Mark Sturza, Viasat; Mark Dankberg, Viasat; William Blount, Viasat
Keywords: LEO, debris, resilience, robustness, redundancy, constellation, fragmentation
Abstract:
Low Earth orbit (LEO) space remains a precarious domain to operate in irrespective of the steps taken to mitigate against risks. Even with best-in-class Space Surveillance and Tracking (SST), Space Situational Awareness (SSA), and Space Traffic Management (STM), collisions will inevitably occur as large constellations continue to deploy. The fragments created by each collision increase the debris flux, increasing the likelihood of future collisions. If the debris creation rate exceeds the debris removal rate (due to atmospheric drag or active debris removal means) positive feedback (cascading collisions) occurs, leading to exponential growth in the debris flux and eventually resulting in loss of access to a region of LEO. This paper develops a model that helps examine the extent to which a particular design may increase a LEO constellation’s resilience, or, alternatively, increase its fragility, in the face of different types of intentional fragmentation events or other interventions.
Proliferated LEO constellations consisting of thousands of satellites have been developed to provide continued coverage even if an appreciable fraction of satellites is no longer functional. However, this redundancy comes with an often-unconsidered cost, the increased sensitivity of large constellations to fragmentation events. These events can occur either through accident or through intention. Accidental events include satellite explosions and collisions. The probability that at least one satellite in a constellation will be fragmented by an accidental collision increases with the size of the constellation. The increasing numbers of satellites stresses SST, SSA, and STM systems (including autonomous collision avoidance systems). Intentional events can result from kinetic (ASAT), energy (EMP), or cyber interventions. A recent ASAT test shows that hostile activities by sovereign actors in space represent a very real threat to safe space.
While debris spread above and below the collision altitude by fragmentation, the most significant increase in probability of a subsequent collision occurs for other satellites with the same altitude and inclination as those of the fragmented object. By design, large LEO constellations concentrate satellites into a few, often only one, altitude/inclination pair. This rigid constellation structure makes them less resilient to cascading collisions than the same number of satellites in random orbits. Constellation designs with satellites partitioned among multiple altitude/inclination pairs may also increase the fragility of the overall constellation to fragmentation events affecting a subset of its satellites.
Previous work has focused on using “source-sink” models, systems of non-linear differential equations, to model the carrying capacity (i.e., the sustainable satellite population distribution in LEO) of various combinations of orbits and satellite characteristics (such as mass and cross-sectional area). These models have provided an approach to better estimate future debris propagation using a residual carrying capacity metric that enables comparing holistic global contributions to debris propagation as a function of specific system characteristics and deducing the incremental impact of individual systems and characteristics on LEO carrying capacity. While useful, these models address population dynamics, not the dynamics of individual objects required to assess resilience or fragility to specific fragmentation events.
In this paper, a Monte Carlo simulation is used to explore the sensitivity (i.e., resilience or fragility) of various constellation architectures to intentional fragmentation events or other interventions. All objects, including debris, are propagated discreetly. The propagator uses semi-analytic methods based on a general perturbation theory to model gravity (through J2) and atmospheric drag (Jacchia 1977 atmospheric model with Blitzer decay model). Fragmentation events are modeled using the EVOLVE 4.0 NASA Standard Breakup Model (SBM) suitably modified to conserve mass. Sufficient Monte Carlo trials are generated to provide high, 95%, confidence in the results.
A baseline is established by using the simulation to evolve reference large LEO constellations over a 5-year timeframe with nominal failure rates initialized with the current debris flux background. Once the baseline has been established, the effect of intentional fragmentation events, such as by an ASAT, is evaluated as the constellation size is increased to determine the point at which increasing size results in less resilience (i.e., increased fragility) to intentional fragmentation events. The simulation start time is adjusted through a full solar cycle to determine sensitivity. The effect of atmospheric contraction is also investigated.
The simulation is additionally used to model the sensitivity of individual, and combinations of, current and proposed large LEO constellations with respect to three types of intentional interventions. The first type, in which individual satellites are fragmented, such as by an ASAT, at various rates and in various combinations. The second type, where all of the satellites in a specific region of space are simultaneously rendered non-maneuverable, for example by an EMP event. The third type, in which all of the satellites in a specific constellation are rendered non-maneuverable simultaneously, such as from the result of a cyber-attack. Non-maneuverable satellites cannot avoid collisions, and thus pose a significantly higher risk of being fragmented by an accidental collision.
The simulation results provide guidelines for designing LEO constellations targeted to applications where resiliency is paramount, including commercial, civil, defense, and security applications. They also inform governments and regulators with respect to matters such as the risks to other uses of space created by certain LEO constellation designs. Additionally, they provide input to determining future SST, SSA, and STM requirements.
Date of Conference: September 17-20, 2024
Track: Space Debris