9.2 Controlled atmospheric re-entry

Controlled atmospheric re-entry is a crucial strategy for safely disposing of spacecraft at the end of their missions. This process involves carefully planning and executing the spacecraft's descent through Earth's atmosphere to minimize risks to people and property on the ground.

Re-entry techniques include propulsive methods using thrusters and passive systems relying on natural forces. The dynamics of re-entry, including the re-entry corridor and breakup process, are key factors in ensuring a safe disposal of space debris.

De-orbit Techniques

Propulsive De-orbit Methods

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• Deorbit maneuvers involve changing the orbit of a spacecraft to cause it to re-enter the Earth's atmosphere
• Propulsive de-orbit uses thrusters or engines to perform a retrograde burn, reducing the spacecraft's velocity and lowering its perigee into the dense atmosphere
  • Requires the spacecraft to have sufficient remaining propellant and functional propulsion system at the end of its mission
  • Commonly used for large spacecraft (International Space Station modules) and those in higher orbits (geostationary satellites)

Passive De-orbit Systems

• Passive de-orbit systems rely on natural perturbations and forces to gradually lower the spacecraft's orbit over time
• Atmospheric drag is the primary passive de-orbit mechanism for low Earth orbit (LEO) satellites
  • As the spacecraft encounters drag forces from the thin upper atmosphere, its orbit decays until it re-enters
  • Effectiveness depends on factors such as the spacecraft's altitude, cross-sectional area, and mass
• Other passive de-orbit techniques include using deployable structures (drag sails or balloons) to increase the spacecraft's area-to-mass ratio and accelerate orbital decay

Re-entry Dynamics

Re-entry Corridor and Breakup

• The re-entry corridor is the range of angles and velocities at which a spacecraft can enter the atmosphere to ensure a controlled and safe re-entry
  • Too steep of an angle can cause excessive heating and dynamic pressure, leading to disintegration
  • Too shallow of an angle may result in the spacecraft skipping off the atmosphere and failing to re-enter
• Ablation is the process of the spacecraft's surface material vaporizing and eroding away due to extreme heat during re-entry, acting as a protective heat shield
• Breakup altitude is the point at which the spacecraft or its components disintegrate due to re-entry forces, typically around 40-80 km altitude depending on the vehicle's design and re-entry conditions

Re-entry Survival Analysis

• Re-entry survival analysis assesses the likelihood of spacecraft components or debris surviving the re-entry process and reaching the ground
• Factors influencing survival include the material properties, size, shape, and mass of the components
  • Dense, high-melting-point materials (titanium or stainless steel tanks) are more likely to survive than lightweight, low-melting-point materials (aluminum or plastic)
  • Smaller, more aerodynamic shapes are more likely to burn up completely compared to larger, blunt objects
• Surviving debris poses risks to people and property on the ground, necessitating careful design and disposal planning to minimize hazards

Risk Assessment

Ground Track Prediction and Casualty Risk

• Ground track prediction involves modeling the spacecraft's re-entry trajectory and determining the potential impact locations of any surviving debris
  • Takes into account factors such as the spacecraft's orbit, attitude, and breakup characteristics
  • Uncertainties in the prediction arise from limitations in atmospheric density models, solar activity effects, and the complex dynamics of the disintegrating spacecraft
• Casualty risk assessment quantifies the probability of a person being struck by re-entering debris based on the predicted ground track and population density data
  • Typically expressed as the expected number of casualties per re-entry event
  • International guidelines recommend limiting the casualty risk to less than 1 in 10,000 for controlled re-entries
• Mitigation strategies to reduce casualty risk include performing controlled re-entries over uninhabited areas (oceans), designing spacecraft for more complete breakup, and scheduling re-entry events during times of low population density along the ground track