Juice: running on solar power in the dark
Welcome to Jupiter space: to one side looms the vast cloudy face of the largest planet in our Solar System; in the other appears a shrunken Sun, like a spotlight in the sky, with just 3% of the illumination from Earth orbit arriving here. This basic fact presented a major challenge to those planning ESA’s Jupiter Icy Moons Explorer, Juice, mission: how to make solar power work in such a gloomy environment, located an average 778 million km away from our parent star?
It was possible in theory. In the previous decade, ESA’s solar-powered Rosetta mission had ventured out to the distance of Jupiter orbit on its mission to rendezvous with a comet – but it had to enter almost total hibernation over 31 months to conserve scarce power.
“This was the concern – we were headed to a faraway, dark place,” notes Christian Erd, Juice spacecraft manager. “One of the first technology development activities raised for the mission was to develop solar cells that could definitely go on working around Jupiter. The good news was that the technology had moved on a great deal since the days of Rosetta.”
State-of-the-art solar cells
Solar cell engineer Carsten Baur was tasked with finding a solution: “Rosetta had flown at a time when silicon solar cells were still the state of the art. Since then, the standard solar cells used for space missions have moved on to more efficient gallium arsenide-based units, using a triple junction cell design – meaning three layers of cells are laid atop each other, each generating power from differing wavelengths of sunlight.”
The result is that while Rosetta’s solar cells achieved around 20% efficiency, the latest GaAs triple-junction cells reach around 30%. But it was not a matter of simply transplanting solar cells from a generic mission to Juice. They needed to be specifically tested for performance at the ‘low intensity, low temperature’ – or LILT – conditions prevailing around Jupiter, where the temperature of Juice’s solar panels as the spacecraft comes out of eclipse can fall to just 30 degrees C short of absolute zero.
“Change the environment and behaviour changes too,” adds Carsten. “So we had to adapt our test setups to low light and cold. We started with the latest version of the European solar cell, the 3G30 from Azur Space in Germany, which has much better performance at room temperature than its predecessor 3G28. But the same was not true at lower temperatures – they had specific thermally activated defects that meant we had to switch to the 3G28.”
And once the type was selected, the individual cell batches still needed detailed scrutiny.
Testing for performance
“The power we receive at Earth is about 1360 watts per square metre,” explains Carsten. “Out at Jupiter it is more like 50 watts per square metre, like going indoors. It’s still not nothing, but not standard conditions to operate solar cells in. Any flaws in the semiconductor making up the solar cell will immediately lead to a drop in performance.”
No semiconductor is pristine, and small ‘shunt path’ imperfections can drain away some of the current generated from sunlight. Solar cells engineers can detect those shunt paths by measuring this so-called ‘dark current’.
“If you have 2 milliamps of loss at 500 milliamps of current from one Solar Constant in Earth orbit, that’s not a problem. But if you are down to 16 milliamps at Jupiter then 2 milliamps would be quite a significant loss, especially because when we group cells together into a string then the lowest cell current will dominate the current outputs of the string.”
The dark current of cell batches was systemically measured by industry under ESA supervision, with around 25% of samples failing to make the grade.
Radiation: the invisible enemy
Another challenge was to assess the effects of another major factor of Jupiter’s environment: high radiation.
Carsten comments: “The solar cells of geostationary telecom satellites are exposed to radiation of course. What we find is that, as they are continuously exposed to sunlight, high temperatures lead them to a degree of self-healing from radiation damage. But out at Jupiter such self-healing is not available.
“Accordingly we worked with a team at Ecole Polytechnique in France who had a portable cryostat and solar simulator, to reproduce illumination conditions as they are experienced at Jupiter. This allowed us to perform low temperature radiation testing and – without increasing the temperature in between – to perform in-situ performance measurements to assess the loss factor. Against that, solar cells do operate more efficiently at low temperatures.”
Slimming down
Overall, around 24 000 solar cells are needed in total to cover Juice’s 85 sq m of solar arrays – half the area of a volleyball court, or the average living space of a UK home.
This sheer quantity of solar cells meant that any reduction in their size could slim down mission mass in a meaningful way.
Operating with lower currents than the standard design meant the thickness of solar cell ’metallisation’ on their front side, used to transfer those currents, could be reduced without impeding functionality, while the Germanium backing that the cells were laid down on was also ground away – thinning each one from 150 down to 100 micrometres.
Conversely, the cover glass was thicker than normal, to protect the solar cells against radiation, coated with a nanometre-scale layer of Indium Tin Oxide and interconnected by tiny copper wires to prevent buildups of electrostatic charge from the energetic particles encountered in space – which might otherwise end up influencing results from Juice’s sensitive magnetic and plasma instruments.
Expertise passed on to Europa Clipper
The Azur Space 3G28 solar cells – with substrate panels from Airborne in the Netherlands, laid down by Leonardo in Italy and integrated by Airbus Defence and Space in the Netherlands – ended up becoming the best characterised solar cells for LILT conditions. Accordingly NASA’s Europa Clipper mission to Jupiter made the decision to use exactly the same solar cells – representing not only a technical achievement for Europe but also a notable export success.
Beyond Juice, Carsten and his colleagues are looking into how much further solar power might yet be expanded into the outer Solar System: “We can still increase efficiency by various means, and also deploy larger areas, for instance using flexible solar cells and solar panels which have been developed for the latest telecom missions anyway. So we have yet to hit any absolute distance barrier.”
