Deciding to go to Jupiter was the easy part: it’s by far the largest planet around the Sun, resembling a Solar System in its own right with its many moons, three of which may well be home to hidden oceans beneath their icy surfaces.

Then came the practical question: how do we actually put together a mission to go there? It was here that ESA’s Directorate of Technology, Engineering and Quality lent its support to the Juice team, tackling numerous technical challenges that threatened to bar Europe’s way to the king of planets. 

Juice prepared for launch
Juice prepared for launch

“When it comes to planning any new mission, you start by looking at the place it’s going to,” says Christian Erd, ESA’s Jupiter Icy Moons Explorer (Juice) spacecraft and system manager. “In the case of Jupiter, that’s very far away from the Sun, in relatively gloomy illumination, with a high level of radiation.” 

Jupiter’s surrounding magnetic field forms the largest structure in the Solar System, nearly 15 times bigger than the Sun. If it were visible to the naked eye it would appear larger than the full Moon in the night sky. And, in the same way that Earth’s magnetosphere traps belts of radioactive particles – the Van Allen Belts – Jupiter has its own equivalent radiation belts around it, except these are thousands of times more intense than those surrounding our homeworld. 

Jupiter's magnetosphere
Jupiter’s magnetosphere

The most energetic parts of these radiation belts form effective no-go zones for either robotic or human explorers. 

Christian adds: “One of the first things we needed was a detailed radiation model for Jupiter, which could then serve as the basis of our mission requirements and analysis. This was where ESA’s Space Environment and Effects section came in, creating what became known as the the ‘Jovian Specification Environment’ or JOSE model.

“A key achievement of this model for us was to show that what at first seemed to be a dangerous place was not completely out of reach. Around three and a half years at Jupiter will involve the equivalent radiation exposure of a telecommunications satellite in geostationary Earth orbit for 20 years – which we have plenty of experience in managing.” 

Sealing one of Juice's lead-lined vaults
Sealing one of Juice’s lead-lined vaults

“Once we had that model, we could move towards implementation, planning orbital trajectories to minimise radiation doses, setting rad-hard requirements for payloads and subsystems, and plan the testing of candidate components.” 

Sensitive electronics are protected inside a pair of lead-lined vaults within the body of the Juice spacecraft, whose mission trajectory has been set out with survivability in mind. So for example, Juice will fly past 21 Callisto times, and end up in orbit around Ganymede, but will only fly past Europa twice, because this icy moon orbits closest to Jupiter and its halo of radiation. Even so, these two flybys will cause Juice to sustain around a third of its overall radiation exposure in one go. 

Operating in an extreme environment
Operating in an extreme environment

Power in a cold, dark place 

Another priority was to investigate if Juice could receive adequate solar power out at Jupiter – an average 778 million km from Earth, receiving just 3% of the solar illumination available at Earth orbit. The projected operating temperature of the solar arrays will drop as low as 30 degrees from absolute zero at points when the arrays are coming out of an eclipse state. 

ESA’s Rosetta spacecraft ventured out to an equivalent distance during its mission to rendezvous with a comet, but had to enter a 31-month hibernation phase before reaching its target. 

The good news was that solar cell technology had moved on a generation since then, but ESA’s Solar Generators section still needed to tailor today’s state-of-the-art gallium arsenide triple-junction cells (actually three layers of cells working together) would operate in Jupiter’s cold darkness.

Juice solar panels
Juice solar panels

Juice will fly 10 instruments in all, ranging from optical and radar observation through to magnetic and plasma sampling, so that lorry-sized spacecraft bristles with booms and antennas.  

That threw up another challenge – to ensure that the spacecraft itself remained as ‘clean’ as possible in magnetic and plasma terms, to be sure that Juice’s instruments are actually gathering data on Jupiter space, rather than perturbations from the spacecraft hosting them.

Unstoppable software 

Once Juice is at Jupiter it will have to be self-reliant like few European missions before it. “It will take about 45 minutes to send a one-way signal to Juice, so if something goes wrong we cannot recover it in anything like real time,” adds Christian.  

Magnetic testing of Juice spacecraft
Magnetic testing of Juice spacecraft

“In particular we have two pivotal orbital insertion manoeuvres – first to enter Jupiter’s orbit and then to orbit Ganymede in turn – which absolutely have to happen as planned.” 

The concern of the Juice team: what if something goes wrong? When a standard mission experiences sufficient system failures then it enters ‘safe mode’, switching into a secure fall-back mode where it waits for a restart from the ground. 

This will not be an option during Juice’s orbital insertion phases, since any interruption could lead to mission loss. So ESA’s Flight Software Systems section worked on a ‘Failure, Detection, Isolation and Recovery’ strategy based on a hierarchical approach, trying wherever possible to achieve a local or subsystem reconfiguration limited to the area having trouble.  

Jorge Lopez Trescastro of ESA’s Flight Software Systems Section explains: “Only when this fails to solve a faulty situation would the spacecraft go into safe mode. This is called ‘fail-operational behaviour’ – meaning that a single failure should not interrupt the main engine or thruster firings. And if going into safe mode ends up being unavoidable, then the spacecraft should still be capable of resuming its critical manoeuvres autonomously.”

Safe mode and how to avoid it 

Juice is also able to switch automatically between nominal and redundant units to avoid switching into safe mode – for instance if the gyroscopes used to measure spacecraft attitude go out of action for any reason, then the spare gyroscopes can begin operations within 20 seconds – considered the maximum time that the spacecraft can go without such vital inputs. During crucial mission phases the redundant units will be kept ‘warm’ – ready to activate – to facilitate such a speedy switch.

The same approach is also being taken for other key systems during these phases, such as the antenna pointing, solar array drive mechanism, star trackers used for navigation and reaction wheels, employed to change attitude. In the event both sets of reaction wheels fail, then the spacecraft would switch to using thrusters in their place. 

Juice in orbit around Ganymede
Juice in orbit around Ganymede

Finally, the mission has not one safe mode but would switch into multiple safe mode configurations keeping different units active, to try and avoid a total shut down. A full Juice safe mode would trigger a reboot of the mission’s Command and Data Management Unit – the spacecraft’s main computer and mass memory.  

The software would then use a minimum of context data for its configuration: its main antenna would attempt to pick up the signal from Earth while also applying its star trackers to orient itself, to try and speed up its return to nominal functioning. 

Juice’s legacy
Juice’s legacy

Coming next? 

The full range of ESA Directorate of Technology, Engineering and Quality contributions to Juice ranged from components shielding to atomic oxygen countermeasures, onboard data processing to customising the SpaceWire interface linking instruments and subsystems. 

Juice will become ESA’s reference mission for deep space, just as Rosetta was before it, serving as a starting point for planning still more ambitious future efforts headed further into the outer Solar System. 

Watch The Making of Juice video series

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