Artemis II Crew to Face Extreme Hypersonic Re-entry Upon Earth Return
The Artemis II astronauts are poised for a dramatic conclusion to their historic mission as they prepare for a high-speed, hypersonic re-entry into Earth's atmosphere. After setting a new record by travelling 406,771 kilometres from our planet, the four crew members will confront one of the most challenging phases of their ten-day journey: a controlled descent that pushes the limits of human endurance and engineering.
A Blazing Fast Descent Through the Atmosphere
The Orion capsule carrying the Artemis II crew will be travelling at an astonishing velocity of more than 11 kilometres per second, equivalent to 40,000 kilometres per hour, when it first encounters Earth's atmosphere. This remarkable speed is approximately forty times faster than a typical passenger jet aircraft. In terms of kinetic energy, which represents the energy an object possesses due to its motion, the Orion capsule will have nearly two thousand times more kinetic energy per kilogram than a commercial aeroplane.
To achieve a safe landing, the spacecraft must dramatically reduce this kinetic energy to almost zero, allowing parachutes to deploy effectively. Unlike conventional aircraft that are designed to minimise aerodynamic drag for fuel efficiency, re-entering spacecraft are engineered to maximise drag. This deliberate design choice helps slow the vehicle down as it plunges through the upper atmosphere, using the air resistance as a natural braking system.
Managing Intense Deceleration Forces
The deceleration experienced during re-entry is measured in g-forces, which compare the acceleration or deceleration force to standard Earth gravity. While a Formula One driver might endure around five g-forces during sharp cornering, which is near the maximum a human can withstand without losing consciousness, uncrewed re-entry capsules can experience forces exceeding one hundred g. These robotic vehicles complete their atmospheric entry in less than a minute, but such extreme forces are unsuitable for human astronauts.
Crewed vehicles like NASA's Orion capsule employ lift forces to extend the re-entry process over several minutes, thereby reducing the g-forces to levels that are survivable for humans. This careful management of deceleration is crucial for ensuring the crew's safety during the intense atmospheric passage.
Surviving Temperatures Hotter Than the Sun
As the Orion capsule re-enters the atmosphere at more than thirty times the speed of sound, it will generate a shock wave that heats the surrounding air to temperatures exceeding 10,000 degrees Celsius. This is approximately twice as hot as the surface of the Sun. The extreme heat ionises the air, creating an electrically charged plasma that temporarily blocks radio communications, leaving the astronauts unable to communicate during the most severe parts of their descent.
To protect against these phenomenal temperatures, the spacecraft is equipped with a sophisticated thermal protection system. This system functions as an insulating blanket, shielding the vehicle and its occupants from the hypersonic flow outside. Materials capable of withstanding intense heat are strategically placed on surfaces expected to encounter the harshest conditions, with thicknesses precisely calibrated for optimal performance.
Advanced Heat Shield Technology
The thermal protection materials are designed to glow red-hot and degrade slightly during re-entry, yet they remain intact throughout the process. This red-hot glow helps radiate heat back into the atmosphere rather than allowing it to penetrate the spacecraft. Through this meticulous design, the Artemis mission can navigate through air at 10,000 degrees Celsius while maintaining a maximum heat shield surface temperature of only about 3,000 degrees Celsius.
Most spacecraft utilise ablative heat shields made from carbon fibre and phenolic resin glue. These shields absorb energy and release a relatively cool gas along the vehicle's surface, aiding in cooling. The Orion capsule employs a specific ablative material called AVCOAT, a modern version of the heat shield used on the Apollo missions that returned from the Moon in the late 1960s and early 1970s.
Learning from Previous Missions
Following the uncrewed Artemis I test flight, engineers observed that the heat shield ablation during re-entry was more extensive than anticipated, with large chunks of material detaching in certain areas. After thorough analysis, they concluded that this occurred due to pressure buildup during the "skip" phase of entry, where the spacecraft briefly exited the atmosphere to cool down before making a second entry for landing.
For the Artemis II mission, engineers have opted to retain the same AVCOAT heat shield but have slightly modified the trajectory. The revised flight path still utilises lift forces but incorporates a less defined "skip" to mitigate the risk of material separation. This adjustment reflects NASA's commitment to continuous improvement and safety in human spaceflight.
The achievements of NASA and the Artemis II astronauts thus far are truly remarkable, capturing global attention. However, many experts and observers will breathe a collective sigh of relief only when the crew is safely welcomed back on Earth, having successfully navigated the extreme conditions of hypersonic re-entry.
