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The European Space Agency (ESA) is sending an orbiter to explore Jupiter’s strange moons. The Jupiter Icy Moons Explorer (JUICE) is scheduled to launch on 13 April from Kourou, French Guiana, and begin its eight-year journey to Jupiter.

The orbiter is designed to explore Europa, Ganymede and Callisto, three of Jupiter’s largest moons. It will make two close passes by Europa and 12 by Callisto during the first phase of its mission, from 2031 to 2034, and then orbit Ganymede for the remainder of its mission.

While Ganymede may seem like an unconventional choice – Jupiter’s smaller moon Europa is commonly regarded as more likely to have the right conditions for life – it is the largest moon in the solar system, making it relatively easy to reach and orbit. Like Europa and Callisto, it is thought to have a liquid ocean beneath its icy shell.

“Ganymede in principle is less interesting than Europa, but because it’s a bigger moon, also potentially with water inside, and with a magnetic field, Ganymede has a lot of mysteries to solve,” says ESA’s , the mission’s project scientist. “One of the big questions is whether we could have around Jupiter a place where there could be habitable conditions, and Ganymede is one of those places.”

The possibly habitable environment lies in Ganymede’s liquid ocean, which is thought to be the largest in the solar system. But we know very little about it, so it is unclear whether it could have the right conditions for life. JUICE will measure the water’s location, composition and depth in the hopes of finding enough promise to warrant a future mission under the ice to look for actual signs of life. “If you want to search inside the ocean, you need to know where the ocean is,” says Witasse.

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JUICE’s measurements of the ocean will be aided by Ganymede’s internally generated magnetic field, the only one on any moon. Studying the magnetic field should also help unravel Ganymede’s internal structure.

The mission will also aim to map all three moons and study the composition of their icy crusts, searching for any signs of geological activity. It will use radar to penetrate the crusts and measure the depths of the moons’ seas and how far underground they lie for the first time.

Because the goal of the mission is partly to understand the Jupiter system as a whole and the potential for life around similar exoplanets, JUICE will also take a look at the other moons from afar and make detailed observations of Jupiter itself. Those observations will focus on the planet’s powerful magnetic field and how it affects the moons.

These measurements are expected to solve a host of mysteries about Jupiter and its moons – if all goes well at the launch, the next step is a long wait. “The next most exciting phase will be in 2031 when we arrive at Jupiter and we start the planned mission,” says Witasse. “We start to get a little bit excited, but that’s still eight years from now.”