flux to the surface of the vehicle from the flow of hot gases, which dominates for slower entries such as return from low Earth orbit. As I mentioned, the hotter post-shock temperature creates more plasma to be managed, meaning the communications blackout will be longer and harder to manage.

NASA is using a type of skip re-entry for Artemis II – what does that actually mean, and why is it so important for crew safety?

Artemis II is using less of a skip-entry profile than Artemis I due to issues with the heat shield, but they will still use something called aerodynamic lift to protect the crew.

For context, the simplest type of planetary entry is called a ballistic entry, where no aerodynamic techniques are used to reduce the velocity. This is used for the return of robotic sample return capsules such as the JAXA Hayabusa and Hayabusa2 capsules, and the NASA Stardust and OSIRIS-REx missions. Ballistic entry is fast, but can result in g-forces of up to 100 g – very unsafe for crewed missions.

To allow a crew to survive re-entry, aerodynamic lift must be used to spread the entry out in time, which greatly lowers the g-forces. The Space Shuttle did this by ‘gliding’ into the atmosphere with a maximum g-force of around 1 g (even though I'm sure they were shaken around a lot!). While capsules can't produce as much lift as the Space Shuttle did for crew comfort, they can still lower the g-forces significantly by flying off-axis at what’s called an angle of attack. This is done routinely by capsules that return from the International Space Station such as the SpaceX Crew Dragon.

A full skip-entry results in the spacecraft having enough lift that it leaves the atmosphere fully before performing a second entry starting from a lower re-entry speed. That is, it ‘skips’ once before completing re-entry.

The Apollo missions instead did what they called a double dip where there was a defined pull up, but the spacecraft didn’t fully leave the atmosphere before second entry. I imagine something similar will be done for Artemis II.

What are engineers most closely watching during re-entry, and what would signal that something isn’t going to plan?

Artemis II engineers will be focused on the heat shield which covers the bottom surface of the spacecraft and protects it from the extreme atmospheric entry conditions which will surround it during entry. Engineers will always be focused on the heat shield, as it burns away during the entry and reaches surface temperatures of over 3,000 degrees! However, Artemis II engineers will be specifically focused on it this time due to issues with the heat shield on the Artemis I mission.

The Artemis II Orion capsule uses a heat shield material called AVCOAT which is a new version of the heat shield material which was used on the Apollo capsule in the 1970s. After the Artemis I entry, many of these tiles showed much more wear than had been expected.

This was believed to have been caused partially by pressure building up inside the tiles as they cooled during the 'skip' part of the re-entry before the spacecraft re-entered for the second time. This is why the full skip re-entry is not being performed this time.

What does successfully bringing Artemis II home tell us about the future of human missions to the Moon and beyond?

It has been about 50 years since we last sent people this close to the Moon. We semi-routinely bring small spacecraft back from other parts of the solar system now in missions such as the JAXA Hayabusa and Hayabusa2 missions and the NASA Stardust and OSIRIS-REx missions. These return at really high speeds of up to 13 km per second, but it's a bit more exciting (and scarier!) when humans are up there doing it.

Because these Moon missions require the largest rockets ever built just to send a small spacecraft to the Moon and back, I don't think the average person will be going to the Moon for a holiday anytime soon.

However, I feel that we have mastered low Earth orbit now – we do it routinely and quite safely – so it’s nice to see us pushing the limits again. I hope that this mission will open up the future of crewed Moon exploration for scientists from around the world, hopefully letting us learn a lot more about this famous rock that we see in our skies.

With future rockets such as the SpaceX Starship on the way, hopefully many more people will get to the moon in the not-too-distant future!