The Jupiter moons

Jupiter’s four biggest moons, Callisto, Europa, Ganymede and Io, are of significant interest to scientists and have been for a number of centuries ever since their discovery by Italian astronomer Galileo in the 17th Century.

Ganymede is the largest moon in our solar system (even larger in diameter than the planet Mercury), and it is unique in having its own magnetic field. In the 1970s, scientists began to theorise that Ganymede could be home to large oceans of water under its thick ice crust and in the early 2000s NASA’s Galileo mission to Jupiter identified and measured the moon’s magnetic field and provided evidence that this was likely the case.

In 2015, NASA’s Hubble Space Telescope captured images of Ganymede’s auroral belts and their “rocking motion” when Jupiter’s magnetic field changes. By measuring this motion and the suppression of the rocking, it gave more evidence that the moon’s oceans were fighting against Jupiter’s strong magnetic field. It is estimated that Ganymede’s oceans are 60 miles (100km) deep and under an icy crust that is 95 miles (150km) in thickness. 

Like Ganymede, Callisto and Europa are also thought to have water present under their rocky, icy surfaces, which could be home to organic life that has existed since the beginning of our solar system. But questions still persist, and there are still many uncertainties around Jupiter and its ocean-bearing moons.

Getting the JUICE flowing

In an effort to answer many of these questions, the European Space Agency (ESA), US space agency (NASA) and the Japanese space agency (JAXA) have been collaborating on a new mission to design, build and launch a spacecraft that will travel to Jupiter to collect important scientific data. Known as the Jupiter Icy Moons Explorer (JUICE), it is the first major mission under the ESA Cosmic Vision 2015-2025 programme, the current planning cycle for the agency’s space missions.

From launch, it will take over seven years to reach Jupiter. As it does not have enough fuel to reach Jupiter directly, the spacecraft will perform a series of complex “slingshot” gravity-assisted manoeuvres around several planets in the solar system to reach its destination. In 2031, JUICE will fly by Ganymede for the first time and later that year will enter into Jupiter’s orbit. By 2034, JUICE is expected to begin an orbit of Ganymede, and eventually the probe will go out of service with a controlled descent into the moon.

With 10 state-of-the-art instruments – including remote sensing, geophysical and in situ instruments – JUICE will be able to investigate the structure, dynamics and composition of Jupiter’s atmosphere, as well as the magnetic fields of the gas giant planet and its moon Ganymede.

JUICE will be the first time a spacecraft has orbited the moon of another planet and will be able to give new insights into the liquid water oceans underneath the crusts of Jupiter’s moons and whether they are habitable environments.

JUICE – built by a consortium led by Airbus Space and Defence – will allow scientists to investigate and characterise various aspects of the Jovian System. The geophysical instruments include the Ganymede Laser Altimeter (GALA), which will allow scientists to study the tidal deformation of Ganymede and the surface topography, as well as the Radar for Icy Moons Exploration (RIME), which can penetrate the icy surface to study the subsurface structure. The Gravity & Geophysics of Jupiter and Galilean Moons (3GM) instrument will also study the Ganymede’s gravity field.

JUICE’s in situ instruments include a magnetometer (J-MAG), the Particle Environment Package (PEP), a Radio and Plasma Wave Investigation (RPWI), and a radiation monitor (RADEM). Remote sensing instruments on JUICE include its camera (JANUS), the Moons and Jupiter Imaging Spectrometer (MAJIS), the Sub-millimeter Wave Instrument (SWI), and a UV imaging spectrograph (UVS).

Instruments including the GALA, SWI, PEP, J-MAG, RIME and 3GM as well as the camera system JANUS have been developed with German participation, including support from the German Space Agency at DLR and funds from the Federal Ministry of Economics and Climate Protection (BMWK). The GALA and SWI instruments specifically have been developed and manufactured under German leadership, with the former receiving input from institutes and industry in Germany, Japan, Switzerland and Spain.

HENSOLDT’s role in the GALA instrument

HENSOLDT is proud to have played a central role in the development of GALA, a key instrument for scientists to determine whether there is water beneath Ganymede’s surface. HENSOLDT has supplied essential items for GALA including the laser transmit and receive unit, and the laser control system. In 2021, a major milestone was achieved when the GALA instrument was successfully delivered to the ESA for final integration into JUICE.

GALA works by measuring the transit time of laser pulses emitted by the instrument, backscattered at the lunar surface and detected in the instrument’s receiver telescope. JUICE will fly several times over predefined points during its orbit of Ganymede, so it will be possible to identify changes in the elevation profile that could indicate water beneath.

It’s not all smooth sailing, though. JUICE – and its array of instruments, including GALA – has to overcome many challenges over its lifetime and must function reliably in order to fully realise the aims of the mission.

GALA will be the furthest a laser instrument has ever flown and will operate in extremely remote and high radiation conditions that are present around Jupiter, including an intense radiation belt. This means that it must be fully protected and ruggedised to deal with the extreme conditions not only in space, but also those experienced during launch. The distance from the sun also impacts the power available to instruments, meaning they have to use electrical power from the craft’s solar cells as efficiently as possible. Sunlight at Jupiter is the equivalent of a lamp on your office desk!

As a leader in laser altimeters for space applications, HENSOLDT was able to bring together all the required technologies for GALA in a single design and a single flight hardware item that was fully qualified for space flight. This included hermetic sealing (which is important to avoid laser damage), laser opto-mechanical structures, radiation hardness, electromagnetic compatibility and more. HENSOLDT was also able to integrate and test major subassembly items from the other nations that contributed to the GALA instrument, which was a significant achievement.

HENSOLDT has also previously contributed its BELA laser transmitter to ESA’s BepiColombo satellite, which is currently on its way to Mercury to discover more about the planet including its surface. HENSOLDT’s expertise in laser rangefinders and its long experience in space applications means that it can implement and customise qualified laser ranging instruments in space to meet the most demanding customer requirements.