CALLISTO is the joint CNES, DLR and JAXA reusable-stage technology demonstration project. It combines vehicle development, progressively more demanding flight tests and refurbishment research to evaluate guidance, propellant management, engine maintenance and the operating economics of reuse. CNES studies began in 2015, with DLR and JAXA joining in 2017. The planned test campaign uses the refurbished Diamant launch site at the Guiana Space Centre. JAXA’s RV-X precursor completed its July 2026 flight-test series, with the resulting operational and technical knowledge feeding CALLISTO research.