Mark A. Sturza, 3C Systems Company; Gemma Saura Carretero, Viasat, Inc.
Keywords: LEO; debris; Kessler Syndrome; collisions; maneuverability
Abstract:
Humanity faces an existential crisis; space debris are at risk of becoming the equivalent of a drifting island of plastic. New large constellation systems plan to operate tens of thousands, or even hundreds of thousands, of satellites in low Earth orbit (LEO), posing the threat of an inglorious end to the Space Age. A take risks and fail often approach to new technology is being extended to space without considering that mistakes in space cannot be cleaned up as easily as they can on Earth.
Satellites that cannot maneuver, cannot avoid collisions. Loss of maneuverability can result from failures of satellite sub-systems in the maneuver chain or from collisions with small (untracked) objects that disable these subsystems. Passive deorbit times can be minimized with lower LEO altitudes and larger area-to-mass ratios but can still require years depending on the solar cycle.
Even satellites that can maneuver, can be involved in collisions. Every time a satellite is not maneuvered in response to a low probability conjunction warning, there is a non-zero collision risk that depends on the space situational awareness (SSA) accuracy. Additionally, every time a satellite is maneuvered, there is another non-zero probability that the maneuver will result in a collision. In both cases, with a sufficient number of conjunctions occurring, even six sigma events can become likely.
Collisions between LEO satellites tend to be catastrophic resulting in large numbers of new debris objects spread across LEO altitudes. For example, there are still 1439 tracked debris objects in orbit from the 2009 collision between Iridium-33 and COSMOS-2251. While the collision occurred at 800-km, the debris apogees range from 400 km to over 1600 km. While satellite can be designed with high area-to-mass ratios to reduce passive deorbit times, fragmentation objects often have lower area-to-mass ratios and can persist for decades, even centuries.
Constellations are appropriately analyzed over their orbital lifecycles. Large constellations are incrementally deployed, and satellites are replenished as they fail, reach end-of-life, or are replaced with more capable models. This replenishment can be reasonably modeled to continue until the constellation is no longer economically viable. The result is a continuing process of orbit raising and phasing, and a combination of active and passive deorbiting.
Sub-system failures and small object collisions can be mitigated with sub-system redundancy, and small objects collision can be further mitigated with shielding. Operational techniques, such as initiating deorbit immediately after the (N 1)-th failure with N-th redundancy, can be used to improve the effective satellite reliability.
The debris environment naturally evolves over time as objects decay and new objects are created by collisions between existing debris objects. Satellite collisions can cause step increases in the debris population. In addition, there are over 900 derelict rocket bodies remaining in orbit which pose further debris generating risk.
Design trades are investigated for increasing the time to Kessler Syndrome (self-sustaining collision cascade). A Markov model is developed taking account of the changing debris environment during the orbital lifecycle of a constellation. Monte Carlo simulations of the model are used to identify a safe space boundary in the parameter space. It is shown that with an appropriate focus on design and operational reliability, it is possible to deploy broadband LEO systems with reasonable risk profiles.
Date of Conference: September 14-17, 2021
Track: Conjunction/RPO