Defining the Strawman Payload for ESA’s L4 Mission to Enceladus

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IntroductionIn 2021, the ESA Voyage 2050 Senior Committee recommended a mission to the “Moons of the Giant Planets” as one of the upcoming ESA Large missions. Subsequently, ESA performed two Concurrent Design Facility studies that explored both Enceladus and Titan as potential landing targets. In 2024, the L4 Expert Committee (EC) released a report recommending Enceladus to be the target [1].Science Traceability MatrixAn L4 Science Traceability Matrix (STM) has been defined and refined during discussions between ESA, the L4 EC, and the L4 PWG. Its main science themes are:(A) Habitability and surface–interior interactions.(B) Enceladus interaction with the external environment and the Saturnian system.(C) Prebiotic chemistry and biosignature detection.Each Science Theme is further derived into Science Questions, and then Science Objectives. The STM up until the science objectives can be found on ESA’s Cosmos website [2]. The EC and PWG then derived potential physical parameters and observables that could address each science objective and the potential performances needed. Finally, the PWG identified and traded off existing payload technologies that could be used for compiling a strawman payload. The STM defines what science that shall be done, and ESA wants the community to provide ideas how it can best be achieved. The strawman payload only represents one potential complement that the PWG thought could fulfil as many science objectives as possible, within the available resources, but it does not in any way reflect any kind of down selection of the eventual final L4 payload complement.Scientifically Attributed Mass AllocationInitial industrial feasibility studies were already conducted in 2025, showing that, with the assumptions at that time, the available total payload mass of the mission, termed the Scientifically Attributed Mass Allocation (SAMA), is 950 kg. The SAMA encompasses all payload-related components, including the entire lander itself. The SAMA also includes the orbiter propellant needed to perform scientifically interesting plume sampling “flythroughs”, and any additional Saturnian Moon Tour flybys not included in the baselined tour otherwise optimized to reduce delta-V needed to enter orbit around Enceladus. Finally, the orbiter payload mass is also encompassed by the SAMA. A potential split of the SAMA is the following:Lander allocation: 600 kgMoon Tour flybys and Plume Flythrough Propellant allocation: 230 kgOrbiter Payload Mass allocation: 120 kgWhile the above SAMA split is baselined for the strawman payload, the mass allocation could be varied as long as the total 950 kg budget is respected. For example, the lander’s battery mass could be decreased, freeing up more mass for flyby propellant or orbiter payloads, at the expense of surface operation time. ESA notes that the assumptions resulting in a SAMA of 950 kg may change going forward.Strawman PayloadThe orbiter strawman payload option A and B are presented below. The main difference between them is that option A has a more extensive plasma payload suite and a more complex camera suite, while option B has a reduced plasma suite, simplified camera suite, but adding a VIS-IR spectrometer instead:Table 1: Orbiter Strawman Payload OptionsOrbiter Option AOrbiter Option BSubsurface Sounder RadarSubsurface Sounder RadarCamera SuiteSimple Camera SuiteRadiometer/Thermal Infrared ImagerRadiometer/Thermal Infrared ImagerPlume Remote Sensing InstrumentPlume Remote Sensing InstrumentRadio Science ExperimentRadio Science ExperimentGas AnalyzerGas AnalyzerIce grain analyser and mass spectrometerIce grain analyser and mass spectrometerMagnetometerMagnetometerE-field & Plasma InstrumentVIS-IR SpectrometerPlasma & Neutral Spectrometer Table 2 below shows the two identified strawman payload options for the lander payload complement. The main difference between them is that Option A has an expanded Ice bulk and gas sample mass spectrometer along with a microfluidics system, Option B has a descoped ice bulk mass spectrometer with a Surface-Enhanced Raman Spectroscopy payload instead. Both options feature imagers and compact surface sensors, along with a sampling handling system.Table 2: Lander Strawman Payload OptionsLander Option ALander Option BSampling Handling SystemSampling Handling SystemIce Bulk and Gas Sample Mass SpectrometerIce grain bulk mass spectrometerMicrofluidics SystemSurface-Enhanced Raman Spectroscopy (SERS)Compact Surface SensorsCompact Surface SensorsCamera Suite (Descent+High resolution imager)Camera Suite (Descent+High resolution imager) Summary and ConclusionsA few potential strawman payload complements have been presented which will be used for accommodation constraints by the industry in the second round of feasibility studies starting in June 2026. These strawman payloads may also be used to identify potential technological developments needed in order to mature it sufficiently by the eventual call for the L4 payloads. The L4 call for payloads is planned no earlier than end of 2027, and thus potential payload providers should already now start to get ready to define a credible plan for their payload to achieve TRL 6 by mission adoption.An L4 Payload Community Workshop is being planned for December 8th-10th at ESTEC in Noordwijk, The Netherlands. AcknowledgementsThe ESA L4 team would like to thank the efforts of the L4 Payload Working Group and L4 Expert Committee.References[1] Martins, Z. et al.: Report of the Expert Committee for the Large-class mission in ESA’s Voyage 2050 plan covering the science theme “Moons of the Giant Planets”, , 2023.[2] ESA Science Directorate.: Science Traceability Matrix (STM), 2026. URL: https://www.cosmos.esa.int/web/l4/science-traceability-matrix

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