Trending and analysis of payload vs. All low earth conjunction data messages below 1000 km, from 2016 through 2021

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) previously generated by the 18th and 19th Space Defense Squadrons (18 SDS, 19 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 orbital environment, should it occur. To accomplish this, the team analyzed all reported CDMs of conjunction events below 1000 km in altitude, between 1 Jan 2016 and 31 Dec 2021. 18 SDS reported Pcs were re-computed using the Foster-92 method. A circular 2-dimensional 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 Corporation's IMPACT 8.0 model. The model accounts for relative velocity, object masses, mass distribution, and structure. Object distribution and structure were inferred from the object type field in the CDMs. Relative velocity was also 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 launch of Starlink and OneWeb satellites into orbit, and analysis confirmed that the uptick in risk is attributable to the launch of those large constellations into 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 launch of large constellations, resulting in a higher and more evenly distributed risk of debris production between 350 and 1,000 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 suggesting the trackable debris production rate estimated here is within a plausible range. Planned future work looks to address known assumptions and limitations. This work is the first known effort to quantitatively analyze historical risks of conjunction events using full sets of CDM data and fuse them with the latest semi-analytical breakup algorithms. Analyses such as these are necessary to better inform ongoing and future Space Safety and Space Traffic Management policy development efforts within the US Government and internationally.