AAS Nova

Plunging Through the Plumes of Enceladus

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A computerized rendering of a small spacecraft with extended solar panels shown above the surface of a grey moon. The surface is erupting with white jets of material in front of and behind the spacecraft.
An illustration of the proposed mission flying through the plumes of Enceladus. [Adapted from Mousis et al 2022]

Daring high-speed, low-altitude maneuvers and scientists wrestling with questions of life elsewhere in the universe: these are both elements found not only in Hollywood blockbusters, but also in a recently proposed mission to Saturn’s moon Enceladus.

But Why, Some Say, (This) Moon?

An illustration of Enceladus with a quarter of the moon removed to reveal an inner layered structure. Each layer and several other notable features are labeled. The layers are: ice shell, global ocean, rocky core.

A schematic of the interior structure of Enceladus as best understood today. The plumes are launched from the “Tiger Stripes” in the southern hemisphere. [Cable et al. 2021]

At barely 300 miles across, tiny Enceladus is not the most dominating exploration target in the outer solar system. What it lacks in size, though, it makes up for with panache. Enceladus simply cannot contain itself, and it makes its presence known by spewing forth plumes of ice and gas from an immense reservoir of liquid water trapped beneath the surface ice. This dramatic performance was intriguing enough to convince the Cassini mission to take a closer look, and what it found convinced planetary scientists that Enceladus was more than just a showboat: it’s a complex system that just might offer habitable conditions at the bottom of its ocean.

Some of Cassini’s most exciting finds were specific particles and molecules in the plumes that are usually associated with hydrothermal vents here on Earth, where strange life forms survive just fine even without sunlight. However, Cassini was limited both by the technology of its time and by its design: since its engineers didn’t know the plumes existed before it launched, it carried no instruments designed specifically to investigate them.

A top-down sketch of ellipses around Saturn. Titan's orbit is the furthest out, Enceladus's is the closest. The spacecraft trajectories are eccentric and pass between the two distances set by the other moons.

Orbital trajectory around Saturn, which is represented by the green circle. Enceladus’s orbit is marked in pink, Titan’s is shown in red. Two sets of spacecraft orbits which would allow for close flybys of Enceladus at different mission phases are marked in purple and cyan. [Mousis et al 2022]

Recently, a team of international collaborators led by Olivier Mousis (Aix-Marseille University) decided it is time to pick up where Cassini left off. In response to the European Space Agency’s (ESA) call for proposals for €550 million Medium-class missions, they submitted a concept for a spacecraft designed to fly straight through the plumes, one which would sniff out gases and ices as it buzzes less than 100 km over the surface at 4 km/s. The mission, called Moonraker, would spend more than a decade cruising to Saturn and would arrive no earlier than 2048.

Mixed Blessings

Unfortunately for the proposal team, ESA did not select Moonraker as one of the four mission concepts for further development, citing difficulties keeping the cost and mission duration within the scope of a Medium-class mission. However, what was surely disappointing news at the time seems much rosier in hindsight: since crafting this initial modest mission plan, ESA announced that they’ll soon seek proposals for much larger, €1 billion missions reliant on larger rockets. The team now plans to regroup, add a few more instruments and science goals to their concept, then resubmit a more ambitious proposal for this latest opportunity.

While it’s impossible to say which missions will ultimately get selected, it remains possible that we’ll celebrate the earliest part of the second half of this century with a robotic dive through these distant, frigid jets.

Citation

“Moonraker: Enceladus Multiple Flyby Mission,” O. Mousis et al 2022 Planet. Sci. J. 3 268. doi:10.3847/PSJ/ac9c03