Introduction

Humanity may be tempted to view the cosmos as a rich reservoir of infinite resources. However, the practical reality of space exploration tells a different story. The need for sustainability in space exploration, as well as the exploitation of space, is becoming more evident with the strengthening desire for the expansion of human activities beyond Earth-orbit, pursued by public and private sectors alike. When applied to space, the concept of sustainability has often been understood as "ensuring that all humanity can continue to use outer space for peaceful purposes and socioeconomic benefit now and in the long term"1. Until now, it mainly referred to the need to control, regulate, and remove space debris from low Earth orbit (LEO)2,3 and planetary protection (which promotes the implementation and development of the responsible exploration of the solar system, in order to protect the space environments and Earth)3. As humans aspire to venture into deep space, the definition of this concept shifts and expands, and the self-sustainability of mission operations becomes a critical aspect. Loop-closure, which indicates the recycling and the reuse of resources toward the establishment of a circular economy, could greatly enhance the sustainability of space exploration, and it is key not only to minimise the costs of resupply of resources from Earth but also for ethical considerations associated with space waste generation and the preservation of extra-terrestrial environments4,5,6,7,8,9,10. The United Nations resolved that outer space activities should minimise impacts on the space environment, as well as on Earth, taking into account the 2030 agenda for Sustainable Development11,12.

The biggest impediment to progress on this frontier is the lack of deployable technologies enabling outposts, extended missions and, in the future, settlements, to sustain themselves through in situ resource utilization (ISRU) and maximised recycling of resources5. In addition to mechanical/physical/chemical approaches, biotechnologies broadly and microorganisms specifically will help enable long-term life-support and habitat systems’ performance (loop-closure), as well as ISRU, manufacturing and energy collection/storage6,7,13,14,15,16,17. Microbiological approaches can be self-sustaining with occasional monitoring and maintenance, owing to their resilience, and could overall require less energy than physicochemical approaches16.

Here, we focus on the pivotal roles that microorganisms can play in the development of technologies for sustainable human exploration of deep space, considering two main aspects: (i) the requirement for mature biotechnologies and bioprocesses to allow near closed-loop operations of mission functions, such as life-support, to increase autonomy and sustainability; and (ii) the need to reduce supply chain dependency for the expansion of human presence in space. The approaches presented here are based on processes and technologies currently implemented on Earth at different technology readiness levels (TRL), which must be adapted to meet the specific requirements and challenges of the space environment. Selecting the most suitable bioprocess and most applicable microorganism for any given space application is non-trivial, as terrestrial technologies are rarely readily adaptable to the harsh conditions of space18. Therefore, extensive research and development are compulsory to increase TRL to the point where these technologies can be successfully implemented in space16. Finally, microbial biotechnologies aimed to increase the sustainability of space exploration may be translatable to Earth applications for advancement towards a circular economy, further supporting the United Nations Sustainable Development Goals (SDGs)11,12.

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