Jupiter is a planet in the same sense that the Sun is a star: it dominates everything around it. The four Galilean moons discovered by telescope in 1610 are each, in their own right, as geophysically interesting as any terrestrial planet. Every serious mission to Jupiter since Galileo in the 1990s has really been a mission to at least one of those moons. Two flagship missions are in flight right now.
The Planet You Cannot Land On
Jupiter has no solid surface. Its atmosphere just gets progressively hotter and denser as you descend, transitioning into a liquid metallic hydrogen ocean, then a dense core of heavier elements that is still an area of active research. There is no altitude at which a spacecraft can exist for long. The Galileo probe, released from the Galileo orbiter in 1995, transmitted data for 58 minutes during descent before pressure and temperature destroyed it at about 22 bar, roughly 150 km below the cloud tops. Every future Jupiter atmospheric probe will end the same way.
The planet's radiation environment is the other hard problem. Jupiter's magnetic field is enormously powerful — about 20,000 times Earth's surface field at its equator — and it accelerates charged particles to energies that will kill unshielded electronics in days. Any spacecraft operating near Jupiter lives inside a tank of radiation-hardened shielding and plans its orbit around minimising total dose.
The Four Galilean Moons
Each of the four large moons is a distinct destination. They vary more from each other than the rocky planets of the inner Solar System do.
Io
The most volcanically active body in the Solar System. Io's volcanism is driven by tidal heating: Jupiter's gravity and resonant tugs from Europa and Ganymede flex the moon enough to keep its interior partially molten. The surface is resurfaced by sulfur and silicate lava on timescales short enough that no impact craters persist. Io is not a future base, but it is one of the most important natural laboratories for tidal heating anywhere, which matters directly for whether moons like Europa can be habitable.
Europa
The moon most often described as the second-best place in the Solar System to look for life. Europa has a smooth water-ice surface cracked into a global jigsaw, strong evidence of a liquid-water ocean beneath that ice, and a heat source (again, tidal) to keep it liquid. Europa has its own deep article — see Europa.
Ganymede
The largest moon in the Solar System — larger than Mercury. Ganymede is the only moon known to generate its own magnetic field, suggesting a partially molten iron core. It probably has a subsurface ocean sandwiched between ice layers. ESA's JUICE mission (Jupiter Icy Moons Explorer), launched in 2023, will eventually enter orbit around Ganymede, the first spacecraft ever to orbit a moon other than Earth's.
Callisto
The outermost and least-geologically-active Galilean moon. Callisto sits outside the worst of Jupiter's radiation belts, which makes it one of the few places in the Jovian system where a long-duration crewed base would be survivable on paper. It does not seem to have the active geology of Europa or Ganymede, but it has its own likely subsurface ocean and a long-term stability that the other three lack.
Current Missions
Two major missions are currently working the Jupiter system, and a third is en route.
- Juno (NASA). In a highly elliptical polar orbit around Jupiter since 2016, studying the planet's interior structure, gravity field, magnetic field, and polar auroras. Juno is scheduled to end its mission with a controlled deorbit into Jupiter's atmosphere.
- JUICE (ESA). Launched 2023, arrives in the Jovian system in 2031, followed by multiple moon flybys and eventual Ganymede orbit insertion in 2034. JUICE's science focus is the three icy moons — Europa, Ganymede, and Callisto — and specifically whether they are habitable.
- Europa Clipper (NASA). Launched October 2024, arrives at Jupiter in 2030. Europa Clipper orbits Jupiter, not Europa, and makes repeated close flybys of Europa to characterise the ice shell and subsurface ocean while staying out of the worst radiation for most of each orbit. See the Europa article for details.
Why the Jupiter System Matters
If you are interested in life beyond Earth, the outer-moon ocean worlds are arguably a better bet than Mars. They are covered in water, heated from within rather than from a fading sun above, and have been in roughly their current state for billions of years — long enough for any biology that did emerge to have had time to do so. The Jupiter system holds three candidate ocean worlds (Europa, Ganymede, Callisto) close together, making it the single richest target for astrobiology in the Solar System.
If you are interested in planetary physics, Jupiter itself is the nearest example of a gas giant, and every rocky exoplanet characterisation programme is calibrated against the much easier target of hot Jupiters. The more we understand about the real Jupiter, the better we read everything we see around other stars.