The Science Behind Orbital Velocity and Safe Reentry

Understanding Orbital Velocity

To stay in a stable orbit above Earth, an object must travel at a specific velocity relative to Earth's surface. This velocity ensures that the object is moving fast enough to counteract the gravitational pull of the planet, effectively allowing it to "fall" around Earth rather than directly towards it. At high altitudes, where the atmosphere is thin, a stable orbit can be maintained without the need for propulsion due to the lack of friction that would otherwise slow the object down.

For instance, satellites and the International Space Station (ISS) orbit the Earth at speeds of approximately 17,500 miles per hour. This high velocity is crucial for maintaining their orbit and avoiding a gradual descent back to the planet's surface. The balance between gravitational force and the object's forward momentum is what keeps it in a stable orbit.

Reentry and Velocity Reduction

To safely descend to Earth's surface from orbit, an object must reduce its orbital velocity significantly. This process is complex and requires precise calculations to ensure that the object reenters the atmosphere at the correct angle and speed. An astronaut ejected from a spacecraft would still travel at nearly the same speed as the spacecraft, meaning their velocity does not disappear upon ejection.

Using the atmosphere to reduce velocity subjects the astronaut to friction and heat generation. Without heavy heat protection, the astronaut would most likely burn up during reentry. The heat generated during reentry is due to the object's velocity, not the object itself. At orbital speeds, the atmosphere in front of the astronaut compresses enough to heat up to a plasma, which can reach temperatures of thousands of degrees Fahrenheit.

The Dangers of Reentry

The heated plasma would burn a hole in the astronaut's suit, leading to fatal consequences. Light objects sometimes survive reentry, but humans are too heavy and their suits not heat-resistant enough. The MOOSE program in the 1960s aimed to create a device for a single astronaut to deorbit safely. The MOOSE system included a rocket motor, heat shield, parachute, radio, and other survival gear, but it never fully flew on any mission.

Joe Kittinger, part of the MOOSE program, held the record for the highest parachute jump for several decades. The program tested various components in space flights, but the challenge of safely reentering the atmosphere remained significant. Jumping into water from a great height can feel like hitting concrete due to high speed, similar to reentry speeds in air. At reentry speeds, the friction with air generates enough heat to burn up the astronaut.

Energy Requirements for Safe Reentry

Slowing an object from 17,500 mph to 0 mph requires an insane amount of energy, equivalent to the energy needed to speed it up. Rubbing against the air at high speeds creates immense heat, which would burn the astronaut. The recent Starship reentry, a steel vehicle with heat shield tiles, showed that even with protection, hot gas can cause severe damage, indicating a human in a spacesuit would not survive.

These challenges highlight the importance of advanced technology and precise engineering in ensuring the safe reentry of astronauts and spacecraft. The development of heat shields, reentry capsules, and other protective measures are critical to overcoming the extreme conditions encountered during reentry. As space exploration continues to advance, understanding and mitigating these dangers will remain a top priority for ensuring the safety of astronauts and the success of future missions.