Identifying hidden nuclear material in orbit from its own neutron spectrum

<title>Abstract</title> <p>The Outer Space Treaty bans nuclear weapons from orbit but lacks a dedicated means of verifying what a closed spacecraft contains. The orbital environment supplies a natural interrogation field: trapped protons induce neutron emission, allowing passive inspection without an active source. Detection can flag an anomalous object from kilometre standoff, but a count does not determine composition. Here we construct a conventional aluminium spacecraft and a uranium-bearing object with nearly identical detector-accepted neutron counts, whose reconstructed spectra remain distinct. In the baseline geometry the full spectrum reaches an area under the receiver-operating characteristic of 0.90 after about 70 accepted neutrons. A tungsten configuration matched to the uranium count rate preserves only about 2.2 per cent of the per-neutron spectral separation, raising the requirement to about 3200 neutrons for discrimination and 8200 for 90 per cent power at 5 per cent false attribution against that alternative. Under a published detector-rate benchmark, ten inspectors at 500 m require about one week for discrimination and less than three weeks for controlled attribution. These thresholds define an inspection claim envelope: detection, discrimination and attribution are distinct measurements with distinct range–dwell regimes. The environment supplies the interrogation; proximity determines what may responsibly be claimed.</p>