Cryogenic propulsion, using liquid oxygen, methane, and hydrogen, offers significant improvements in rocket performance compared to traditional propellants that can be stored at room temperature. A key advantage of cryogenic systems is their compatibility with In-Situ Resource Utilisation (ISRU)—a strategy that involves sourcing and processing propellants directly from extraterrestrial environments, such as the Moon or Mars, rather than transporting them from Earth. This approach not only reduces launch mass and cost but also makes ambitious missions, like sustained lunar bases or journeys to Mars, more feasible than with conventional storable propellants. For instance, a Mars return mission utilizing LOX/LH2 and sourcing propellant directly from the planet could deliver a payload up to six times larger than a mission that does not employ in-situ resource utilization.

As global strategies shift towards a long-term presence on the Moon, the space industry faces new challenges and opportunities. Notably, the United States has prioritised technologies such as cryogenic refuelling in orbit, which enables spacecraft to replenish their fuel supplies during missions and thus remain operational for longer periods (Artemis III and beyond). These developments underscore the essential role of advanced cryogenic and ISRU technologies and highlight how Europe stands to benefit from accelerating its own technology development in this area. For instance, European industry could contribute to lunar logistics and support international efforts for sustained exploration.
In response to these evolving demands, the European Space Agency (ESA) ESTEC has developed a comprehensive roadmap for advancing propulsion technologies, with a focus on long-term in-space cryogenic propulsion and ISRU-derived propellants. This roadmap aims to encourage and coordinate European research and development and raise awareness on the importance of these technologies.

This paper provides an overview of the current European state of the art in long-term in-space cryogenic propulsion and ISRU propellants. It outlines the necessary steps to achieve a mature suite of technologies, describes ESA’s strategic roadmap, and discusses the challenges and opportunities Europe is facing, as it adapts to the global shift towards enhanced space exploration.
 

Keywords
Space Propulsion|ISRU|Space Exploration|Cryogenic Propellants|Propellant refilling