CALLISTO is a reusable VTVL demonstrator propelled by a small 40 kN Japanese LOX-LH2 rocket engine. It is being developed jointly by the German Aerospace Center (DLR), the French Space Agency (CNES), and the Japanese Aerospace Exploration Agency (JAXA).

Vertical takeoff, vertical landing (VTVL) is a form of takeoff and landing for rockets. Multiple VTVL craft have flown. The most successful VTVL vehicle was the Apollo Lunar Module which delivered the first humans to the Moon. Building on the decades of development, SpaceX utilised the VTVL concept for its flagship Falcon 9 first stage, which has delivered over two hundred successful powered landings so far.