Shoot for the moons: inside ESA's JUICE mission

Among the many intriguing aspects of Jupiter, drawing fascination from scientists eager to discover more about our solar system’s largest planet, are its large icy moons. More than 75 moons are known to orbit the planet, but three of its biggest– Europa, Callisto and Ganymede –  are the focus of the European Space Agency (ESA)’s Jupiter Icy Moons Explorer mission, dubbed ‘JUICE’.

The ’gas giant’ Jupiter has a complex environment, with no Earth-like surface and an atmosphere made up mainly of hydrogen and helium. While this makes the planet itself hostile to life, these three major moons are thought to bear oceans beneath their icy crusts which could be capable of supporting it.

JUICE will see a particular focus on the largest of these, Ganymede – the largest moon in the solar system and, uniquely, the only one with its own magnetic field.

“To understand this question of habitability, we need to explore the Jupiter system…we study Jupiter, its atmosphere, its weather, its magnetic field, the volcanic moon Io, the other moons … and how all these bodies are connected to each other,” said Olivier Witasse, JUICE project scientist at ESA, during a pre-launch briefing event in April.

“Jupiter is really a miniaturised solar system, and to understand it better is key to understanding our own solar system and how it was formed.”

The JUICE spacecraft, built by Airbus, launched on an Ariane 5 from Europe’s spaceport in Kourou, French Guiana on 14^th April – a significant moment as the final ESA mission to launch on the Ariane 5 launch vehicle, before its successor, the Ariane 6, takes over.

After an eight-year journey, JUICE is set to reach Jupiter in July 2031 ready for its planned four-year exploration of the Jovian system.

The JUICE spacecraft had to be designed with complex engineering challenges in mind. These were mainly around ensuring the spacecraft, and its ten scientific instruments, could withstand the brutal conditions it’s set to face on its epic voyage across the solar system.

“The instruments are very complex, very sensitive, producing lots of data that we need to get back to earth and there are significant periods of autonomy needed for the spacecraft to look after itself,” said Justin Byrne, head of science programmes at Airbus Defence & Space, during the media event.

He explained that in addition to grappling with extreme temperatures and low light levels, Jupiter’s extreme radiation environment and strong magnetic fields meant that special considerations had to be taken when designing the spacecraft, one of which was to shield the electronics in specially designed vaults implemented to protect the most sensitive components from radiation damage.

“The vault is very new, it’s not been tried on any other missions other than Jupiter because of the heavy radiation requirements,” Byrne said. “The radiation is a killer for the electronics and unless you protect [them], they will start failing with mini corruptions and then catastrophic failure of the computers eventually.”

While Jupiter’s temperature conditions are extremely cold, the JUICE mission journey will involve a Venus flyby where conditions will be quite the opposite – above 250°C, compared with -230°C at Jupiter. Balancing these conflicting requirements was also an engineering challenge for the spacecraft, which has been designed with a blanket of novel Multi-Layer Insulation (MLI) to keep internal temperatures stable.

Additionally, huge solar arrays with a total surface area of 85m₂ – the largest ever to fly on a planetary exploration mission – were required to collect as much light and power as possible on Juice’s journey to Jupiter, the environment of which is very low-light at over 740 million km from the Sun.

To send data back to Earth over such a long distance, a 2.5m High Gain Antenna is also included alongside powerful onboard computers with complex software autonomy designed to address any challenges encountered with the spacecraft along the way.

“The spacecraft is fully redundant, there’s two computers, two of everything,” Byrne explained. “Because Jupiter’s a long way away, we can’t directly control it, there’s a long time cycle so the spacecraft has to monitor itself.

“If it detects any system failures it will reconfigure the spacecraft autonomously to bring in the spare computers…a lot of the work on Juice was to prove that in any scenario, any failure, the spacecraft can reconfigure itself and not put itself into any jeopardy.”

A mixture of powerful remote sensing, in-situ and geophysical instruments are onboard, nine of which are led by European partners and one by NASA. These include an optical camera system, visible and infrared imaging spectrometer and UV imaging spectrograph among others. An experiment, PRIDE (Planetary Radio Interferometer & Doppler Experiment) will also utilise ground-based radio telescopes on Earth to perform precise measurements of the spacecraft’s position and velocity through very-long-baseline interferometry.

Alongside novel technologies and the most powerful payload ever flown to the outer solar system, the journey will feature several world-firsts – one of these will be to perform a Lunar-Earth gravity assist (LEGA) manoeuvre, whereby JUICE will take advantage of a gravity assist flyby of the Moon, then just 1.5 days later by one of Earth. The purpose of this, followed by a sequence of further gravity assists from Venus and Earth before arriving at Jupiter, is to gain extra energy and save as much propellant as possible.

It will also be the first spacecraft to change orbit from another planet to one of its moons when it moves from orbiting Jupiter to Ganymede – in fact, it will be the first time a spacecraft has orbited any moon besides our own. The spacecraft will orbit Ganymede at 500km, but there is potential to get even closer, in a lower altitude orbit at 200km in an extended mission phase if the spacecraft ends up with more fuel than expected toward the mission’s final stages.

Previous missions to study Jupiter, such as NASA’s Galileo (from 1989 – 2003), have laid the foundation for JUICE to probe further into the Jovian system’s exploration.

At the same time as JUICE, NASA’s Europa Clipper spacecraft will focus on the Europa moon specifically, launching early next year and arriving in 2030 – one year before JUICE’s arrival. This is due to a combination of factors, ESA’s Olivier Witasse explained, which include the alignment of Jupiter with respect to the launch dates, and the Europa Clipper having slightly more power at launch – therefore a less complex trajectory with fewer gravity assist manoeuvres, meaning it will travel for five and a half years compared with JUICE’s eight.

Findings from NASA’s Juno mission, which arrived at Jupiter in 2016 and is currently still active in its extended phase, are also being used to inform plans for JUICE and Europa Clipper. In its extended mission, Juno has already achieved the highest resolution image ever taken of Europa’s icy surface.

JUICE will address two themes of ESA’s Cosmic Vision 2015-2025, delving deeper into what the conditions are for planet formation and the emergence of life. From further investigation into the chemical processes taking place inside Jupiter’s ‘Great Red Spot’ storm system, to finding out how Jupiter’s magnetic field shapes conditions on its icy moons, JUICE is expected to provide the scientific community with a wealth of further information on this fascinating mini solar system.