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The biggest space missions gestate for the longest time. Today, the European Space Agency (ESA) revealed the three broad science themes it wants to pursue for large-scale missions of €1 billion or more that would launch between 2035 and 2050. They include a close look at icy moons around Jupiter and Saturn, dissecting the atmospheres of nearby exoplanets, and new ways to study the formation of the universe’s first stars, galaxies, and black holes. “We must start planning the science and the technology we’ll need for the missions we want to launch decades from now,” Günther Hasinger, ESA’s director of science, said in a statement.

ESA refreshes its slate of science missions roughly every decade or two. The current program, called Cosmic Vision, has three flagship missions that will launch before 2034: a spacecraft to study Jupiter’s moons, an x-ray telescope, and a gravitational wave detector. 

The next round, dubbed Voyage 2050, kicked off in 2019 with almost 100 suggested missions or themes from teams of researchers. Those ideas that could achieve breakthrough science were whittled down into three broad categories by 75 researchers split into six committees. ESA’s Science Programme Committee approved the categories this week. Although the themes do not explicitly call for missions, some translate into fairly specific mission possibilities.

The first theme calls for a mission to look for life in the Solar System, with a visit to either Jupiter’s Europa or Saturn’s Enceladus, moons thought to have water oceans hidden beneath their frozen shells. ESA says the mission could include not just an orbiter, but also a lander or drone to sample materials in situ, as on some moons water, presumably from the underground ocean, has been seen spurting out through cracks in the ice. ESA previously landed the Huygens probe on Saturn’s moon Titan in 2005 and will be visiting the jovian moons Ganymede, Callisto, and Europa with the Jupiter Icy Moons Explorer (JUICE) mission, due for launch in 2022. NASA has already committed the Europa Clipper, due for launch in 2024, that will perform flybys of that moon when it arrives in 2030.

The second theme calls for new probes of the early universe. One possibility is a mission that builds on techniques used by the Planck mission, which mapped the relic microwave radiation from the big bang to learn about the expansion of the universe and the initial clumping of matter that seeded the formation of galaxies. Another possibility is a successor to the Laser Interferometer Space Antenna, a fleet of three spacecraft flying in formation, due for launch in 2034, which aims to detect gravitational waves with long wavelengths, such as those from the merger of supermassive black holes—events that detectors on Earth cannot catch. But Fabio Favata, head of ESA’s Strategy, Planning, and Coordination Office, says officials are open to other ideas. “We’re not ruling out anything,” he says. “We pick certain science goals and then ask what is the best way to address this with a space mission.”

The third theme calls for either a probe of the Milky Way or an exoplanet hunting telescope. Under the first scenario, a mission could continue the work of ESA’s Gaia telescope, mapping the precise positions of billions of stars to understand the evolution of our Milky Way Galaxy—but shifting attention to near-infrared light, to gain additional stellar information and get a fix on stars obscured by clouds when viewed in visible light. If ESA decides to build an exoplanet hunter instead, it could be a single large telescope focusing on midinfrared wavelengths—like NASA’s James Webb Space Telescope, but at longer wavelengths that hold additional clues to a planet’s atmosphere. Another possibility is to launch several scopes that fly in formation, which would offer sharper sight able to pick out smaller exoplanets. Sascha Quanz of ETH Zurich, who heads a team developing such a fleet, called the Large Interferometer for Exoplanets, welcomes the inclusion of exoplanets in the themes. He isn’t deterred by the thought of working on a mission that won’t launch for 2 decades. “I’m happy to devote the rest of my career to this,” he says. “This is what we really need to understand exoplanets.” 

ESA has decided it will build the icy moon mission first, because after JUICE launches next year, teams with related expertise will be free to work on a new mission. Favata says the next step is to form a committee to pick the target moon and decide what is technologically possible within the time and cost constraints. That process will take up to 5 years before teams will be invited to propose missions for the slot. Then it takes roughly 15 years to design, build, and launch a large mission. And missions to Jupiter or Saturn can take many years of travel time. European planetary scientists will need to settle in for the voyage of a lifetime.