Jose Pedro Ferreira, University of Southern California; Joseph Wang, University of Southern California; Ken-ichi Nomura, University of Southern California
Keywords: Spacecraft Demise, Atmospheric Reentry, Atmospheric Pollution, Mega-Constellations, Climate Change
Abstract:
The number of orbiting satellites has increased significantly in an unrestricted and unregulated manner over the last decades, threatening the sustainable access to space in the future. Ongoing plans from the commercial space sector to build mega-constellations of microsatellites will inevitably increase the number of orbiting bodies, adding up to the ever-growing number of pieces of debris, in spite of numerous claims of skepticism concerning its impact on ground- and space-based scientific assets [1].
Reentry rates are expected to continue growing as the number of orbiting bodies increase along with the envisioned need to deploy active debris removal solutions. While it is widely understood that most objects will completely burn during reentry, the effect of spacecraft demise on Earth’s atmosphere has only been lightly studied and the long-term impact remains unknown with possible consequences to the ozone layer [2].
We developed what is, to our knowledge, the first Molecular Dynamics (MD) simulation study on the atmospheric chemical mechanisms and byproducts generated by satellite reentry. This tool emulates the interaction between atoms, considering the interatomic potentials and external forces at each time step which are directly simulated at very reduced length scales. These simulations rely on empirically generated force fields that describe the behaviour of a given set of elements. The Reactive Force Field (ReaxFF) is used to describe the chemical bond breakage and formation based on a charge-equilibration approach, having been extensively used in the material science domain to study the oxidation process of metals and hypervelocity impacts.
MD simulations are then carried out to resolve chemical reactions and byproducts for Aluminum – a typical satellite structure constituent – under mesospheric reentry conditions (T > 2000 K). Results are subsequentially used to estimate the presence of oxides after the hypervelocity impingement of molecular oxygen and predict the accumulated increase of reentry byproducts in the mesosphere when compared with that from meteoroids entering the atmosphere and other natural sources [3]. This research presents results of large-scale supercomputer runs of our MD simulations to show consistency of our extrapolation methodology for larger time and length scales.
Resorting to the particle size distribution of reentry byproducts, we estimate the settling time of such compounds in the atmosphere using a 1D model that accounts for viscous forces as per Stokes’ Law corrected for high Knudsen numbers by the Cunningham Correction factor. Preliminary results show that pollutants may react with stratospheric ozone only decades after the reentry event [4], making this a problem that would remain unnoticed for longer than it should.
Publicly available reentry data are used to extrapolate our results to the entire reentry population. Past figures suggest that the total mass of re-entering objects summed up to 332 tonnes in 2022, resulting in a 21 % increase when compared with the previous year [5]. On its own, it already consists in a 87 % increase of Aluminum injected at the top of the atmosphere when compared with natural sources. Furthermore, it is worth noting that the injection of such chemicals may also influence the radiative forcing in the atmosphere. As pollutants tend to endure for decades in the atmosphere, they may also increase shortwave radiative forcing.
References:
[1] NASA, “Space Station Applications Accepted for Filing, Space Exploration Holdings, LLC (SAT-AMD-20210818-00105),” Report No. SAT-01598 , 2022.
[2] Federal Communications Commission, “Space Exploration Holdings, LLC Request for Modification of the Authorization for the SpaceX NGSO Satellite System,” Order Authorization IBFS File No. SAT-MOD-20200417-00037, 2021.
[3] J. P. Ferreira et al, “Quantifying Spacecraft Demise Byproducts in the Era of Mega-Constellations,” in 73rd International Astronautical Congress, France, 2022.
[4] J. P. Ferreira et al, “Impacts of Satellite Reentry on Atmospheric Composition in the Era of Mega-Constellations: Molecular Dynamics Simulations,” in AGU Fall Meeting 2022, United States of America, 2022.
[5] ESA Space Debris Office, “ESA’s Annual Space Environment Report,” GEN-DB-LOG-00288-OPS-SD, 2023.
Date of Conference: September 19-22, 2023
Track: Atmospherics/Space Weather