Juice will perform 35 close passes of the moons - getting to within 400km of their surfaces on occasion - before settling into orbit around Ganymede.
The spacecraft carries a total of 10 instruments. There are various cameras, particle detectors, a radar to map sub-surface features; there's even a lidar, which is used to make 3D maps of surface terrain.
But it is the UK-provided magnetometer that could provide some of the most influential data. The Imperial College London-built experiment will tell us about the properties of the moons' hidden oceans. And at Ganymede, in particular, the information should be quite detailed.