Abstract
The Outer Space Treaty (OST) was opened to signatures in 1967 and, since then, 117 countries, including China, the USA and Russia, have become part of it1. Among other stipulations, the treaty bans the placement of nuclear weapons in outer space. Recently, the US government has raised worries that Russia is testing nuclear-armed anti-satellite weapon (ASAT) components, with the possibility that it will place a nuclear weapon in space. Such a device, if detonated, would destroy most of the satellites in the low Earth orbit. This danger is compounded by the lack of a verification mechanism for the OST. No methodologies of verification have been proposed in the open peer-reviewed literature. Here a concept and feasibility study is presented for verifying a satellite’s compliance to the OST by observing the neutrons induced by spallation from the approximately GeV protons in the inner Van Allen radiation belts2. The calculations show that a 9U-CubeSat-sized detection platform can identify a thermonuclear weapon from a distance of 4 km in approximately one week of observation. This conceptual study will stimulate and inform future research and development of verification platforms for the OST.
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Code availability
Analysis of the output was performed using Python scripts and Jupyter notebooks with the numpy, scipy and matplotlib libraries. The simulations were performed using the grasshopper/Geant4 toolkit18. The Geant4 simulation input and macro files, the grasshopper geometry definitions and the complete Python/Jupyter analysis package used to produce all results and figures in this work are publicly available and can be found at https://github.com/ustajan/kosmos, comprising a listing of the most important parts of the modelling toolkit. All of the GitHub sub-directories contain a README file with detailed information relevant to the particular directory.
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Acknowledgements
I would like to thank G. Tukharyan for generating the IRENE AP9/AE9 model output used for Fig. 1. Special gratitude is due to J. C. Nino, I. Jovanovic and K. Hartig for some early discussions about the use of GCR-induced signatures. Thanks are due to J. Hecla for many valuable discussions. V. Narang, A. Long, P. Vaddi, G. Ginet, E. Evans, B. Parham, G. Stokes and S. Van Broekhoven provided great feedback on the ideas used in this study. I thank C. Nitta from Lawrence Livermore National Laboratory (LLNL) and S. Kemp for introducing me to the problem of OST verification. The GPT-5.2 large language model was used for the development of the analysis code, which then underwent thorough testing and verification by the author.
Funding
This work was in part funded by the NNSA NA-221 award DE-NA0003920. This publication was made possible in part by a grant from Alfred Carnegie Foundation and Longview Philanthropy USA Inc. The statements made and views expressed are solely the responsibility of the author.
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Danagoulian, A. Verification of the Outer Space Treaty with cosmic protons. Nature (2026). https://doi.org/10.1038/s41586-026-10783-2
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DOI: https://doi.org/10.1038/s41586-026-10783-2
