Reusability—launching a vehicle, bringing it back to Earth and then sending it into space again—is one possible way of reducing the cost of space launch. This is the path being explored by U.S. launch providers SpaceX and Blue Origin. CNES has been working on reusability concepts since the 1980s, but none has resulted in a real-world demonstration. To fill this gap, the agency has therefore decided to launch a small, fully reusable spacecraft called Callisto with international partners.

Standing 15 metres tall and with a diameter of one metre, Callisto will be powered by a reusable cryogenic engine running on hydrogen and oxygen. It is not intended to become an operational launcher, but is rather a demonstrator designed to conduct flight testing and mature the complex technologies required to return a launcher to Earth, as well as refurbishment operations between flights, and to precisely cost an operational European launcher with a reusable first stage.

Standing for “Cooperative Action Leading to Launcher Innovation in Stage Toss-back Operations”, Callisto is a project pursued since November 2015, chiefly by CNES and the German space agency DLR. CNES is in charge of architecture studies for the launcher and ground systems. A large number of international contractors are contributing to the experiment.