Jupiter’s icy moon Europa is one of the most tantalizing worlds for exploration—which is why NASA scientists are deep in the process of designing Europa Clipper, a spacecraft meant to crack its secrets.

Europa Clipper will launch as soon as 2023, then trek out to the Jupiter system for about 40 close passes over the mysterious icy moon. Once it arrives, the spacecraft will gather vital information about the moon’s geology, composition and hidden interior ocean. But before the team can get to work building the spacecraft, it has one final review to pass.

“Europa we don’t really get—there are these really key mysteries we’re trying to understand,“ Robert Pappalardo, a planetary scientist at NASA’s Jet Propulsion Laboratory and project scientist for the mission, told Space.com. “[Europa Clipper] is going to tell us so much about how icy moons work … and icy moons are probably the most common habitable environments in the universe, so that’s exciting." [Photos: Europa, Mysterious Icy Moon of Jupiter]

Any life on an icy moon wouldn’t be on the surface: It would be hidden in interior oceans, where the pull of a nearby planet’s gravity keeps water liquid. All life as we know it right now needs water. In addition, geologic activity on the seafloor could provide chemicals to feed microorganisms. and the ice would block dangerous radiation that pummels the surface. So, while there’s plenty of other science to do at Europa, understanding its habitability—or lack thereof—is a key piece of the mission. “People care about it, people want to know about this mysterious world that might harbor life,” Pappalardo said. “That’s a really important reason to do it.”

The spacecraft will carry a set of nine instruments designed to work together to solve some of those big mysteries and to assess how habitable the moon really is. During its visit, the spacecraft will come within just 15.5 miles (25 kilometers) of Europa’s surface, and that alone bodes well for scientists on the mission. “We’re going to get fantastic images back,” Christina Richey, staff scientist on Europa Clipper, told Space.com.

That ice shell has plenty of secrets, not the least of which is how thick it is. The most common features on the surface are ridges, and scientists aren’t sure how they form. Gaps in the ice sheet may let plumes of seawater shoot out into space, as also happens at Saturn’s most famous icy moon, Enceladus.

And there seems to be some sort of process that involves moving chunks of ice along Europa’s surface, but the details of what’s driving it are still a mystery. “There’s so much from a geology standpoint we have not seen yet, and we’re just getting inklings,” David Senske, deputy project scientist for Europa Clipper, told Space.com.

In order to tackle those mysteries, the spacecraft, its instruments and its managers are all building on tactics developed by previous missions. Like Juno, the spacecraft will rely on solar power in the cold reaches of the Jupiter system. And like Cassini at Enceladus, the Clipper could fly through plumes. [Water Plumes on Europa: The Discovery in Images]

But there are new challenges to tackle as well, like selecting a path for the spacecraft to follow around the Jupiter system, in a process called trajectory planning, which Senske jokingly refers to as “black magic.” (The spacecraft won’t orbit Europa directly, because it would receive too much radiation if it did. But that restriction offers benefits as well—like catching a peek at other moons. “Io happens to be right there,” Richey said. “Who doesn’t want to look at the planetary body that looks like a pox-ridden abyss?”)

Trajectory planning is a complicated mathematical endeavor, and the final choice will determine precisely what science can be done during the mission’s passes over Europa. That’s why the trajectory team is coming up with a suite of options for the teams on each instrument to evaluate, in order to select the one with the best science potential overall.

That’s a separate process from determining the spacecraft’s route to the Jupiter system in the first place, which is waiting on a decision from NASA about the mission’s launch vehicle. Clipper will ride either the agency’s own Space Launch System for a three-year journey or a SpaceX Falcon Heavy, which would stretch the journey to five or six years.

While they’re waiting on a decision, the team is focused on the current hurdle: addressing questions raised on reviews of individual components of the project and how they interact. Once those are addressed, the project will enter what mission designers call phase C, which includes setting the final budget for the project and beginning to build the real spacecraft. “That’s when the fun starts,” Richey said.

In the meantime, a key risk is that one piece of the spacecraft might get a little too far ahead of others, making it harder for engineers to pull everything together. “It’s always a stressful time for any team, but I think this team is handling it quite well, and I think we’re all excited to start building,” Richey said. “It’s kind of like a complicated version of Tetris, where this row is lining up really great and other swathes are starting to line up and work in, but you want to make sure you don’t get this one too far ahead of the rest of them.”

For Pappalardo, who was involved with the mission in its earliest days and remembers the struggle to build support for it, the process has become a whirlwind.

“It’s amazing, we have to move so fast to get to a launch that at the earliest will be 2023,” he said. “Sometimes, you stop and realize you’re in this river that’s rushing along, and it just carries you with it because there are so many things to do.”

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