The European Space Agency’s JUICE mission (Jupiter Icy Moons Explorer) starts today at 22:15 AEST from the European Spaceport in French Guiana.
JUICE will focus on three aquatic worlds – Jupiter’s moons Ganymede, Europa and Callisto – to explore potential habitats and evidence of past alien life, both on and below the surface. There’s an excellent reason why these worlds in particular are the target of the mission – they could be habitable for life as we know it.
Jupiter’s moons
Though we have only one moon lighting up our night sky, Jupiter has at least 92. Some, including the four Galilean moons (the largest Jovian moons) formed with Jupiter in the early solar system nearly 4.5 billion years ago. Others have been drawn to and captured by this massive planet and added to the collection over time.
These moons are made of vastly diverse materials, and some are thought to have, or may have had, favorable conditions for life.
Fewer than ten interplanetary missions have ever flown past Jupiter, with only two NASA missions stopping to orbit and investigate the planet further: the Galileo mission between 1995 and 2003, and the current Juno missionlaunched in 2011. These are the only two that have also made special passes of the moons and collected valuable information for upcoming missions.
ESA, CC BY-SA
Life as we know it
The moons of Galilee are of particular interest. The second smallest, Io, may not be habitable, but it has some of the largest active volcanoes in the solar system (with eruptions that can be seen from Earth!).
The other three, Ganymede, Europa, and Callisto, are all believed to have large amounts of liquid water beneath their icy surfaces, and may even have thin atmospheres.
Ganymede’s liquid iron core as well gives it a magnetic field, the only known moon in the solar system to have one. Our own magnetic field shields the Earth’s atmosphere from the harsh solar winds and shields us from solar radiation. These are factors we associate with nurturing and protecting life on Earth.

NASA, ESA and A. Feild (STScI)
We only know about life on Earth, so when we look for where life might exist (or once existed) somewhere else, we look for factors we consider essential to life as we know it.
Watery or icy worlds are the first targets, because we know that life on Earth originated in and around water. A rocky surface with warm temperatures would be even more ideal. Jupiter itself is a complete write-off: its crushing pressure, toxic gases, freezing temperatures, and lack of a stable surface would never support life as we know it. But the large, icy moons have good protection deep beneath the ice, potentially liquid water and elements such as carbon and oxygen.
JUICE will use its suite of scientific instruments to check the thickness of the moons’ icy crusts, which they are made of, and to search for liquid water beneath the surface. Europe will be particularly sought for evidence of organic molecules.
Read more: The search for life under the ice: why we’re heading back to Europe
An extremely efficient journey
After launch, the solar-powered JUICE will take nearly eight years to reach Jupiter. The spacecraft uses minimal propulsion, instead using other planets to give it speed and set its course.
These maneuvers aregravity helps”. This essentially means that JUICE will purposefully fly to a planet, just missing it, only to be pulled in by its gravity and “slingshot” down the other side. It may take some time, but it is extremely efficient.
Juice’s first gravity assist in 2024 is going around both Earth and our Moon – the first time this has ever been done. Other gravity assists will take it around Venus in 2025, and Earth (only) in 2026 and 2029, before being thrown to Jupiter for a mid-2031 arrival.
What Happens When JUICE Meets Jupiter?
Fun fact: It will be the first spacecraft to orbit any other than ours!
Usually, a spacecraft will orbit the main planet (Jupiter in this case) and fly past the moons only as it passes by. JUICE will soon begin flying past Callisto, Europa and Ganymede a total of 35 times on its three-year tour of the moons. It will come together shortly NASA’s Europa Clipper mission around Europe, a nice addition to this mission.
But in 2034 JUICE will actually change orbit around Jupiter to go around Ganymede. This gives it an exceptional view and nearly a year to study this fascinating moon and probe its internal, surface and atmospheric systems.

ESA/S. Corvaja, CC BY-SA
Of particular importance is the magnetic field. Ganymede is one of only three rocky bodies in our solar system known to have one (Earth and Mercury being the other two). Questions that JUICE will be able to explore include not only the fundamental question of what creates Ganymede’s magnetic field, but what happens to it as Ganymede travels through the larger field produced by Jupiter itself, and how their complex interactions create auroras on both Jupiter and Jupiter. affect Jupiter. Ganymede.
JUICE will also have the opportunity to study Jupiter itself, looking at features of giant gas planets that may be universal. Could Jupiter hold the key to understanding other solar systems and the hundreds of exoplanets we’ve discovered orbiting other stars?
So we may be waiting a while for JUICE to arrive at Jupiter, but it will be well worth the wait. Could any of these moons have ever supported extraterrestrial life, and what can we learn about our own Earth, its early oceans, and the conditions necessary for life to emerge?