Daniel Moomey, Air Force Institute of Technology; Rachael Falcon, Headquarters Air Force Safety Center; Arbab Khan, Booz-Allen-Hamilton
Keywords: Conjunction Assessment, Conjunction Data Message, Space Environment, Sustainability, Space Policy, Orbital Collision
Abstract:
The Air Force Safety Center is developing a program to quantify the distribution of risks to the orbital environment across time and space by trending and analyzing orbital conjunction events. Previous efforts described automating the intake of Conjunction Data Messages (CDM) generated by the 18th Space Defense Squadron (18 SDS) and developing pre-processing scripts necessary to populate information germane to orbital collision risk assessment. The intent of the current effort is to account not only for the probability of collision (Pc), but also the consequence of the debris products from a collision to the low Earth environment, should it occur. To accomplish this, CDMs of conjunction events below 1,000km in altitude, between 1 Jan 2016 and 31 Dec 2021 were analyzed. 18 SDS reported Probabilities of Collision (Pc) were re-computed using the Foster-92 method. A circular interaction area was applied instead of a square, to reduce conservatism and lend closer towards realism. Hard Body Radii estimates were pooled from externally sourced information, including The Aerospace Corporation and various open sources. When not available, radius estimates were imputed based on available data. Covariance information was sourced from CDMs. For consequence, the amount of damaging, lethal non-trackable and trackable debris products were estimated using The Aerospace corporations IMPACT 8.0 model. The model accounts for relative velocity, object masses, distribution, and structure. Object distribution and structure were inferred from the object type. Relative velocity was sourced from CDMs. The object masses were sourced from data furnished by The Aerospace Corporation and Union of Concerned Scientists satellite database. The expected number of damaging, lethal-non-trackable, and trackable debris products were multiplied by the Pc to produce a quantitative risk metric for each conjunction event. Risks were adjusted to account for likely collision avoidance maneuvers based on satellite age, mass, and Pc. The results show a logarithmic increase in total risk over time, particularly after 2020. This change correlates to the arrival of Starlink and OneWeb satellites on orbit, and analysis confirmed that the uptick in risk is attributable to the arrival those large constellations on orbit. The risk metric for Starlink vs Starlink conjunctions was further adjusted to more accurately account for proprietary processes and high accuracy ephemerides which enables better information for performing collision avoidance maneuvers. This adjustment reduced the slope of the risk trends starting in 2020, but they are still positive and logarithmic. Spatial domain analysis revealed that the preponderance of debris production risk for reported conjunctions below 1,000 km resides between the altitudes of 450 km and 850 km. The spatial distribution of risk by altitude changed starting in 2020 due to the arrival of large constellations at both their operational altitudes, and as a function of orbit raising operations, resulting in a higher and more evenly distributed risk of debris production above 350 km. Results also indicate that more trackable debris would have likely been generated by collisions between payloads and trackable objects, with perigees below 1,000 km than the 37 that have been confirmed and cataloged during that time period. However, analysis of the four confirmed payload collision events with cataloged debris produced a bootstrapped confidence interval of trackable debris generated per year suggests the trackable debris production rate estimated here is within a plausible range. Planned future work looks to address known assumptions and limitations. The processes and metrics demonstrated herein will be improved upon and repeated on an annual basis, with the aim to inform risk trends of environmental hazards to space operations and the sustainability of the low Earth orbital environment. Analyses such as these are important to better inform ongoing and future Space Safety and Space Traffic Management policy development efforts within the US Government and internationally.
Date of Conference: September 27-20, 2022
Track: Conjunction/RPO