Artemis II re-entry: how Orion will bring astronauts safely back to Earth

Following the successful completion of their lunar mission, the Artemis II crew are preparing to come back to Earth.

The four astronauts have set a new record for the greatest distance humans have travelled from Earth, reaching 406,771 kilometres from their home planet.

Their return journey will end with a rapid, hypersonic and intensely hot plunge into Earth’s atmosphere, followed by a splashdown in the Pacific Ocean off California at approximately 8 pm local time on 10 April.

Re-entry is the final ordeal of the crew’s extraordinary ten-day mission. Although it presents numerous hazards, their spacecraft carries a range of technologies intended to protect them.

A speedy re-entry

As Orion reaches Earth’s atmosphere with the Artemis II astronauts aboard, it will be travelling at over 11 km/s (40,000 km/h). That is 40 times faster than a passenger aircraft.

You can watch a livestream of the crew’s return here:

Measured by kinetic energy - the energy an object has because it is moving - Orion will have nearly 2,000 times more kinetic energy for every kilogram of vehicle at re-entry than a passenger jet.

As with every spacecraft returning to Earth, it must shed almost all of this kinetic energy before deploying its parachutes and landing safely.

Spacecraft do this through a controlled descent into Earth’s upper atmosphere. Aerodynamic drag acts as a brake, slowing the vehicle down.

Aircraft are usually shaped to be aerodynamic, reducing drag and therefore fuel use. Re-entering spacecraft are designed in the opposite way: they are made as un-aerodynamic as possible, maximising drag to help reduce their speed.

The resulting deceleration can be exceptionally severe.

Acceleration and deceleration are commonly measured in g-forces, or "g's". This describes an acceleration or deceleration force divided by the standard acceleration caused by Earth’s gravity. A Formula One driver experiences more than 5 g's when cornering, close to the highest g-forces a person can withstand without losing consciousness.

Small uncrewed return capsules, including NASA’s OSIRIS-REx capsule that delivered samples from asteroid Bennu, simply barrel into the atmosphere and decelerate rapidly. Such entries take under a minute, but can produce g-forces exceeding 100 - acceptable for robotic spacecraft, though not for people.

Crewed spacecraft such as NASA’s Orion capsule instead use lift to extend their entry. This keeps g-forces at levels humans can tolerate and means re-entry takes several minutes.

A very hot Artemis II re-entry

Orion will enter the atmosphere at more than 30 times the speed of sound.

A shock wave will surround the spacecraft and generate air temperatures of 10,000°C or higher - roughly double the temperature at the Sun’s surface.

That intense heat converts air passing through the shock wave into electrically charged plasma. Radio transmissions are temporarily interrupted as a result, leaving the astronauts unable to communicate during the most punishing phase of descent.

Making sure it’s a safe re-entry

To withstand the extraordinarily harsh conditions of re-entry, spacecraft trajectories are carefully designed to limit heating as far as possible.

The vehicle also has a thermal protection system: in effect, an insulating blanket that shields the spacecraft, its crew and any cargo from the hypersonic flow outside.

This system is precisely tailored to both the spacecraft and its mission. More heat-resistant materials are placed where conditions will be most severe, while material thickness is also carefully specified.

During entry, these materials are intended to glow red hot and deteriorate - but remain intact. Their red-hot glow also sends heat back into the atmosphere, rather than allowing the spacecraft to absorb it.

This careful engineering allows Artemis to travel through air at 10,000°C while keeping the heat shield’s maximum surface temperature to only about 3,000°C.

Most spacecraft use protective materials known as ablatives, which are generally composed of carbon fibre and a glue-like material called phenolic resin.

Ablative heat shields take in energy while releasing comparatively cool gas into the airflow over the vehicle’s surface, helping cool the system.

Orion’s ablative heat shield uses a material called AVCOAT. It is a form of the material that protected Apollo capsules returning from the Moon in the late 1960s and early 1970s.

Although Artemis I, an uncrewed test mission, was highly successful, its heat shield experienced substantially more ablation during re-entry than anticipated. In several areas, large pieces of material broke away from the shield.

Following extensive inspections and analysis, engineers nevertheless chose to proceed with the same type of heat shield for Artemis II.

They consider that Artemis I shed pieces of its heat shield because pressure built up within the material during the "skip" section of entry. During this phase, the spacecraft left the atmosphere to cool before making a second entry for landing.

For Artemis II, engineers have opted to adjust the trajectory slightly, continuing to use lift but with a less pronounced "skip".

It is remarkable to see what NASA and the astronauts have accomplished on the mission so far. Yet, like many others, I will be relieved to see them safely welcomed home on Earth.

Chris James, Senior Lecturer, Centre for Hypersonics, School of Mechanical and Mining Engineering, The University of Queensland

This article is republished from The Conversation under a Creative Commons licence. Read the original article.