Space Exploration Technologies

Europa and Enceladus are the Perfect Targets for a Lightsail Mission

There’s always a need for new technologies or for novel uses of existing technologies to lower the cost of space exploration and extend our reach. Lightsails are a novel type of spacecraft that could eventually be our first visitors to nearby stars like the Alpha Centauri system. But they could be put to productive use right here in our Solar System.

Lightsail technology is not exactly new. The Planetary Society proved that the technology is feasible as propulsion for small satellites when they relied on sunlight alone to change the orbit of their LightSail 2 spacecraft in 2022. Japan’s IKAROS 3 spacecraft successfully demonstrated lightsail technology on an interplanetary spacecraft when it flew by Venus in 2010.

Those missions were relatively modest, but they provided critical proof that lightsail technology can work.

These missions relied purely on sunlight for their energy. That can restrict their use to inner solar systems, where sunlight is abundant. But some engineers think we can augment the Sun’s energy with powerful lasers aimed at lightsails. With that additional directed energy, lightsails could travel further into the Solar System.

A new paper shows how lightsails could be used to visit the icy moons Europa and Enceladus, two prime targets in the search for life. The paper is “A Light Sail Astrobiology Precursor Mission to Enceladus.” It’s in its pre-print stage and hasn’t been peer-reviewed yet. The authors are Manasvi Lingam from the Florida Institute of Technology, Adam Hibberd from the University of Texas, and Andreas Hein from the Institute for Interstellar Studies and the University of Luxembourg.

Jupiter’s second Galilean moon, Europa. What’s under all that ice? (Credit: NASA/JPL/Galileo spacecraft)

“Icy moons with subsurface oceans of liquid water rank among the most promising astrobiological
targets in our Solar System,” the authors write. “In this work, we assess the feasibility of deploying laser sail technology in lieu of conventional chemical propulsion to instantiate precursor life-detection missions.”

Note the word precursor. That word is aimed at the limitations of lightsail spacecraft. Most importantly, they’re unable to decelerate like spacecraft with other types of propulsion systems. But lightsail spacecraft are simpler and less expensive than other spacecraft designs. How can they fit into our exploration of these important Solar System bodies, where life might find refuge in warm, salty oceans buried under kilometres of ice?

Lightsail spacecraft may struggle to decelerate, but they can control their attitude, as Japan’s IKAROS spacecraft demonstrated. It used LCDs with controllable reflectivity to alter its attitude and trajectory. So while a lightsail can’t enter orbit around moons like Europa and Enceladus, they could perform flybys. That’s why the authors call them “precursor” missions.

The researchers have their eye on the plumes that come from the buried oceans on both moons. Sampling the chemistry of both of those plumes would be a huge step forward in understanding their potential habitability. “We principally investigate such laser sail missions to Enceladus and Europa, as these two moons emit plumes that seem accessible to in situ sampling,” they write.

What they’re proposing are more correctly called laser sails because it’s powerful lasers that make them feasible. They point to the potential of GigaWatt Lasers, a technology being researched and developed but still out of reach. With powerful lasers like these, the authors say a laser sail spacecraft could reach Europa in 1 to 4 years of travel and Enceladus in 3 to 6 years of travel. These numbers are for a 100 kg (220 lb) spacecraft travelling at about 30 km s?1.

But GigaWatt lasers are no ordinary lasers. These are super-villain-scale machines that could burn down a city if one were aimed at Earth. Not only that, but they may have to be built in the Arctic or Antarctic for a mission to Enceladus. The implications of building powerful energy infrastructure in either location are unclear at best.

This figure from the research shows regions in the Antarctic compatible with achieving an optimal laser sail encounter velocity with Saturn’s moon Enceladus for a 2045 launch date. Image Credit: Lingam et al. 2023.

There are specific launch windows to reach Europa and Enceladus, according to the authors. The windows depend on where Earth and the target moons are but also on spacecraft velocity. And that depends partly on laser power.

The team also calculated the minimum encounter velocities for each moon. These velocities allow the laser sails to encounter both moons and fly past them without collisions. Different moons have different masses and gravitational relationships with their planets, so each mission would have a different minimum velocity based on its destination.

This figure from the research shows the minimum velocity needed for a 100 kg laser sail to visit some of Saturn’s moons. Image Credit: Lingam et al. 2023.

The spacecraft also can’t be travelling too quickly. If its velocity is too high, it won’t be able to detect biological building blocks like amino acids in the plumes coming from both moons.

When the researchers combined all of these factors and others, they came up with some launch windows.

This figure from the research is focused on a laser sail mission to Saturn’s moon Enceladus. It shows encounter velocities, flight durations, and launch dates in the year 2045. Image Credit: Lingam et al. 2023.

Space travel is complicated stuff, even though we’re growing accustomed to overcoming the complexity. But laser sails introduce even more complexity, and it’s apparent when you look at launch windows and how they relate to minimum velocities and destinations. Minimum velocities are moving targets that depend largely on the destination and the launch dates.

This figure from the research shows the minimum velocities for different launch dates during a 33-year window for a mission to Enceladus. Image Credit: Lingam et al. 2023.

Europa and Enceladus are critical targets in the search for life. These types of moons are not as exotic as we once thought, and we need to learn more about them. This paper is an initial look at using laser sails to get there as precursor missions. The ESA and NASA are already working on missions to Jupiter’s icy moons that are based on chemical propulsion. But there may be a role for laser sails in exploring these moons.

When launched at the right time, they could reach Europa and Enceladus more quickly. They’re also cheaper to build, and the paper contains a loose estimate for a mission to Europa of $320 USD.

But there are obstacles.

We don’t have a laser powerful enough to propel them yet. And even when we do, can we really construct one on Antarctica? In the Arctic? Maybe that won’t be necessary in the future, but as it stands now, it doesn’t seem likely to gain approval. Activities in the Antarctic are governed by the Antarctic Treaty, but it was written decades ago before astronauts landed on the Moon and the Space Race got going. Nobody envisioned a GigaWatt laser in 1959.

Artist’s rendering of possible hydrothermal activity that may be taking place on and under the seafloor of Enceladus. If we could sample these plumes and similar ones on Europa, we’d learn a lot. Image Credit: NASA/JPL

But the authors point out that a laser sail could also be combined with chemical propulsion to help overcome some of the obstacles. They also point out that the continued miniaturization of electronics could change this entire picture in a decade or two. That might mean that we could send a fleet of 1kg (2.2 lb) laser sail SmallSats to Enceladus with far less power.

No single paper can address every aspect of a topic like this, and the authors don’t claim to have covered every factor. But they point out that laser sail missions are theoretically feasible and could be advantageous in the near future.

“Hence, we conclude by advocating that forthcoming mission concepts by governmental agencies and
private enterprises (e.g., Breakthrough Starshot) should accord light sail architectures serious consideration,” they conclude.

Evan Gough

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