9.1 Graveyard orbits and re-orbiting techniques

Graveyard orbits and re-orbiting techniques are crucial for managing space debris. These strategies involve moving defunct satellites to higher orbits, reducing collision risks with active satellites. It's a balancing act between fuel reserves, orbital mechanics, and long-term stability.

Re-orbiting requires careful planning and execution. Satellites need enough fuel for end-of-life maneuvers, and operators must consider factors like timing, collision risks, and international guidelines. It's all about responsible satellite disposal to keep space safe and usable.

Graveyard Orbits

Definition and Characteristics

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• Graveyard orbit refers to a disposal orbit where defunct satellites are placed at the end of their operational life to reduce the risk of collisions with active satellites
• Super-synchronous orbit is an orbit above the geostationary orbit (GEO) altitude of 35,786 km, typically used for graveyard orbits
• Orbital stability is a crucial factor in selecting graveyard orbits to ensure long-term containment of disposed satellites and minimize the risk of their re-entry into the protected regions
• Long-term evolution of graveyard orbits is influenced by various perturbations such as solar radiation pressure, third-body effects (lunar and solar gravity), and the Earth's oblateness

Advantages and Challenges

• Graveyard orbits provide a safe disposal option for satellites at the end of their operational life, reducing the probability of collisions with active satellites in the geostationary orbit region
• Placing satellites in graveyard orbits requires additional fuel reserves and careful planning of end-of-life operations to ensure successful re-orbiting maneuvers
• The selection of appropriate graveyard orbits involves considering factors such as the satellite's mass, area-to-mass ratio, and the desired long-term stability of the disposal orbit
• Challenges associated with graveyard orbits include the need for accurate orbit determination and control, as well as the potential for long-term orbit instability due to perturbations

Re-Orbiting Techniques

Maneuvers and Requirements

• Re-orbiting maneuvers involve raising the satellite's altitude to a graveyard orbit at the end of its operational life
• Delta-v requirements for re-orbiting depend on the satellite's initial orbit, mass, and the desired graveyard orbit altitude
• Fuel reserves must be allocated and managed throughout the satellite's mission to ensure sufficient propellant is available for end-of-life re-orbiting maneuvers
• End-of-life operations include planning and executing the necessary maneuvers to raise the satellite's orbit, as well as deactivating and passivating the spacecraft to minimize the risk of explosions or fragmentation

Strategies and Considerations

• Re-orbiting strategies may involve a single burn or a series of smaller maneuvers to gradually raise the satellite's altitude over time
• The timing and sequence of re-orbiting maneuvers are critical to minimize the risk of collisions with other satellites during the disposal process
• Spacecraft design considerations for re-orbiting include the placement and sizing of propulsion systems, as well as the optimization of fuel consumption throughout the mission
• International guidelines and best practices, such as the Inter-Agency Space Debris Coordination Committee (IADC) recommendations, provide guidance on re-orbiting altitudes and post-mission disposal procedures

Risk Assessment

Collision Probability and Mitigation

• Collision risk assessment involves evaluating the probability of collisions between the disposed satellite and other objects in the geostationary orbit region
• Factors influencing collision risk include the satellite's size, shape, and altitude, as well as the population of objects in the surrounding orbital environment
• Mitigation measures to reduce collision risk include selecting appropriate graveyard orbit altitudes, minimizing the satellite's cross-sectional area, and ensuring the spacecraft is passivated and stabilized after disposal
• Regular tracking and monitoring of disposed satellites in graveyard orbits are essential to assess their long-term evolution and identify any potential risks or instabilities

Long-Term Orbit Evolution and Stability

• Long-term orbit evolution of disposed satellites in graveyard orbits is influenced by various perturbations, such as solar radiation pressure, gravitational effects, and the Earth's oblateness
• Assessing the long-term stability of graveyard orbits is crucial to ensure that disposed satellites remain confined within the designated disposal region and do not pose a risk to active satellites
• Numerical simulations and analytical models are used to predict the long-term behavior of disposed satellites and evaluate the effectiveness of graveyard orbit selection
• Regular updates and refinements to collision risk assessment methods are necessary to incorporate new data, improved models, and changes in the orbital environment over time