The JUICE spacecraft

On 2 December 2012, the European Space Agency (ESA) decided to prepare an L-class mission – L for large – to Jupiter and its moons. The Jupiter Icy Moons Explorer (JUICE) mission was born – a Jupiter orbiter that will end up circling a moon of another planet for the first time. JUICE is one of the missions in ESA's Cosmic Vision Programme 2015-2025. The mission aims to answer many fundamental scientific questions.

JUICE has been able to adopt many elements and features from ESA's precursor missions: Mars Express, Venus Express and Rosetta. However, each mission is a 'one-off', with new developments, adapted to its respective target in detail. JUICE will be ESA's first mission to the outer Solar System, apart from the journey of ESA's lander Huygens, which travelled on board NASA's Cassini spacecraft to Saturn's moon Titan, and the ballistic, unguided, two-and-a-half-year flight phase of the comet exploration spacecraft Rosetta, which extended beyond the orbit of Jupiter.

In the case of JUICE, the special requirements are derived from the eight-year journey to the mission destination, and the associated, more difficult communications with the spacecraft at distances of up to almost one billion kilometres. Power generation with only one twenty-fifth of the Sun's energy when compared to Earth and protection from the enormous radiation exposure for the spacecraft and instruments, as well as extreme temperature differences, also had to be addressed.

Spacecraft and power supply

The main body of the orbiter, referred to as the 'bus', has a cuboid shape with external dimensions of 4.09 by 2.86 by 4.35 metres excluding the solar arrays. During the mission, the probe will be three-axis stabilised. This means that in the microgravity of space, a number of reaction wheels will be used to change the orientation of the spacecraft in three axes and in relation to other celestial bodies. This will allow JUICE to look 'down' at the surfaces of the moons or Jupiter's cloud cover to make planned science observations during flybys. In addition, the spacecraft can effectively orient its large solar arrays as needed or point its antenna precisely towards Earth.

Power is generated by ten, side-mounted, cross-shaped solar panels, each 2.5 by 3.5 metres in area, giving a total area of 85 square metres. This means that 2356 Gallium-arsenide solar cells can generate between 700 and 900 watts of electrical power photovoltaically at Jupiter. Solar radiation is 25 times weaker at Jupiter than at Earth due to the distance, which is five times greater. This makes JUICE the largest 'solar power plant' in the Solar System – with its solar panels deployed, JUICE measures 16.8 by 21.7 by 13.7 metres.

The technical progress in power generation through solar technology has made it possible for the first time to supply spacecraft so far away from the Sun with solar power, for example with NASA's Juno spacecraft (in Jupiter orbit since July 2016). The batteries on board will allow JUICE to endure eclipses by moons and planets lasting up to five hours.

Propulsion

With fuel for course corrections and orbit changes, the JUICE spacecraft has a mass of 5963 kilograms; its dry mass is 2420 kilograms. This includes the payload adapter that connects JUICE to the Ariane 5 launcher. Of this, 280 kilograms are reserved for the 'payload', which consists of ten scientific instruments. The approximately three and a half tonnes of liquid propellant carried on board consists of monomethylhydrazine (MMH) as propellant and mixed oxides of nitrogen oxides (MON) as the oxidiser.

The propulsion system can produce 425 newtons of thrust in a vacuum to keep the spacecraft precisely on course to its destination and in the desired position from time to time during its eight-year flight to Jupiter. Its position and orbital parameters can also be adjusted at Jupiter and Ganymede. The spacecraft mass-to-fuel ratio is comparatively high for JUICE because more orbital manoeuvres are required during the mission phase 2031-2035 than for other missions. This applies in particular to the phase of changing from a Jupiter orbit with various orbital inclinations to an initial high orbit around Ganymede, which will then be reduced to 500 kilometres.

Communication with Earth

Jupiter's orbit around the Sun is on average approximately 630 million kilometres away from Earth's orbit but it has an eccentricity of 0.05. Jupiter's orbit is thus slightly elongated into more of an ellipse than Earth's almost circular orbit, which is about 150 million kilometres away from the Sun (eccentricity 0.017). The duration of the transmission of radio signals from the spacecraft to Earth or from there to JUICE can therefore be between 33 and 53 minutes for each transmission. The varying signal transit times result from Jupiter's position – depending on whether it is at its closest point to the Sun, perihelion, at 740 million kilometres, or at its furthest point from the Sun, aphelion, at 817 million kilometres. In addition, if Earth is in opposition to Jupiter (Earth is directly between Jupiter and the Sun), or in conjunction with Jupiter (Jupiter is on the far side of the Sun when viewed from Earth), the transit time can vary enormously. The maximum signal transit time – spacecraft-Earth-spacecraft – is one hour and 46 minutes.

This requires very careful planning of operations and orbital manoeuvres for the JUICE spacecraft at Jupiter. The mission is designed so that JUICE receives its commands well in advance and these commands are then executed by the spacecraft with extensive autonomy. For communication, JUICE is equipped with a non-steerable, high-performance parabolic antenna with a diameter of two-and-a-half metres. In addition, there is a medium-gain antenna on board, which will be mainly used during flybys in the inner Solar System, when the main antenna is turned towards the Sun to protect the spacecraft from solar radiation, particularly during the Venus flyby.

JUICE transmits in the Ka band (26.4 to 40 gigahertz) and the X band (8.2-12.4 GHz). This should enable at least 1.4 gigabytes to be transmitted from the spacecraft to Earth every day. ESA's three 35-metre antennas in Spain (Cebreros), Argentina (Malargüe) and Australia (New Norcia), each separated by about 120 degrees of longitude, are available to receive these data. This enables continuous 24-hour communication from the rotating Earth to spacecraft in the depths of the Solar System. From the deep-space antennas, the radio signals reach the ESA European Space Operations Centre (ESOC) in Darmstadt within seconds. In the event of network congestion or problems, NASA's three large deep space network antennas in California, Australia and Spain can also provide support. JUICE's data storage system has a capacity of 1.6 terabytes. All spacecraft data as well as observations, images and experimental data will be stored by ESA in the Planetary Science Archive and mirrored in NASA's Planetary Data System.

In addition to the antennas, several booms attached to JUICE carry sensors to measure the magnetic field of Jupiter and Ganymede as well as the planet's plasma environment. For their measurements, they must be distanced from the spacecraft body with its electric fields to avoid interference and disturbances. Among them is an antenna rod that can be unfolded to a length of 10.6 metres for the J-MAG (JUICE Magnetometer) and RPWI (Radio and Plasma Wave Investigation) experiments, as well as an antenna rod for the Radar for Icy Moon Exploration (RIME) experiment that can be unfolded to a length of 16 metres. Two additional 2.5-metre antennas will be deployed for RPWI.

Thermal control

JUICE will experience high temperatures of 250 degrees Celsius in the inner Solar System and during its flyby of Venus. Later, it will be exposed to temperatures of down to minus 230 degrees Celsius, far from the Sun. These temperatures are quite extreme for most of the components of the spacecraft, particularly the sensitive electronic components of the experiments and sensors. Therefore, JUICE is 'wrapped' in 500 protective, thin thermal insulation sheets ('multi-layer insulation', with a total mass of 100 kilograms) to regulate its temperature and keep its interior stable in the face of changing conditions.

Protecting the spacecraft from radiation

During its time in the Jupiter system, JUICE must also protect the parts that are exposed to space, as well as almost all of the electronic components inside, from radiation. The radiation environment on Jupiter is – apart from around the Sun – the most extreme in the entire Solar System.

The planet has a magnetic field up to 20 times stronger than Earth's – 400 microteslas at the equator and 1000 to 1400 microteslas at the poles. On Earth, these values are about 30 microteslas at the equator and 60 microteslas at the north and south poles. As on Earth, the axis of Jupiter's dipole magnetic field is tilted by 10 degrees relative to its axis of rotation. Jupiter's magnetic field probably originates in the planet's low-lying layers, where hydrogen has metallic properties under high pressure, as well as from the planet's rapid rotation.

In the direction of the Sun, the magnetic field extends five to seven million kilometres into space. On the side facing away from the Sun, the magnetic field extends as much as 100 times further, almost as far as the orbit of Saturn. The interaction with the solar wind leads to strong fluctuations in the magnetic field, which is why it is compressed 'in front' of the planet and extended 'behind' it.

In the Jovian magnetosphere, the flux of bound electrons dominates the proportion of protons and ions by several orders of magnitude. These electrons come partly from the solar wind and to a greater extent from the moons – most likely primarily from the substances released during volcanic eruptions on the moon Io. Besides the Sun's magnetosphere, Jupiter's magnetic field is the largest structure in the Solar System.

The electrons are strongly accelerated by Jupiter's magnetic field and thus attain high energies, so that they act like projectiles in collisions with a spacecraft. The energy spectrum ranges from low, easily controllable energies to significantly higher energies, which satellites in geostationary orbit are exposed to 36,000 kilometres above Earth. With JUICE, the electrons are mainly absorbed on the surface. Materials that have been used on geostationary satellites to withstand such doses will be used there; these are conductive layers that dissipate the electron flow.

Protection against bombardment by high-energy electrons is more difficult. The instruments are directly exposed to them during their use. This makes it more difficult to protect the experiments and their many electronic, mechanical and optical components. The high doses of radiation can damage the surfaces or cloud optical components. The surfaces exposed to space, such as optics, cannot be protected and are therefore made of radiation-resistant materials.

Unlike protons and ions, the strongly accelerated electrons can also penetrate deeper into the spacecraft and thus also into the instruments. This requires more shielding material for the sensitive parts inside the instruments. This is particularly difficult for space missions to distant targets because the instruments have to be constructed to be as light as possible due to the payload mass limitations – 280 kilograms for JUICE. But since during JUICE's shortest encounter with Jupiter the radiation flux will be more than half a million times higher than the average value on Earth, a suitable compromise had to be found between 'radiation hardening' and low mass. This is described vividly and in detail in this DLR blog entry using the example of the radiation shield for the Ganymede Laser Altimeter, GALA.