🛰️Space Debris Mitigation Unit 9 – Post–Mission Disposal Strategies

Key Concepts and Definitions

• Post-mission disposal (PMD) involves the safe and responsible removal of spacecraft from orbit at the end of their operational lifetime
• Space debris consists of non-functional spacecraft, abandoned launch vehicle stages, mission-related debris, and fragmentation debris
• Orbital lifetime refers to the amount of time an object remains in orbit before re-entering the Earth's atmosphere
• Deorbit is the process of intentionally maneuvering a spacecraft to re-enter the Earth's atmosphere and burn up or crash into the ocean
• Graveyard orbit is a disposal orbit above the operational orbit where spacecraft are moved at the end of their mission to reduce the risk of collisions
  • Typically used for geostationary orbit (GEO) satellites
• Passivation involves removing stored energy from a spacecraft, such as depleting propellant and discharging batteries, to reduce the risk of explosions
• End-of-life (EOL) disposal planning is the process of determining the appropriate PMD strategy for a spacecraft during its design and development phase

Importance of Post-Mission Disposal

• Mitigates the growth of space debris in Earth's orbit, which poses risks to operational spacecraft and future space activities
• Reduces the likelihood of collisions between defunct spacecraft and operational assets, preventing the generation of additional debris
• Ensures the long-term sustainability of space activities by preserving the orbital environment for future generations
• Complies with international guidelines and regulations, such as the Inter-Agency Space Debris Coordination Committee (IADC) guidelines and the United Nations Committee on the Peaceful Uses of Outer Space (COPUOS) guidelines
• Demonstrates responsible behavior and stewardship of the space environment by spacecraft operators and space-faring nations
• Minimizes the risk of uncontrolled re-entry of large spacecraft, which could pose a threat to human life and property on Earth
• Enables the efficient use of limited orbital resources, particularly in highly congested regions like low Earth orbit (LEO) and GEO

Types of Post-Mission Disposal Strategies

• Direct re-entry involves maneuvering a spacecraft to re-enter the Earth's atmosphere at the end of its mission, ensuring complete destruction
  • Suitable for spacecraft in LEO with sufficient propellant and control capabilities
• Controlled re-entry is a type of direct re-entry where the spacecraft is guided to a specific location (usually over an ocean) to minimize risks to human life and property
• Orbital decay relies on natural atmospheric drag to gradually lower the spacecraft's altitude until it re-enters the atmosphere
  • Appropriate for spacecraft in LEO without sufficient propellant for direct re-entry
• Graveyard orbits are used for spacecraft in GEO, where they are raised to a higher altitude (typically 300 km above GEO) to reduce the risk of collisions
• Lunar disposal involves sending a spacecraft to a collision course with the Moon, effectively removing it from Earth's orbit
• Solar disposal is a theoretical strategy where a spacecraft is sent into a heliocentric orbit or to collide with the Sun, but it requires significant propellant and is not yet practical

Technical Requirements and Challenges

• Spacecraft design must incorporate PMD considerations, such as sufficient propellant reserves, robust control systems, and the ability to passivate
• Accurate orbital lifetime prediction is necessary to plan PMD strategies and ensure compliance with guidelines (25-year rule for LEO)
• Re-entry survivability analysis is required to assess the risk of debris reaching the Earth's surface during uncontrolled re-entry
• Passivation procedures must be reliable and effective in preventing post-mission explosions or fragmentation events
  • Includes depleting propellant, discharging batteries, and venting pressurized systems
• Collision avoidance maneuvers may be necessary during the PMD phase to prevent impacts with other objects
• Tracking and monitoring of disposed spacecraft is essential to verify the success of PMD operations and update orbital debris models
• International cooperation and data sharing are crucial for effective PMD planning and execution, particularly for spacecraft in shared orbital regions

Regulatory Framework and Guidelines

• The Inter-Agency Space Debris Coordination Committee (IADC) provides a set of international guidelines for space debris mitigation, including PMD
  • IADC Space Debris Mitigation Guidelines (2002, updated 2007) serve as the foundation for many national and international regulations
• The United Nations Committee on the Peaceful Uses of Outer Space (COPUOS) has adopted the Space Debris Mitigation Guidelines (2007), which are based on the IADC guidelines
• National space agencies and regulatory bodies have developed their own space debris mitigation standards and requirements, often aligned with the IADC and COPUOS guidelines
  • Examples include NASA's Technical Standard 8719.14 and the European Space Agency's (ESA) Space Debris Mitigation Policy
• The International Organization for Standardization (ISO) has developed a series of standards related to space debris mitigation and PMD, such as ISO 24113:2019 (Space systems - Space debris mitigation requirements)
• Compliance with PMD guidelines and regulations is increasingly becoming a prerequisite for obtaining launch licenses and operating permits from national authorities

Implementation and Best Practices

• Incorporate PMD planning early in the spacecraft design and development process to ensure compatibility with mission objectives and regulatory requirements
• Conduct regular spacecraft health monitoring and orbit determination to inform PMD decision-making and ensure the availability of necessary resources
• Develop and test robust passivation procedures to minimize the risk of post-mission explosions or fragmentation events
• Plan for contingencies and fault scenarios that may affect PMD operations, such as loss of communication or control
• Engage in international collaboration and data sharing to improve the accuracy of orbital debris models and facilitate the coordination of PMD activities
• Implement a comprehensive space situational awareness (SSA) program to track and monitor disposed spacecraft and validate PMD success
• Regularly review and update PMD plans and procedures to incorporate lessons learned and adapt to evolving best practices and regulatory changes

Case Studies and Examples

• Iridium 33 and Cosmos 2251 collision (2009) highlighted the importance of PMD and the risks posed by non-functional spacecraft in LEO
• ESA's GOCE satellite successfully performed a controlled re-entry in 2013, demonstrating the feasibility of direct re-entry for LEO spacecraft
• The Mir space station was deorbited through a controlled re-entry in 2001, setting a precedent for the PMD of large, complex spacecraft
• Intelsat 901, a GEO communications satellite, was successfully moved to a graveyard orbit in 2020 using Northrop Grumman's Mission Extension Vehicle (MEV-1)
• The Chinese Tiangong-1 space station experienced an uncontrolled re-entry in 2018, raising concerns about the risks posed by unplanned PMD events
• NASA's Cassini spacecraft was intentionally disposed of through a controlled descent into Saturn's atmosphere in 2017, preventing post-mission contamination of potentially habitable moons

Future Trends and Innovations

• Development of advanced propulsion technologies, such as electric propulsion and solar sails, to enable more efficient and cost-effective PMD strategies
• Increased adoption of on-orbit servicing and active debris removal (ADR) technologies to extend spacecraft lifetimes and remove non-compliant objects
  • Examples include the ESA's ClearSpace-1 mission and the ELSA-d (End-of-Life Services by Astroscale) demonstration
• Integration of artificial intelligence and machine learning techniques to optimize PMD planning, monitoring, and execution
• Establishment of international frameworks for space traffic management (STM) to coordinate PMD activities and ensure the safe and sustainable use of Earth's orbits
• Development of new materials and designs for spacecraft to minimize the generation of debris during PMD operations and improve re-entry survivability
• Increased focus on the development of space sustainability rating systems and incentives to encourage responsible PMD practices among spacecraft operators
• Growing recognition of the importance of PMD in the context of the long-term sustainability of space activities and the preservation of the space environment for future generations