The Argonaut mission: Paving the way for European nuclear use in space

April 18, 2023, 7:00AMNuclear NewsGrzegorz Ambroszkiewicz, Alexander Getimis, and Paloma Villar

Long-duration missions with limited solar exposure need a reliable power source to operate. This makes nuclear power sources (NPSs) an attractive alternative to solar energy for such missions. The implementation of the ESA Safety Policy on the Use of Nuclear Power Sources by the European Space Agency’s Independent Safety Office (ISO) provides a framework for ensuring the safe use of NPSs and sets a standard for future ESA missions. This article provides an overview of how the ISO is implementing the policy in the development and operation of the Argonaut mission, which serves as a valuable case study for understanding the practical application of the ESA safety policy and the importance of ensuring the safe use of NPSs in space.

The Argonaut mission

The ability to land on the moon’s surface is a key aspect of ESA’s Terrae Novae 2030+ Strategy Roadmap.1 Argonaut (formerly called the European Large Logistics Lander, or EL3) is a planned space mission by the ESA to land a large spacecraft on the surface of the moon. Part of ESA’s vision for human and robotic exploration of the moon, the aim of the mission is to deliver supplies and support infrastructure for long-term, sustainable exploration and utilization of the moon with a focus on international cooperation.

Argonaut’s design overview

Fig. 1. Concept art of the Argonaut lunar lander. (Image: ESA/ATG-Medialab)

Argonaut is a robotic multimission that enables a series of proposed ESA missions to the moon that could be configured for different operations such as cargo delivery, returning samples from the moon or prospecting resources found on the moon. The spacecraft is composed of a lunar descent element (LDE) and a cargo platform element (CPE).

The LDE is the element responsible for propulsion from launcher separation in lunar transfer orbit all the way to the surface of the moon. It has an interface for a lunar landing (legs mechanism) and interfaces with the CPE with cargo or scientific instruments, depending on the mission.

Radioisotope heater units

Fig. 2. The 3-watt European RHU. (Image: ESA)

The function of the Argonaut’s radioisotope heater units (RHUs) is to keep propellants or electronic equipment within a temperature range that will ensure their operation.2 Heat production is ensured by radioisotope decay. After evaluating production capacities and estimated costs, americium-241 was chosen as the European NPS. The chemical form of the fuel is a ceramic form of uranium doped americium oxide.

The radioisotope fuel is encapsulated in a system of physical barriers to prevent the risk of radioactive material release during the nominal environments of launch and to minimize the risk of dispersal under accident environments. Both the RHU design (which would produce a minimal thermal power of 3 watts) and the European Large Heat Source (ELHS) design (which would produce a minimal thermal power of 200 watts) have been considered.

ESA safety policy

The safety assessment of radioactive material use—and their potential harm to humans and the environment in Earth’s biosphere—will always be an inherent part of the design and operation of space missions that make use of an NPS. Therefore, the ESA missions utilizing NPSs are subject to independent nuclear safety evaluations, including operations and end-of-life phases, to assess the risk to people and the environment.3

ESA policy principles

ESA NPS policy has introduced principles to secure nuclear safety that ensure that a mission utilizing an NPS is appropriately justified; that alternative power sources have been considered; and that the project is managed by developing, implementing, and maintaining a safety culture.

To ensure the protection of people and the environment, the potential impact of NPSs on human health and the environment will be assessed. This includes evaluating the potential for exposure to ionizing radiation and the potential for contamination of the environment through the release of radioactive material. Additionally, the assessment will address nominal and accidental scenarios, including transport of the radioisotope material, integration, and all mission phases.

All ESA missions are subject to launch approval. Missions that include an NPS must demonstrate compliance with all relevant safety regulations and requirements. This includes ensuring that the spacecraft and its systems are designed, constructed, and tested to meet safety standards and that the launch vehicle and its systems are suitable for the safe deployment of the spacecraft.

Further, ESA missions utilizing an NPS will be designed, developed, and conducted to ensure that radiation risk to people and the environment arising from normal operations and potential accidents is acceptable and as low as reasonably achievable (ALARA). Potential risks during all mission phases will be assessed and quantified to the extent possible.

Consideration of nuclear safety during ground operations, launch, and in-flight operations (including accidents involving fallout) is a necessity. The design and development of the missions with NPSs will include identification, evaluation, and implementation of the design features, controls, and preventive measures that reduce the probability of potential accidents as well as the magnitude of potential radiological release. Those features will be verified and validated though tests and analyses, as appropriate, and effectiveness will be assessed through risk analysis.

In addition to safety processes to ensure contained risk, measures to handle potential consequences of accidents that may release radioactive material into Earth’s environment will also be evaluated and implemented, as required. This evaluation will include a crisis management approach with identification of various possible radiological crisis scenarios and the criteria for triggering them. The associated resources required for all situations from an accident during ground operation to fallout in ground areas inside and outside the launcher site needs to be provided.

Missions with NPSs must comply with all relevant national regulations for the terrestrial and launch phases. The aspects of NPS handling and transportation prior to arrival at the launch site will be assessed, taking into account the sovereignty of countries. International regulations, such as resolutions of the United Nations and International Atomic Energy Agency4,5 must also be followed.

A project including NPSs will also need to provide a clear definition of the responsibility for nuclear safety authorization processes with the relevant nuclear authorities in order to avoid conflicting or duplicative requirements.

One crucial aspect of the safety and success of ESA missions utilizing NPSs is the nuclear safety files (NSF). A preliminary NSF is subject to review as a part of the integrated project review process, and the final NSF is reviewed by an independent organization when the ESA seeks launch authorization.

Before any ESA mission that involves an NPS can be launched, it be endorsed by the member states of the agency. This process ensures that the mission aligns with the policies and regulations set forth by the member states and ensures the safety and security of the mission and its stakeholders.

Independent safety assessment

The independent safety assessment provides recommendations to the director general of the ESA before authorization is given to proceed with the mission.

It is important to note that ISO assessments are conducted independently and are not influenced by the spacecraft’s manufacturer or mission team. Therefore, assessments remain impartial and unbiased and the results accurately reflect the spacecraft’s nuclear safety status.

Due to the potential radiation risk to humans and the environment, the use of NPSs in ESA missions must have a strong justification. In the assessment, benefits of mission results are weighed against the risks. It also considers the use of NPSs against the use of an alternative nonnuclear design.

Radiation level scenarios, likelihood, and consequences for both nominal and accidental situations are reviewed and assessed. This risk assessment identifies potential hazards and consequences as part of the safety assessment process and helps define emergency response related to the use of radioactive materials.

The ESA and any partner organization must agree to a clear definition of responsibilities regarding nuclear safety, requirements, and processes to ensure adequate safety coverage and avoid conflicting or duplicative requirements and unnecessary burden. NPS-containing ESA missions will comply with relevant national and international regulations for the terrestrial and launch phases. All activities occurring during the terrestrial phase—such as development, testing, manufacturing, handling, and transportation—must comply with the applicable national and international standards and regulations relating to terrestrial nuclear installations and activities.

In addition to the safety process that ensure a contained risk during the use of NPSs, measures must be put in place as required to handle the consequences of accidents that could potentially release radioactive material into the environment. This includes developing and implementing contingency plans, determining the occurrence of radioactive material release, characterizing the areas and nature of release and contamination by radioactive material, and recommending protective measures to limit exposure of population groups in affected areas. Strict safety protocols and guidelines—both international and national—must be followed.

The contingency plan in the event of an NPS-related accident needs to outline all necessary steps to minimize harm and inform the public. It should identify key stakeholders, including international organizations, governments, non-governmental entities, and the general public. It should also ensure transparency in information sharing surrounding the accident with stakeholders and the public.


As the use of NPSs in space missions becomes more prevalent, it is important that their implementation be carried out with the utmost care and attention to safety. The Argonaut mission is a significant step forward for the safe use of NPSs in Europe and demonstrates Europe’s commitment to safe and responsible use of NPSs in space, paving the way for future missions and the advancement of space technology. The Argonaut mission marks a critical moment in the history of European space exploration and sets the stage for continued safe and responsible use of NPSs in space.

Furthermore, close collaboration with international organizations, governments, and space agencies is essential to ensure that best practices in nuclear safety are shared and adopted globally. As such, it is crucial to work together to establish and enforce global standards for the safe use of NPSs in space. This includes sharing best practices and technical expertise and collaborating on research and development efforts to continuously improve the safety of NPSs in space operations. International cooperation promotes transparency and builds trust between countries and organizations, which is critical for ensuring the safe use of NPSs in space.


We would like to express our gratitude to the Argonaut team and industrial partners for their invaluable contributions to space nuclear safety, who all demonstrated exceptional technical expertise and dedication, providing crucial support in implementation of safety measures. We also would like to acknowledge ESA and member states for funding and initializing Argonaut program. This work would not have been possible without the contributions of all those involved, and we would like to extend our sincerest thanks to each and every one of them.

Grzegorz Ambroszkiewicz, Alexander Getimis, and Paloma Villar are all with the European Space Agency.


  1. European Space Agency, Terrae Novae 2030+ Strategy Roadmap, June 2022.
  2. A. Barco et al., “Impact tests and modelling for the ESA radioisotope power systems”, Journal of Space Safety Engineering9, 1 (2022);
  3. European Space Agency, “Implementation of the guidelines provided for in the international safety framework for nuclear power source—The ESA safety policy on the use of nuclear power sources,” United Nations Committee on the Peaceful Uses of Outer Space Scientific and Technical Subcommittee, Vienna (Feb. 11–22, 2019).
  4. United Nations Office for Outer Space Affairs, “Resolution: Principles Relevant to the Use of Nuclear Power Sources in Outer Space,” resolution A/RES/47/68 adopted by the General Assembly (1993).
  5. United Nations Committee on the Peaceful Uses of Outer Space Scientific and Technical Subcommittee and the International Atomic Energy Agency, Safety Framework for Nuclear Power Sources Applications in Outer Space, A/AC.105/934 (2009).

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