ESA’s Jupiter Icy Moons Explorer (Juice) will study Jupiter and its ocean-bearing moons with cameras, spectrometers, radar, gravity measurements and instruments that sample the surrounding plasma and particles. Its main targets are Ganymede, Europa and Callisto, with Ganymede the primary scientific target and the planned destination for the mission’s final orbital tour. As of 3 October 2026, Juice is still travelling to Jupiter; its close investigations of the moons are planned for the 2030s, not completed discoveries.
What Juice is designed to find out
Juice addresses two broad questions: what are Jupiter’s ocean worlds like, and could the Jupiter system provide conditions suitable for life now or in the past? The mission is not designed to announce that life exists. It will investigate environments and processes relevant to habitability, including the structure of ice shells and possible oceans, geological activity, chemistry and the effects of Jupiter’s magnetic environment.
Three moons are central to that comparison. Ganymede, Europa and Callisto are thought to have oceans beneath their icy surfaces, but their surfaces, histories and environments differ. Studying them together can help scientists understand how ocean worlds form and evolve, and what conditions might make an environment more or less suitable for life.
How Juice will investigate the moons
Juice combines remote observations with measurements of the moons’ interiors and the space around them. ESA groups its ten dedicated science instruments into three broad types:
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- Remote sensing: cameras and spectrometers observe from a distance, capturing images and studying light from ultraviolet to sub-millimetre wavelengths. They can map surface features, identify materials and observe Jupiter’s clouds and atmosphere.
- Geophysical instruments: radar, laser altimetry and radio science help measure topography, subsurface structure, gravity, atmospheres and ionospheres.
- In-situ instruments: sensors examine the local magnetic fields, plasma, particles and radio emissions around the spacecraft, moons and Jupiter.
The methods complement one another. A surface map can show where a moon has fractures or ancient terrain; radar can probe beneath the surface; gravity measurements can constrain how mass is distributed inside; and magnetic and particle measurements help reveal how the moon interacts with Jupiter’s environment.
Examples of what the instruments will do
- JANUS is the optical camera system. ESA lists a capability of up to 2.4-metre resolution on Ganymede and about 10 kilometres at Jupiter; these are instrument capabilities, not guarantees that every image will have that resolution.
- MAJIS will observe Jupiter’s clouds and atmospheric constituents and characterise ices and minerals on moon surfaces.
- GALA will measure surface topography and Ganymede’s tidal deformation—small changes in shape that can help constrain its interior.
- RIME is an ice-penetrating radar intended to sound the subsurface of the icy moons to around nine kilometres. That is an intended sounding capability, not a promise to image every moon to that depth under all conditions.
- 3GM will contribute gravity and geophysics measurements and study neutral atmospheres and ionospheres.
- J-MAG, PEP and RPWI will investigate magnetic fields, particles, radio emissions and plasma. UVS studies ultraviolet emissions, exospheres, aurorae and upper atmospheres, while SWI studies temperature, composition and dynamics.
Other systems support these observations: RADEM monitors radiation, while PRIDE uses radio telescopes on Earth and Juice’s communications signal for precise position and velocity measurements. The spacecraft’s cross-shaped solar arrays have a total area of 85 square metres. Its 16-metre deployable RIME antenna and 10.6-metre magnetometer boom extend beyond the spacecraft body to support measurements.
Why Ganymede, Europa and Callisto are different targets
Juice will compare the moons along several lines: ice-shell and ocean structure, surface age and composition, geological activity, interior and gravity, and interaction with Jupiter’s magnetic and plasma environment. The planned encounters and distinct scientific roles are:
| Moon | Planned encounters | What Juice will investigate |
|---|---|---|
| Ganymede | Five flybys in 2031–2032, followed by a planned transfer into orbit in late 2034; mission end is planned for 2035. | Its subsurface ocean and ice shell, geological record, magnetic field and dynamo, and interactions with Jupiter’s environment. ESA identifies it as the primary scientific target. |
| Europa | Two planned flybys in July 2032, with a closest approach of about 400 kilometres. | Surface chemistry and geological activity, possible shallow water, the ice shell and exchanges between the subsurface, surface and space. Possible water-vapour plumes are an investigation target, not a confirmed Juice discovery. |
| Callisto | Twenty-one planned flybys between 2032 and 2034, with a closest approach of about 200 kilometres. | Its shell and possible ocean, interior and gravity, surface chemistry and evidence of past activity. Its ancient, heavily cratered surface offers a contrast with the other moons. |
Why Ganymede is the main target
Ganymede is both the largest moon in the solar system and the planned focus of Juice’s concluding orbital tour. The mission will use its flybys to shape the Jupiter tour before transferring into Ganymede orbit. There it can investigate the moon’s ocean, ice shell, geological history and magnetic field in detail, including the dynamo that generates that field.
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What Europa and Callisto add
Europa offers a chance to study a world with signs of geological activity and a potentially active exchange between its interior and surface. Its two planned flybys are designed to examine its chemistry and ice shell, as well as possible shallow water and possible plumes.
Callisto’s old, cratered terrain provides a different record of the Jovian system. Its many planned flybys will allow Juice to investigate the moon’s surface and interior, including the possibility of a subsurface ocean, and look for evidence of past activity.
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Jupiter is part of the investigation
Juice is not simply visiting three moons in isolation. It will study Jupiter’s atmosphere, upper atmosphere, weather and climate, magnetic environment and rings, as well as the ways the planet interacts with its moons. Those interactions shape the conditions around the moons and help explain what instruments measuring plasma, particles and magnetic fields detect. ESA also describes Jupiter as an archetype for gas giants elsewhere, so studying it can inform understanding of giant planets beyond our solar system.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Juice’s route to Jupiter and its current status
Juice launched on 14 April 2023 from Europe’s Spaceport in French Guiana aboard an Ariane 5. ESA’s current mission overview lists 35 planned Jovian moon flybys and an expected arrival at Jupiter in July 2031. The route uses gravity assists to adjust the spacecraft’s trajectory:
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| Encounter | Timing |
|---|---|
| Lunar–Earth flyby | August 2024 |
| Venus flyby | August 2025 |
| Earth flyby | September 2026 |
| Final Earth flyby | January 2029 |
| Expected arrival at Jupiter | July 2031 |
On 28 September 2026, Juice passed 8,640 kilometres above the Indian Ocean at closest approach during its Earth gravity assist. ESA reported that the encounter increased the spacecraft’s velocity by 3.5 kilometres per second and deflected it 20 degrees from its pre-flyby path, improving its route to Jupiter. The flyby also gave instrument teams a chance to calibrate instruments using Earth, the Moon and Earth’s magnetotail. These were calibration observations, not Juice’s main science investigations of Jupiter’s moons.
Juice Spacecraft Operations Manager Angela Dietz said the flyby required “ultra-precise navigation in real time” and that careful planning meant the team used only a small amount of the propellant reserved for the manoeuvre, leaving more available for observations at Jupiter. The mission factsheet gives Juice a nominal science lifetime of four years; that is a planned duration, not a guarantee of how long the spacecraft will operate.
What Juice’s findings can—and cannot—tell us
Juice will make measurements that help scientists assess the moons’ environments and how they have evolved. Radar, gravity, surface composition and magnetic-field data can each constrain different parts of the picture; together, they can improve understanding of whether and where conditions relevant to habitability may exist. No single instrument’s capability—for example, RIME’s intended subsurface sounding depth—means it will directly reveal an ocean or establish that life is present.
As of 3 October 2026, Juice has not yet begun its close observations of Jupiter’s moons. The planned flybys, orbital tour and science objectives describe what the mission intends to do after reaching Jupiter; they should not be mistaken for results already obtained there.
Sources: ESA mission overview; ESA mission factsheet; ESA science objectives; ESA spacecraft specifications; ESA moon-by-moon science plan; ESA update on the 28 September 2026 flyby; ESA instrument descriptions.
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