Cryogenic propellant management in space: open challenges and perspectives This paper presents open challenges and perspectives of propellant management for crewed deep space exploration. The most promising propellants are liquid hydrogen and liquid methane, together with liquid oxygen as an oxidizer. These fluids remain liquid only at cryogenic conditions, that is, at temperatures lower than 120 K. To extend the duration of space exploration missions, or even to enable them, the storage and refueling from a cryogenic on-orbit depot is necessary. We review reference missions, architectures, and technology demonstrators and explain the main operations that are considered as enablers for cryogenic storage and transfer. We summarize the state of the art for each of them, showing that many gaps in physical knowledge still need to be filled. This paper is based on recommendations originally proposed in a White Paper for ESA's SciSpacE strategy. Since the first crewed mission reached the surface of the moon more than 50 years ago, many nations strive for more open and broader access to space. The research and development to enable and improve space accessibility by robotic machines and human beings led to a discovery boost in several fields. Physical and biological sciences have had a new ground of exploration, but new technologies have also enabled a better life on Earth. After more than 50 years, research and development is still needed to push the boundaries of space exploration. Human space endeavors beyond low Earth orbit (LEO) are now the new target. Propulsion is an important subsystem of a spacecraft or launcher, and propellant is by far its largest mass fraction. For instance, ~98.5% of the Saturn V launch mass was propellant and propulsion systems. Many national space agencies are working to enable deep space exploration. One example is NASA's exploration program, which outlines the basic requirements for future exploration missions. On 14.10.2020, one of NASA's tipping point selection was: "Cryogenic Fluid Management Technology Demonstration: NASA and industry partners have developed and tested numerous technologies to enable long-term cryogenic fluid management, which is essential for establishing a sustainable presence on the Moon and enabling crewed missions to Mars. Implementation of the technologies in operational missions requires further maturation through in-space demonstrations." The awarded companies are Eta Space of Meritt Island, Florida, Lockheed Martin of Littleton, Colorado, SpaceX of Hawthorne, California, and United Launch Alliance (ULA) of Centennial, Colorado. In the last two decades, several propulsion systems have been proposed and analysed for deep space exploration. The most promising ones are those fully based on nuclear thermal power (requiring liquid hydrogen) and on nuclear electric power plus cryogenic chemical propulsion for large velocity change maneuvers. The studies show that large amounts of cryogenic fuels need to be stored in space and transferred between spacecraft. For this reason, we focus this review paper on physical problems which are encountered inCryogenic propellant management in space: open challenges and perspectives This paper presents open challenges and perspectives of propellant management for crewed deep space exploration. The most promising propellants are liquid hydrogen and liquid methane, together with liquid oxygen as an oxidizer. These fluids remain liquid only at cryogenic conditions, that is, at temperatures lower than 120 K. To extend the duration of space exploration missions, or even to enable them, the storage and refueling from a cryogenic on-orbit depot is necessary. We review reference missions, architectures, and technology demonstrators and explain the main operations that are considered as enablers for cryogenic storage and transfer. We summarize the state of the art for each of them, showing that many gaps in physical knowledge still need to be filled. This paper is based on recommendations originally proposed in a White Paper for ESA's SciSpacE strategy. Since the first crewed mission reached the surface of the moon more than 50 years ago, many nations strive for more open and broader access to space. The research and development to enable and improve space accessibility by robotic machines and human beings led to a discovery boost in several fields. Physical and biological sciences have had a new ground of exploration, but new technologies have also enabled a better life on Earth. After more than 50 years, research and development is still needed to push the boundaries of space exploration. Human space endeavors beyond low Earth orbit (LEO) are now the new target. Propulsion is an important subsystem of a spacecraft or launcher, and propellant is by far its largest mass fraction. For instance, ~98.5% of the Saturn V launch mass was propellant and propulsion systems. Many national space agencies are working to enable deep space exploration. One example is NASA's exploration program, which outlines the basic requirements for future exploration missions. On 14.10.2020, one of NASA's tipping point selection was: "Cryogenic Fluid Management Technology Demonstration: NASA and industry partners have developed and tested numerous technologies to enable long-term cryogenic fluid management, which is essential for establishing a sustainable presence on the Moon and enabling crewed missions to Mars. Implementation of the technologies in operational missions requires further maturation through in-space demonstrations." The awarded companies are Eta Space of Meritt Island, Florida, Lockheed Martin of Littleton, Colorado, SpaceX of Hawthorne, California, and United Launch Alliance (ULA) of Centennial, Colorado. In the last two decades, several propulsion systems have been proposed and analysed for deep space exploration. The most promising ones are those fully based on nuclear thermal power (requiring liquid hydrogen) and on nuclear electric power plus cryogenic chemical propulsion for large velocity change maneuvers. The studies show that large amounts of cryogenic fuels need to be stored in space and transferred between spacecraft. For this reason, we focus this review paper on physical problems which are encountered in