The Tentative Payload Complement for the L4 Flagship Mission to Enceladus

In the frame of the Voyage 2050 Science Programme, the European Space Agency (ESA) is preparing its next large-class flagship mission to Enceladus (L4), following the launch of JUICE (L1), the adoption of LISA (L2), and the ongoing reformulation phase of the NewAthena (L3) mission. Set to arrive in the early 2050s, L4 aims to explore Enceladus’ potential for habitability, the presence of biomarkers, prebiotic chemistry, and the interaction with the external and endogenic environment, aligning with the Voyage 2050 recommendations for the science theme “Moons of the Giant Planets”.

Initially, the Concurrent Design Facility (CDF) at the European Space Research and Technology Centre conducted four internal feasibility studies, ranging from a solar-powered orbiter at Saturn to landing a probe on one of the moons of the gas giants such as Europa, Ganymede, Titan, or Enceladus [1]. Ultimately, the L4 Expert Committee (EC), an external scientific body appointed by ESA, prioritised landing on Enceladus with prior flybys of other moons of Saturn, considering its significant astrobiological interest as an ocean world and the fact that to date no other space mission has committed to visit this intriguing moon. The tiger stripes, located at the south pole of Enceladus, are renowned for active cryovolcanism, whereby a mixture of vapour and icy grains composed of water, salts, and organics is ejected into space [2, 3]. Consequently, the EC has recommended deploying a lander near this region and an orbiter equipped with plume sampling capabilities.

Currently, ESA is leading efforts with the continued support of the EC to consolidate the science requirements further [4], such that payload technologies with the required performance to fulfill those can be derived, accordingly. The EC Report [1] outlined a preliminary set of payloads that could satisfy the science objectives identified:

Lander Payloads:

• Mass Spectrometer for surface sample analysis,
• Micro-camera,
• Meteorological and geophysical payload suite,
• Miniaturised “Lab-on-a-Chip” laboratories for biomarker detection,
• Descent imagers for digital terrain mapping,
• Sample Acquisition System.
  
  

Orbiter Payloads:

• Remote sensing package (visible, near-infrared, and thermal imaging),
• Magnetometer,
• Ice penetrating radar,
• Dust and gas analysers,
• Gravity and radio science experiment.
  
  

Due to the mission design drivers (e.g., mass, power, and data), new technologies requiring fewer resources than traditional payloads are desired. As of today, no payloads have yet been selected, albeit with allocations for preliminary resource budgets in place to respect the top-level mission constraints. The orbiter will use large solar arrays to provide sufficient power in the Saturnian system, enabling a high-power solar electric propulsion system. The lander is designed to operate for a minimum of two weeks on the surface of Enceladus, powered entirely by batteries, with energy tightly rationed for surface operations. Additionally, the unique environment on Enceladus demands careful design considerations to adhere to planetary protection regulations.

From March 2025 onwards, a Payload Working Group (PWG) has been established to assist ESA in defining a Strawman payload in preparation for a future call, with further details to be announced. Towards this objective, the PWG will support the analysis of various aspects, including but not limited to:

• Payload concepts suitable for achieving the L4 mission objectives for the lander and orbiter, building on the above list of preliminary payload complement,
• The trade-offs of payload concepts and their performance,
• The technical maturity and technology gaps for possible innovation,
• A concept of operations in line with the operational constraints on mission level.
  
  

In parallel to these efforts, the engagement of the broader technical and technology community in Europe is necessary to begin advancing the development of their payloads, thereby enhancing the probability of their selection. For instance, miniaturisation of payload designs is strongly encouraged for resource optimisation. Identifying critical technology gaps and developing and testing prototypes in the laboratory and the relevant environment is essential to achieve the recommended Technology Readiness Level before mission adoption in 2034 (TBC). Further, it is crucial for the reliable detection of biosignatures to study processes preventing spacecraft-induced or forward contamination as well as false positives. To this end, payload teams from European universities and research institutes are invited to explore funding opportunities in coordination with their respective national space agencies and ESA technology funding programmes.

References

[1] Martins, Z., et al.: Expert Committee for the Large mission covering the science theme “Moons of the Giant Planets”, 2024.

[2] Porco, C. C., et al.: Cassini observes the active South Pole of Enceladus, Science, Vol. 311, pp. 1393-1401, 2006.

[3] Postberg, F., et al.: Macromolecular organic compounds from the depths of Enceladus, Nature, Vol. 558, pp. 564-568, 2018.

[4] Helbert, J., et al.: The Mission to Enceladus – The ESA L4 mission, EPSC-DPS2025-1307, 2025.