The JUICE mission launched successfully on 14 April, after being called off the day before due to a risk of lightning. . . .

As is the case for all missions to the outer Solar System, JUICE’s trajectory will be anything but direct. It will first make a Venus fly-by, in 2025 — which will expose it to high temperatures — after which it will fly past Earth twice more, in 2026 and 2029. It will arrive at Jupiter in 2031, at which point its main engine will perform a long retro-burn to slow down and begin orbiting the planet.

Whereas JUICE is scheduled to do 21 fly-bys of Jupiter’s second-largest moon Callisto, it will only pass the smallest Galilean moon, Europa, twice. That’s because Europa will be studied closely by a NASA mission called Europa Clipper, which is due to launch in late 2024.

In 2035, JUICE will fire up its main engine again to enter orbit around Ganymede, at an altitude of 500 kilometres, for at least 9 months. This will be a delicate manoeuvre, says Scott Bolton, a planetary scientist at the Southwest Research Institute in San Antonio, Texas. “You have to really go slow around Ganymede and match its orbital speed, in order to get trapped,” says Bolton, who is the principal investigator for Juno, an ongoing NASA mission that has been orbiting Jupiter since 2016. “If you’re moving just a little bit too fast, Jupiter’s gravity pulls you away.”

Bigger than the planet Mercury, Ganymede is the largest moon in the Solar System, and the only one known to currently have its own magnetic field.

Some of Jupiter’s icy moons are thought to harbour liquid water underneath their predominantly water-ice surfaces — potentially providing an environment for some form of life to have evolved. In the mid-1990s, NASA’s Galileo probe provided hints of such a water ocean on Ganymede and, more convincingly, Europa. Further evidence came in 2015 from Hubble Space Telescope observations of Ganymede’s aurorae1, which are created by its magnetic field similar to how Earth’s geomagnetism produces the aurorae borealis and australis.

One of the most conclusive tests of the ocean theory will come from topographical maps made by JUICE’s laser altimeter. Because Ganymede moves slightly closer to and farther from Jupiter during its weekly orbit, and because it experiences the gravitational pull of the other moons, it stretches and compresses under tidal forces. On a water world with an ice crust, such deformations might make the surface swing up and down by as much as 10 metres, says JUICE project scientist Olivier Witasse, who is based at the ESA in Noordwijk. “If it’s only a solid tide, it would be less than a metre,” he adds.

The Galilean moons(/ˌɡælɪˈliː.ən/),or Galilean satellites, are the four largest moons of Jupiter: Io, Europa, Ganymede, and Callisto. They were first seen by Galileo Galileiin December 1609 or January 1610, and recognized by him as satellites of Jupiterin March 1610. They were the first objects found to orbit a planet other than the Earth.
Galilean moons - Wikipedia
en.wikipedia.org/wiki/Galilean_moons