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#SpaceWatchGL Opinion: JUICE: Europe’s mission to Jupiter’s Icy Moons

By Alessandro Atzei, Giuseppe Sarri, Philippe Garé, Olivier Witasse and Christian Erd

The team working on the final tweaks before launch of JUICE. Credit: CNES/Arianespace/ADS/ESA
The team working on the final tweaks before launch of JUICE. Credit: CNES/Arianespace/ADS/ESA
JUICE, The JUpiter ICy moons Explorer, is the first European mission to study Jupiter and its Icy Moons. This impressive spacecraft (over six metric tons at launch) will embark on an eight-year journey to Jupiter, followed by a four-year mission in the Jovian system, studying Jupiter, Europa, Callisto and especially Ganymede.

JUICE is the first large mission in the European Space Agency’s Cosmic Vision 2015–2025 program. It has two ambitious goals: a) understanding how does the solar system work, and b) exploring  habitable worlds. To achieve this, JUICE carries the most powerful instrument suite to date for the exploration of the Jovian system, comprising of 10 key instruments.

JUICE will explore the Jovian system as a miniature solar system and as a model for gas giants. The mission will study the Jovian atmosphere, examine what makes the climate at Jupiter so different- so exotic and extreme- compared to our own.

It will study the Jovian magnetosphere, to know the particles and fields radiation environment to understand the conditions that may make Jupiter’s moons habitable. JUICE will also explore the Jovian satellites and ring systems, to monitor the volcanic activity of Io and gather important information on the formation and evolution of the Jovian system.

Ganymede and the search for life

JUICE will also study the conditions that may have led to the emergence of habitable environments among the Jovian icy satellites (habitable environments are defined as planetary bodies that could support, or might have had supported, organic life). For this it will focus on the three Icy Galilean moons: Ganymede, Europa, and Callisto. The reason for this choice is that they are believed to be harboring vast internal oceans: each of the icy moons is believed to contain more water than all the oceans on Earth, and identifying liquid water is crucial in the search for habitable worlds beyond Earth and in the search for life as we know it.

To study the science goals described in the previous section, JUICE carries the most powerful instrument suite ever flown to Jupiter.

JUICE will especially focus on Ganymede’s possible habitat, Europa’s recently active tectonic zones and Callisto as a remnant of the early Jovian system. The primary target of the mission is Ganymede, the largest moon in our solar system. With its 100 km thick saltwater ocean (10 times deeper than Earth’s ocean) buried under a 150 km shell of mostly ice, Ganymede provides a natural laboratory to understand the potential habitability of icy and water worlds.

Last but not least, Ganymede is the only moon known to have its own magnetic field (in general having a magnetic field is a rare feature in our solar system – Earth and Mercury are the only other bodies with it). Ganymede’s unique magnetic and plasma interactions with the surrounding Jovian environment are interesting because they may have a role in protecting the moon from radiation, and they can also explain processes at work in extrasolar systems and astrophysical objects.

It is important to point out that JUICE itself will not look for direct signs of life. It will however focus on characterizing the conditions that may have led to the emergence of habitable environments among the Jovian icy satellites.

The JUICE payload

To study the science goals described in the previous section, JUICE carries the most powerful instrument suite ever flown to Jupiter. The instruments can be subdivided in three main categories: remote sensing, geophysics and in situ particles and fields. Each category will study different aspects of Jupiter and Jupiter’s moons.  Among the others, the payload is equipped to study the geology, cloud morphology and atmospheric chemistry of the icy moons, their topography and gravity fields, and their magnetic fields and plasma environments.

Finally, JUICE will also carry a radiation monitor, which will provide information on electron, proton and heavy ion en-route to Jupiter and in the Jovian system. These measurements investigate the radiation environment in the solar system and on Jupiter.

A large spacecraft

JUICE, built by the Prime Contractor Airbus Defence and Space France, is the largest spacecraft that ESA has ever sent into the outer solar system. This is because going to Jupiter requires a very large amount of propellant: as JUICE is very heavy, the launcher cannot give it all the energy required to reach Jupiter and to reach its final destination around Ganymede. This means that JUICE must provide the remaining energy with its own engine. For this it carries nearly 3.7 tons of MMH (mono-methyl hydrazine) and MON (mixed oxides of nitrogen) propellant. This requires larger than usual tanks, which are situated in the central cylinder of the spacecraft and dictates the height of JUICE: nearly 4.5 meters high from nozzle to the tip of the spacecraft.

It will be the first time a combined gravity assist (GA) of both the Moon and Earth will be performed

The other key feature of JUICE is its solar panels: as Jupiter is more than 5 times further away from the sun than the Earth, light intensity is only 1/25th of the intensity on Earth, or just a bit less than 4% of the sunlight reaching Earth (noon on Jupiter would feel like a dim sunset on Earth). This means that to provide JUICE with the power it requires, we need 85 m2 of solar arrays, which results in a nearly 30 m wide spacecraft when the wings are fully deployed.

Despite the very large solar panels, JUICE will generate relatively little power once it reaches Jupiter, which makes it a very efficient machine: to operate all its subsystems and instruments JUICE will have batteries with a capacity of 6 kWh and the solar array will generate 800 W at end of life: less than what an average microwave oven requires.

Key features of JUICE Credit: ESA
Key features of JUICE Credit: ESA

JUICE also has a very large high gain antenna (2.5 m in diameter), required to download science data. The additional benefit of this large antenna dish is that it will be used as a sunshield when the spacecraft passes Venus on its 8-year tour of the solar system.

Other striking features are a 16 m long RIME dipole antenna, which will emit a 9MHz signal to sound the subsurface of the icy moons to detect liquid water, and a very long boom (10.6 m). The boom carries very sensitive experiments that need to be kept far away from the payload, to avoid being disturbed by the electronics on the spacecraft.

A long voyage

JUICE will be launched on April 13th 2023 from the European Spaceport in French Guyana. It will take more than eight years to reach Jupiter. This long journey is required because of the high launch mass of the spacecraft (>6 tons). The most powerful version of the Ariane 5, the ECA, is unable to provide a direct injection to Jupiter; instead, it will use a sequence of gravity assist maneuvers, which is when a planet’s gravity pulls the spacecraft and changes its orbit. This can speed up or slow down the spacecraft – depending on whether it passes behind or in front of the planet.

In order to reach its final destination, JUICE will require four gravity assists (see below).

JUICE trajectory. Credit: ESA
The journey of JUICE. Credit ESA/ADS

The first gravity assist after launch in August 2024 will be a special one, as it will be the first time a combined gravity assist (GA) of both the Moon and Earth will be performed. This is to maximize the energy gain required to ultimately reach Jupiter. This will be followed by a GA at Venus in August 2025, one more GA at Earth in September 2026, and one last Earth GA in January 2029, which will provide the final push to Jupiter, where JUICE will arrive in July of 2031.

A large Jupiter Orbit Insertion maneuver will slow down the spacecraft to the point of being captured by Jupiter’s very strong gravitational field, after which the spacecraft will perform its scientific mission. During the extent of the mission in the Jovian system, JUICE will still need several gravity assist maneuvers in order to reach the icy moons. The mission control team in ESA’s control center in Darmstadt (ESOC) will use all the gravity assists it will be able to obtain from the Galilean moons: a record of 35 GA from Europa, Ganymede and Callisto.

The complex sequence of instrument operation will be coordinated by ESA’s ESAC science operation center near Madrid. Once the propellant required to maintain the orbit around Ganymede will be depleted, the orbit will slowly decay and ultimately the mission will end with an impact on the surface of the moon, after spending at least four years in the Jovian system.

JUICE’s unique opportunities and challenges

JUICE faces several unique challenges due to the harsh environment is going to visit. Such challenges have required extensive design efforts from all parties involved and several years of dedicated work.

LOW SOLAR INTENSITY: A first difficulty to overcome is the low solar intensity of Jupiter, which required specially developed solar cells that can perform at low intensity and low temperatures. Despite these custom cells, the two cross-shaped panels have a huge surface still: 85 m2, to provide sufficient power for the mission.

EXTREME RADIATION: Jupiter’s radiation is also a great challenge. Jupiter’s magnetic field is 14 times greater than Earth, and combined with the particles emitted by Io, generates an extremely harmful radiation field, producing the radiation equivalent of 100 million x-rays. This required a combination of radiation hardened components as well as two large lead-lined vaults to protect the electronics and limiting the end-of-life exposure of the devices to less than 50 krad (for reference, whole body doses of more than 1 krad are almost invariably fatal). Without these vaults, the units would be exposed to doses of well above 2 Mrad. Needless to say, Jupiter is not a pleasant environment for humans.

VERY HIGH AND VERY LOW TEMPERATURES: JUICE not only needs to survive the very low temperatures of the Jovian system, but also the very high temperature of Venus, which it will fly by during its second gravity assist maneuver. For this, a thermal design was developed capable of withstanding both environments. The very large High Gain Antenna (HGA) is also used as a thermal shield when the aircraft will be near Venus: by painting it white, the 2.5 m disc will absorb most of the flux from the Sun, protecting the rest of the spacecraft by casting a shadow over it. Only the areas that will be exposed to the sun are covered in silver Multi-Layer Insulation (MLI), consisting of several layers of insulating materials (StaMet coated with black Kapton®) that can withstand both the highest (>250 oC) and lowerest temperatures (-230 oC) of the mission. The rest of the spacecraft is covered in black Kapton MLI, which is ideal for cold environments.

NO REAL TIME COMMUNICATION: the long distance from Earth and the impossibility of real time communication presents another struggle. The distance between Jupiter and Earth varies between 4 and 6.5 Astronomical Units. This means that Jupiter can be nearly one billion km distant from Earth, and the signal turnaround time can be more than 90 minutes, thus making real time communications impossible. As a result, JUICE has a high level of autonomy on board, including navigation cameras, which allow the spacecraft a certain degree of independence from mission control during critical maneuvers.

JUICE is the first of the ESA Cosmic Vision Large missions. More than 2000 people from countries all over Europe, but also from the United States, Israel and Japan have worked together for over a decade to build this impressive spacecraft, with the most powerful instrument suite yet, capable of overcoming the extremely challenging environment of the Jovian system. The mission not only will advance our knowledge about our solar system’s largest planet and its fascinating icy moons, but also push the boundaries of Europe’s technical capabilities and advances further. Because of this mission we have now new understanding of radiation resistant materials, more efficient solar cells for extreme environments, a further understanding of spacecraft autonomy and the development of highly efficient subsystems. All these technological advances will be part of humanity’s heritage, contributing to the next step in space and on Earth.


Alessandro Atzei
Alessandro Atzei

Alessandro Atzei holds a master’s degree Space systems and rocket propulsion engineering from TU Delft. He has been working for the European Space Agency since 2003: as study manager for Jupiter feasibility studies until 2007, spacecraft systems engineer for the Gaia observatory until 2014 and currently as payload systems engineer for the JUpiter ICy moon Explorer.



Giuseppe Sarri
Giuseppe Sarri

Giuseppe Sarri holds a master’s degree in Nuclear Engineering from the Politecnico di Torino (I). He joined the European Space Agency in 1989 in the Human Space Flight Directorate and moved to the Science Directorate in 1994.  He has been the Payload Manager of the Integral mission, the Study Manager of Eddington, now called Plato) and he was involved in the development of Planck. He was the Project Manager of ESA’s cornerstone Gaia. After the launch of Gaia in 2013 he was appointed Project Manager of JUICE, the first European mission to Jupiter and its icy moons.



Philippe Garé
Philippe Garé

Philippe Garè holds a master’s degree in electrical engineering from the University of Wyoming (USA). He has been working for the European Space Agency since 1992.  He previously held the post Payload engineer in the earth observation directorate on ENVISAT (MIPAS), Instrument Payload Engineer on INTEGRAL in the Science directorate, Payload Manager for Gaia and has been the JUICE Payload Manager since 2014.



Olivier Witasse
Olivier Witasse

Olivier Witasse holds a PhD in Geophysics from the University of Grenoble (France) and has been working at the European Space Agency since 2003. As a planetary scientist, he has worked on many space projects to Mars, Venus, the Moon, and Titan. For the last 8 years he has been working as the JUICE Project Scientist.



Christian Erd
Christian Erd

Christian Erd holds a PhD in Physics from the Technical University Vienna (Austria) and has been working at the European Space Agency since 1991. Initially he has worked as a calibration scientist in the science operations team of the XMM-Newton mission, and then as a study payload and manager of future mission studies, including the predecessor of the JUICE mission. He has been involved in the preparations of the JUICE mission for the last 14 years and is holding the position of JUICE spacecraft System Manager.

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