Space-based remote sensing now allows detection and monitoring of large, concentrated methane sources worldwide. However, independent evaluation of instruments and algorithms is very limited, and new satellites and AI techniques have not been tested under controlled, single-blind conditions. To address this gap, we conducted a year-long, single-blind testing campaign >15 times larger than prior tests. Testing included 746 events (481 large-scale methane releases and 265 blank controls). We released metered methane quantities in Arizona, USA coincident with overpasses from 12 global satellites (government, commercial, and non-profit). This is the first evaluation of prominent methane instruments EMIT, Tanager-1 (Carbon Mapper), and GHOSt. Fifteen analysis teams from nine countries generated >2,250 estimates. Across all sensors and teams, 70.6% correctly identified the presence or absence of emissions. Performance is heterogeneous: “built for purpose” instruments designed to target methane demonstrate excellent performance, with strong detection rates (92-100% true positive rate, 0-11% false positive rate) and quantification accuracy (unbiased parity slopes 0.83-1.00, 44% mean absolute percent error, MAE). Instruments repurposed for methane sensing have more variable performance (detection rates 62-88%, MAE 46%); still, many teams demonstrate significant skill. Broad land scanners performed poorly but have consistent global coverage and vast historical archives, motivating additional work. AI-based methods show some promise, but caution is warranted; 79% of false positives emerge from these methods. Ground-measured cloud cover exceeding 20% caused strong decline in performance across all instrument classes, raising concerns for use in some locations. Our findings show that high-quality methane measurements from space can allow accurate detection and unbiased quantification, initiating a new era in international greenhouse gas mitigation.