ESA’s L4 Mission Set to Focus on Icy Moons

Across the planet, space agencies are setting their sights on distant worlds – ones that many within the greater scientific community believe could harbor the necessary building blocks for life. For Europe, a bold new series of mission proposals may bring forward new questions and answers to one of humanity’s oldest questions. After meeting to discuss ideas and assess technologies, the European Space Agency has begun the process of designing their next “Large Class” mission – one that could explore Saturn’s moons of Enceladus and Titan up close. 

After the end of the joint NASA-ESA Cassini mission, the outer planets are entering the discussion once more. ESA issued an open call in December 2021 for an Expert Committee for a “Large Class” or “L4” mission covering the science theme “Moons of the Giant Planets”. Missions in this theme would see spacecraft visit moons of the Giant Planets in the outer Solar System, which have quickly become prime targets in the search for life. The Expert Committee considered how a new ESA L4 mission could address priority topics such as habitability, prebiotic chemistry, and biosignatures, providing new knowledge beyond any previous missions through a tailor-made approach to planetary biology. The Committee considered planetary probe missions to one or more of the moons of Jupiter and Saturn, with a targeted launch date in the early 2040s and different possible mission scenarios to ensure the L4 mission would deliver a transformational scientific return. Through this, the Committee ultimately identified Saturn’s moons of Enceladus and Titan as the primary targets.

Enceladus is the sixth-largest moon of Saturn, and the 19th largest in the Solar System. It is about 500 kilometers (310 miles) in diameter, about a tenth of that of Saturn’s largest moon, Titan. It is mostly covered by relatively clean ice, making it one of the most reflective bodies of the Solar System. Starting In 2005, Cassini performed multiple close flybys of Enceladus, revealing its surface and environment in greater detail. In particular, Cassini discovered water-rich plumes venting from the south polar region. Cryovolcanoes near the south pole shoot geyser-like jets of water vapor, molecular hydrogen, other volatiles, and solid material, including sodium chloride crystals and ice particles, into space, totaling about 200 kilograms (440 pounds) per second. In 2014, after a series of plume dives NASA reported that Cassini had found evidence for a large south polar subsurface ocean of liquid water with a thickness of around 10 km (6 mi). Cassini performed chemical analysis of Enceladus’s plumes, finding evidence for hydrothermal activity, possibly driving complex chemistry. Ongoing research on Cassini data suggests that Enceladus’s hydrothermal environment could be habitable, akin to Earth’s hydrothermal vent ecosystems, and that methane in these plumes could be produced by such organisms. The existence of Enceladus’ subsurface ocean has since been mathematically modeled and replicated – only furthering interest in exploration of this mysterious, diminutive moon.

Enceladus has been proposed as a focus for a large scale mission before, such as the NASA-ESA Titan Saturn System Mission concept aiming to explore both Titan and Enceladus through the use of orbiters and landers. Based on the venerated Cassini spacecraft, this mission would aim to further understanding of the two moon’s distinctive qualities with a diverse range of sub-spacecraft. This mission would ultimately not come to fruition, as funding was split to cover both ESA’s Jupiter Icy Moons Explorer (JUICE) mission and NASA’s Europa Clipper, which both aim to explore potentially life harboring worlds in the Jovian system. 

ESA’s new call for missions to Enceladus and Titan were split into three distinct mission types, each centered around three key thematic structures to investigate:

• The issue of habitability of ocean worlds and the interaction between the surface and the interior.
• The issue of habitability of ocean worlds and the interaction with the external environments.
• The identification of prebiotic chemistry and the search for biosignatures on ocean worlds.

With these three themes in mind, mission planners and policy makers looked at both technical requirements and scientific questions to design missions that could potentially cover as many bases as possible. 

In terms of mission design, there exists some commonality between the proposed designs to ensure that the spacecraft or multiple spacecraft can achieve as many science goals as possible with common hardware. The Committee identified a baseline system of requirements that would be packaged on the orbiter, and distinct, destination oriented elements that would aim to answer many of the science goals. Two out of the three of these mission proposals favor Enceladus as a destination, as many biochemistry questions are set to be explored by the NASA-led Dragonfly mission, scheduled to arrive at Titan in 2034. The mission would notionally target launch in the 2040s, well after the arrival of both JUICE and Europa Clipper. 

The first mission type explored by the Committee is an Enceladus focused, single Ariane 64 launch mission which would see a hybrid solar electric-chemical vehicle make the transit to the Saturn system. Once there, the mission would conduct over 50 flybys of Saturn’s moons and several plume sampling dives at Enceladus before culminating in the orbiter impacting the moon as part of its end of life phase. While this answers many of the mission goals, it fails to provide much of the up close context for exploration of the moon that the science goals necessitate. To solve this, a second more complex mission was also proposed by the Committee: a dual Ariane 64 launch campaign to assemble the spacecraft in Earth orbit before its ultimate transfer out to the Saturn system. This option provides the same characteristics for the tour but is much more favorable due to the increased payload performance of a dual launch campaign. This permits a lander on Enceladus of ~800 kg, operating for more than 20 days on the surface to closely investigate the local environment of the icy moon. The third option presented differs significantly from the previous two proposals, using a single launch and opting to explore Titan as the primary target for the mission with a 1100 kg lander.

Internationally, other missions have begun to turn to the Saturn system as their next stop on the search for the building blocks of life. While the Uranus Orbiter and Probe mission was recommended by the 2023 Planetary Science Decadal Survey as the most important mission for NASA to pursue, the Enceladus Orbilander was a close second – a combination orbiter/lander vehicle that would spend time in orbit categorizing the moon before descending to the surface. Given the severe financial constraints that NASA is operating under over the next several years, it seems unlikely that this mission will fly within the same timeframe as ESA’s proposed L4 mission. The China National Space Administration (CNSA) has announced plans for a mission to the Jupiter system, which overlaps with science themes present on both JUICE and Europa Clipper. The Indian Space Research Organisation (ISRO) and the United Arab Emirates Space Agency (UAESA) are also expanding their planetary science programmes, but their focus is on the terrestrial planets and the inner Solar System so far. Without a dedicated focus on Enceladus, many within the European scientific community view these mission objectives as the ideal space for development of a mission. 

While a mission to the outer Solar System is tantalizing, ESA must overcome a more constrained budget and unique method of governance. While the agency has seen a steady increase in funding, global inflation rates and high project costs have resulted in the agency having to stay lean to accomplish their goals. During the initial development of the first “Large Class” mission, JUICE, several elements had to be rescoped and reformatted to better align with budgetary availability. As a result of financial constraints and differing viewpoints of international partners, ESA has implemented a minimization of cost and emphasized maximized science output whenever possible. Ahead of L4, the L2 mission and L3 missions, LISA and NewATHENA, must make it through their build, checkout and launch campaigns on time and on budget – an ambitious ask for large science missions in the modern era. Like UOP, this latest mission requires careful budget formulation and championing from the scientific community to become reality, should it move past this initial early study and be accepted as a proper mission. Further budgetary constraints may also emerge as ESA further integrates itself with the Artemis program, requiring a careful balancing act to ensure that all of the goals of the agency can be met. 

For now, the L4 mission remains a more distant and uncertain goal, with several missions on the docket ahead of it for ESA to accomplish. It remains, however, a sign of changing scientific thinking – no longer are terrestrial planets the only focus on our search for life; now, ocean worlds are emerging as prime candidates in the quest for clues about our place in the solar system. With a focus on ocean worlds unfolding, the European Space Agency may be a step closer to uncovering the mysteries of our past, and generating further questions for the future. 

Edited by Beverly Casillas

Correction: The ESA L2 mission is not ARIEL, rather, NewATHENA