Nature Astronomy, Published online: 21 July 2026; doi:10.1038/s41550-026-02912-8 Based on activity inferred from observed comet-like nongravitational acceleration, we report the detection of a cometary tail on a near-Earth object that was previously classified as inactive. This finding demonstrates that nongravitational accelerations can be used to infer the outgassing of ices on covertly ‘inactive’ objects such as dark comets.