Satellite Collision Avoidance
As more satellites are launched into orbit around Earth, the chances of collisions between satellites and space debris continue to rise. Satellite collisions can generate thousands of pieces of dangerous debris, potentially triggering a chain reaction of subsequent collisions known as the Kessler syndrome. Avoiding collisions in space is therefore critical to ensure the sustainability and safety of space operations.
There are several key aspects to effective satellite collision avoidance:
Monitoring the Space Environment
The first step is monitoring the population of satellites and debris in orbit to identify potential collision risks. Ground-based radars and telescopes track and catalog objects down to 10cm in size, while statistical models estimate the untracked population. The US Space Surveillance Network maintains the most extensive public catalog of space objects. Commercial companies like Leo Labs and ExoAnalytic also track satellites. However, limited sensor coverage and data sharing between agencies hampers space situational awareness.
Identifying Collision Risks
Operators analyze upcoming conjunctions between their satellites and other objects to assess collision risk. The probability of collision is calculated based on the objects’ sizes, predicted miss distance, geometry, and orbital uncertainties. An alert is generated if the probability exceeds a mission-defined threshold, commonly around 1 in 10,000. As the encounter time approaches, the probability estimate is refined with new tracking data. Hundreds of alerts are issued weekly for satellites in low Earth orbit.
Planning Avoidance Maneuvers
If the collision risk remains above the threshold, an avoidance maneuver is planned. The satellite adjusts its orbit to increase the miss distance at the time of closest approach. Executing the maneuver days in advance provides time for proper analysis while minimizing orbital disruption. Maneuver plans are screened to ensure risks from other objects remain low.
Autonomous Collision Avoidance
Future large satellite constellations will require automated collision avoidance. Satellites will autonomously share position data, assess risks, plan maneuvers, and execute after coordinating with other operators. Machine learning can help optimize decision making. Ground systems provide oversight and intervene if necessary. This will prevent delays from ground communications.
Mitigating Orbital Debris
Preventing debris-generating events is crucial to limit collisions long-term. Post-mission disposal procedures direct satellites to reenter Earth’s atmosphere within 5-25 years after end of life. Design measures like passivation and shielding aim to eliminate accidental explosions. International guidelines deter irresponsible activities like anti-satellite tests. Removing existing debris with harpoons, nets, and tugs may help remediate the environment.
Key Collision Avoidance Technologies
Several technologies play important roles in enabling effective collision avoidance:
High-Accuracy Tracking
Precise orbital data on both objects is required for reliable collision risk assessment. Ground-based radars and optical sensors are improving in resolution and sensitivity to detect and track smaller debris. Commercial companies are building large-scale tracking networks with greater coverage, accuracy, and revisit rates. Onboard navigation provides real-time ephemeris data.
Conjunction Analysis Algorithms
Advanced software algorithms accurately model the orbits of space objects and quantify the collision risk for upcoming close approaches. Uncertainties in the orbit state vectors are incorporated using Monte Carlo simulations. The probability estimate is refined as uncertainties reduce over time with new tracking data.
Maneuver Planning Software
Following a collision alert, planning software calculates an optimal avoidance maneuver that maximizes miss distance while minimizing fuel use, orbital disruption, and risks from other objects. The software models the spacecraft’s propulsion system and operational constraints to create a safe and efficient maneuver plan.
Autonomous GNC Algorithms
Guidance, navigation, and control (GNC) algorithms allow satellites to autonomously execute collision avoidance maneuvers once approved. The GNC system tracks the satellite’s state, calculates and performs thruster burns, and handles contingencies. Onboard collision avoidance is critical for large constellations.
Space Traffic Management Systems
As space gets more congested, a traffic management framework will help coordinate collision avoidance across government and commercial operators. STM systems allow position data sharing, maneuver deconfliction, and communication of intent. Global standards and best practices for STM are still in development.
Challenges for Collision Avoidance
While collision avoidance is an established practice today, improvements in certain areas are needed to scale up to the megaconstellations of the future:
- Data Sharing and Space Situational Awareness – Limited data sharing between commercial and government operators hampers space situational awareness. A lack of regulations also enables irresponsible behaviors like anti-satellite tests. Improved data sharing and international norms of behavior are needed.
- Tracking Capabilities – Current sensor networks cannot adequately track the small debris that poses the greatest collision threat. Upgraded tracking capabilities are essential for reliable risk assessment.
- Traffic Management Framework – Clear protocols are needed for exchanging maneuver plans between operators and deconflicting actions to prevent collisions. A traffic management framework will be crucial as space gets more congested.
- Autonomous Systems – Existing collision avoidance relies on time-consuming human-in-the-loop operations. Fully autonomous systems will be needed to scale up for megaconstellations.
- Orbital Debris Remediation – Active debris removal may be necessary to stabilize the growth of debris long-term. Methods like harpoons and tugs show promise but technology is immature. Legal and liability issues also exist.
Summary
Satellite collision avoidance is vital to preserve our access to and use of space. The increasing probability of collisions as orbit becomes more congested threatens the sustainability of the space environment and space missions essential to worldwide infrastructure. While avoidance practices are effective today, improvements must be made to processes, policies, and technologies to safely scale up space operations. Implementing an advanced space traffic management framework, expanding tracking and situational awareness capabilities globally, and transitioning to autonomous collision avoidance systems will help ensure safe satellite operations for decades to come.