At a glance – questions and answers on the European Jupiter mission

How much did JUICE cost (ESA/Germany)?

ESA is responsible for the spacecraft's platform, launch and operation. The payload (ten instruments) were provided by various institutions of various European countries, Japan and the USA. The German financial participation in the platform, the launch and the operation of JUICE is provided by the German ESA contribution (approx. 21 percent of one billion euros). On behalf of the BMWK, the German Space Agency at DLR is funding seven instruments to date with about 80 million euros (cost to launch). It is also supporting the cruise phase to Jupiter and the measurement phase in the Jupiter system. In total, this funding via the scientific institutes will eventually amount to more than 100 million euros. The scientific institutes, in particular the DLR Institute of Planetary Research in Berlin-Adlershof and the Max Planck Institute for Solar System Research (MPS) in Göttingen, but also the universities, are contributing to the financing of the instruments with considerable amounts.

Who built JUICE (ESA/Germany)?

JUICE's platform was built by an industrial consortium led by Airbus Defence and Space (Toulouse, France) on behalf of ESA.

On a major ESA mission like JUICE, many scientific institutes from Europe and also from other parts of the world participate. At the beginning of the programme, the scientists submit proposals for the instrumentation of the space probe. The selection of instruments is based primarily on the expected scientific benefits. In addition, the experience of the participating institutes and the available resources are evaluated. This is not only a question of financial resources, but also of the limited resources of the spacecraft (electrical power, data transmission rates, volume, mass, etc.). Two German proposals were selected for JUICE (GALA and SWI). These two instruments are being implemented under the leadership of German institutes (DLR Institute of Planetary Research for GALA, Max Planck Institute for Solar System Research for SWI). In addition, there are international participations in GALA and SWI as well as German participations in instruments (DLR for JANUS, MPS for PEP, University of Braunschweig for JMAG, University of Dresden for RIME, University of Cologne for 3GM) that are led by research institutes in other countries. The special expertise and experience are always taken into account here as well.

How many countries are involved in JUICE in total?

23 (source: ESA)

What is Japan's share in the mission?

Japanese partners contributed to SWI, GALA and PEP.

What is America contributing to the mission?

The UVS instrument is provided by the Southwest Research Institute in the USA. American institutes are involved in PEP, RIME and MAJIS.

What is Israel's share in the mission?

Israeli partners contribute to the 3GM instrument.

NASA's Galileo spacecraft (launched October 1989) was the first spacecraft to enter orbit around Jupiter to observe both the planet itself and its moons, particularly Io. A daughter probe, uncoupled before arrival, entered Jupiter's atmosphere and provided various data on temperature, pressure, wind speed and chemical composition. Galileo was able to determine through measurements that there are oceans of salt water beneath the surfaces of the moons Callisto, Ganymede and Europa.

NASA's Juno spacecraft (launched August 2011) orbits Jupiter in an elliptical polar orbit. Juno explores not only Jupiter's magnetic field and atmosphere, but also Jupiter's magnetosphere and its outer boundary – the magnetopause – and its interaction with the solar wind.

NASA's Europa Clipper spacecraft (planned for launch in October 2024) will provide detailed information about Jupiter's moon Europa, with a focus on studying its habitability.

Will JUICE complement NASA’s Europa Clipper and Juno missions?

JUICE will study Jupiter and its system in depth in all its interrelationships and complexity, focusing on Ganymede as a planetary body and potential habitat. Europa Clipper will provide detailed information about Jupiter's moon Europa, with a focus on exploring its habitability. Juno is mainly focussing on the planet Jupiter. All missions thus complement each other and will broaden our picture of the entire Jupiter system.

How long does the journey to the Jupiter system take?

The journey will take about eight years. JUICE launched on 14 April 2023 and is scheduled to begin its scientific measurement program in the Jupiter system in July 2031.

Why will it take the JUICE spacecraft so long to arrive at the Jupiter system?

JUICE is fascinating in terms of its scientific question about space, the technology used in its realisation, and the collaboration at the international level. At the beginning of the 17th century, Galileo Galilei discovered the group of moons that went on to be named after him – the ‘Galilean’ moons Io, Europa, Ganymede and Callisto – and thus influenced the view of the world. He used a new technology at that time - a telescope. Today, with JUICE, we are using the latest technologies available to us to gain further knowledge of the Jupiter system. Today, a large number of researchers, engineers and technicians are working together, which also means a very demanding management for the project.

Questions about the spacecraft

How long did it take to develop and build JUICE?

Development and construction of JUICE took approximately ten years.
The development phase for the spacecraft platform began in mid-2015.
The development phase for the instruments began in 2013.

How is JUICE powered?

JUICE is powered by one 400-newton engine. In addition, there are eight 20-newton engines as back-up for the main engine and for smaller manoeuvres and 12 10-Newton engines for attitude control. The propellant consists of nitrogen tetroxide (NTO; N₂O₄) and mono-methyl hydrazine (MMH) CH3NHNH2. JUICE has a tank of 1600 litres for each of the two components.

How is JUICE supplied with energy so far from the Sun?

JUICE uses ten solar modules with an area of about 85 square metres for the energy supply. This is the largest area ever built for an ESA satellite. 85 square metres of solar panels is roughly equivalent to the area that would be installed on three terraced houses. Due to the large distance from Jupiter to the Sun, the light power arriving there is about 30 times less than it would be if the distance was Earth Sun. To obtain as much electrical energy as possible, solar cells made of Gallium arsenide (GaAs) with a very high efficiency of about 30 percent are used, which are also optimised for very low light output. Nevertheless, at the Jupiter system, all solar panels of JUICE together generate only about 900 watts at maximum.

Has the spacecraft been sterilised? Could JUICE bring life from Earth to a Jupiter moon?

JUICE has not been sterilised. Under current planetary protection regulations, an impact on Ganymede is permitted because there is no evidence that the deep subsurface ocean on Ganymede is in contact with the icy surface. An impact on Europa would not be allowed because Europa's subsurface oceans are believed to be less deep below the surface, and therefore contamination from the surface to the ocean would theoretically be possible.

Jupiter has a large, deadly radiation belt. How can the spacecraft survive in this radiation?

JUICE will be exposed to much more intense radiation in the Jupiter system than satellites in low-Earth orbit. This includes gamma rays, electrons, protons and high-energy heavy ions. To ensure that JUICE can withstand this radiation, several design and manufacturing measures were taken. These include:

• The coating of optical surfaces is designed to be radiation hard.
• Only radiation-hard electronic components are used.
• Additional shields are installed. These are to be optimised so that the mass only increases by the necessary amount. Proof is provided by elaborate analyses and tests.
• Software is programmed to detect and, if possible, correct radiation-induced faults.

Jupiter has the strongest magnetic field of any planet in our Solar System. What challenges does this pose for the construction of the spacecraft?

Jupiter has the strongest magnetic field of the planets in our Solar System. In addition, Ganymede has its own magnetic field. The JUICE instrument JMAG for measuring the magnetic field consists of three magnetometers. It will study the interaction between Ganymede's own magnetic field and Jupiter's magnetosphere. In addition, JMAG will provide data to draw conclusions about the depth and extent of the ocean beneath Ganymede's ice sheet. The challenge now is to determine the magnetic field components as accurately as possible without interference from JUICE's internal magnetic fields. For this reason, it was necessary to avoid magnetisable materials such as iron as far as possible in the construction of JUICE. In addition, the JUICE magnetometers are mounted on a fold-out boom to achieve the greatest possible distance between the magnetometers and the platform.

Scientific questions

What is the composition of the gas giant?

Jupiter belongs to the family of planets known as gas giants. This means that it consists almost entirely of gases. About three quarters of Jupiter consists of hydrogen. Just under another quarter is helium. There are also small amounts of methane and ammonia on Jupiter. Inside Jupiter there is probably a core of ice and rock. Computer calculations show that this core alone has about 14 to 18 times the mass of Earth.

Why didn't Jupiter become a star, even though it contains so much hydrogen and helium?

Jupiter therefore did not become a star during the formation of the Solar System, because it has too little mass. To become a brown dwarf star, in which the first fusion processes take place, it would have had to be at least 13 times heavier.

Could JUICE detect life on Jupiter's moons? How can JUICE detect liquid water under the moons' ice crusts?

The JUICE mission cannot directly detect traces of life on Jupiter's moons because it does not carry a lander with appropriate instruments. JUICE will observe and measure Jupiter and its three large moons Europa, Callisto and Ganymede during flybys and from orbit using cameras, spectrometers, radar and lasers. This will be used to investigate whether there are favourable conditions for life. The laser distance meter (the GALA instrument) will be used to measure the deformation of Ganymede's ice crust over a period of months. The deformation follows from the gravitational pull of Jupiter and the rotation of the moon – it is a ‘tidal effect’. The amount of deformation can then be used to infer whether there is an ocean of liquid water beneath the ice crust and how thick the crust is. Life could have formed in this ocean of water. This will help later land missions identify favourable locations where it will be worthwhile to search for signs of life.

What kind of life could exist on the moons?

The building blocks for life (nucleic acids and amino acids) and perhaps the first primitive life forms (single-celled organisms) could have formed in the water oceans of the icy moons. On Earth, we assume the depths of the oceans near thermal heat sources as the origin of life. The water ocean under the ice crust of the moons is heated by tidal action, but there may also be warm springs (such as ‘black smokers’) there – similar to those on Earth.

Why are the icy moons so special?

The Galileo probe was able to determine through measurements that there are likely oceans of salt water beneath the surfaces of the icy moons Callisto, Ganymede and Europa. Since liquid water is one of the prerequisites for life, these oceans make the Galilean satellites perfect targets for the search for extraterrestrial life. We are looking for life similar to ours. Other forms of life are conceivable under certain circumstances, but not yet proven. For life as we know it, water is a basis, plus other elements such as carbon, phosphorus, nitrogen and sulphur. In addition, a favourable temperature range is required, as well as a radiation dose that is not too high. Such conditions exist in the Solar System only on Earth, Mars and in the oceans of the icy moons.

One of JUICE’s instruments (SWI) will also be used to observe ‘weather phenomena’ in Jupiter's atmosphere. What is the weather like on the gas giant?

Cold and stormy with wind speeds of 360 kilometres per hour. The big red spot is the largest storm in our Solar System and the typical band structure is created by jets of strong winds. Hundreds of whirlwinds are constantly forming in Jupiter's dense atmosphere, with an average lifetime of between one and three years. Some can even persist for decades.

What new insights will JUICE deliver?

Understanding the Jupiter system and deciphering its history, from its origin to the possible emergence of habitable environments, will give us better insight into the formation and evolution of gas giant planets and their moons. In addition, new insights should be gained into the formation of the Solar System and the potential for the emergence of life in Jupiter-like exoplanetary systems. The water-containing moons are of great interest for the search for life and the prerequisites for the emergence of life – even if JUICE will not provide direct evidence.

To what extent will JUICE expand knowledge about our Solar System?

Many extrasolar planets like Jupiter have already been discovered. However, almost all of them are very close to their stars, much closer than Earth is to the Sun. These ‘hot Jupiters’ could not have formed so close to their stars and therefore must have migrated inward. Such inward migration probably took place at Jupiter as well. The study of Jupiter should expand our understanding of this migration (grand tack hypothesis). This hypothesis would also explain some other long-standing puzzles of the Solar System, such as the small size of Mars and why the asteroid belt contains both rocky and icy asteroids.

What could JUICE discover about Jupiter's Great Red Spot?

Jupiter's Great Red Spot is the mega-whirlwind of the Solar System: with a diameter of approximately 16,000 kilometres, it has been raging over the gas giant for at least 200 years. Earth could fit within this structure. Although this mega-storm is very stable, it is not unchanging. It changes its colour and in the last 150 years it has shrunk and become rounder. Among other things, JUICE is studying Jupiter's general atmospheric structure, for example temperature, pressure, depth, chemical composition and wind speeds. It is hoped that this will lead to answers to questions about the formation and maintenance of the monster storm that are not covered by previous models, as well as about the future of the Red Spot.

When will the first JUICE results be available?

The first results will be available shortly after arrival in the Jupiter system, in 2031.

Io will also be visited (briefly). What will be investigated there? It is not an icy moon, is it?

Io is also being studied to find out how the Galilean moons – Io, Europa, Ganymede and Callisto – and the planet Jupiter interact with and influence each other. They are considered a mini-planetary system of their own.

How cold is it on Jupiter's moons?

Temperatures on Io vary widely. Some areas of Io dominated by volcanic activity can reach an astonishing 1700 degrees Celsius. Io's surface temperature averages -143 degrees Celsius. The average surface temperatures of Europa and Ganymede are around -160 degrees Celsius and those of Callisto are -139 degrees Celsius. These temperatures make the icy surfaces of the moons appear frozen and as hard as granite.

Do Jupiter's moons have atmospheres?

Io has an atmosphere partly derived from volcanism, composed of sulphur dioxide and possibly traces of other gases. Europa has a weak oxygen atmosphere. Hubble Space Telescope images show water plumping. Oxygen is thought to be formed by the action of solar radiation on the ice crust, splitting the water ice into oxygen and hydrogen. The volatile hydrogen escapes into space, and the more massive oxygen is held in place by Europa's gravity. On Ganymede, astronomers have found evidence of an oxygen atmosphere (using the HST).

How big are Jupiter's moons?

The four Galilean moons Io (3643 kilometres in diameter), Europa (3122 kilometres in diameter), Ganymede (5262 kilometres in diameter), and Callisto (4821 kilometres in diameter) are the largest moons of Jupiter. Ganymede is the largest moon in the Solar System and has a slightly larger diameter than the smallest (though more massive) planet Mercury. All others are relatively small with mean diameters of about one to 200 kilometres.

How many moons does Jupiter have?

Jupiter has a total of 95 moons currently known to us. Of these, the moons Io, Europa, Ganymede and Callisto are called Galilean moons after their discoverer Galileo Galilei. They are also the largest moons of Jupiter. (Source: https://ssd.jpl.nasa.gov/sats/discovery.html; Issued: 2023-02-03)

Why are we only interested in three of the four Galilean moons?

Observations made by the Galileo spacecraft showed that there are likely oceans of salt water beneath the surfaces of the moons Callisto, Ganymede and Europa. Each of these oceans would contain more water than all the oceans on Earth combined, according to typical model calculations. Furthermore, Ganymede is the only natural satellite in the Solar System that has its own magnetic field, normally found only on planets. All these are important prerequisites for life as we know it and make these moons particularly interesting for us.

How did Jupiter get so many moons?

When the Solar System formed 4.6 billion years ago, countless small bodies were formed from the interstellar gas and dust in addition to the sun and the planets. Today, they are mainly located in the asteroid belt between Mars and Jupiter and are continuously pushed out of their orbits by the gravitational pull of the planets. Comets originated in the colder, outer reaches of the solar system and also come closer to the sun and planets through orbital perturbations. When they come close to Jupiter they can be deflected into orbit under certain circumstances and are then moons of Jupiter. Since most of the moons are relatively small, new ones are discovered again and again.

With Jupiter’s immense gravitational pull, how do the moons stay in orbit and not crash into the planet?