Juice — the Jupiter Icy Moons Explorer project of the European Space Agency — needs electrical power to perform its scientific mission. That’s a problem, because Jupiter — at 778 million kilometers (466 million miles) from the sun — receives only 3% as much solar energy as does the Earth. The ambient temperature in the part of the solar system occupied by Jupiter is just 30º C above absolute zero. To make the Juice mission possible, ESA had to invent solar panels that could operate effectively in the cold and dark environment that surrounds Jupiter.
A decade ago, the Rosetta space mission traveled through deep space to rendezvous with a comet that was as far away from the sun as Jupiter. At that time, there were no solar panels available that could power the spacecraft for the 31 months it took to reach its destination, so it had to travel in nearly total hibernation mode until it was time to perform is mission. The Juice mission would require more solar power in order to be successful.
“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.”
Solar Panels For Deep Space
Solar cell engineer Carsten Baur was put in charge of 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.”
While the solar cells for the Rosetta mission were only about 20% efficient, the latest GaAs triple junction cells are close to being 30% efficient. It was not simply a matter of transplanting solar cells from a generic mission to Juice. They needed to be specifically tested for performance at the “low intensity, low temperature” conditions prevalent around Jupiter.
“Change the environment and behavior changes too,” says 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.
“The power we receive at Earth is about 1360 watts per square meter,” explains Carsten. “Out at Jupiter it is more like 50 watts per square meter, 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.”
Small Defects Can Be Critical
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,” Carsten says. During quality control testing under the supervision of the ESA, about 25% of samples were rejected.
Another challenge was to assess the effects of the high radiation levels in the vicinity of Jupiter. “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,” Carsten says.
“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.”
Lots Of Solar Panels
Overall, around 24,000 solar cells are needed in total to cover Juice’s 85 square meters of solar arrays, about half the area of a volleyball court. Because of the number of solar cells needed, any reduction in their size could reduce the total mass of the Juice space exploration vehicle significantly.
Operating with lower currents than the standard design meant the thickness of solar cell ’metallization’ on their front side, used to transfer those currents, was reduced without impeding functionality, while the germanium backing for the cells was reduced from 150 micrometers to 100 micrometers.
On the other hand, the glass used to cover the solar cells is thicker than normal to protect them against radiation. It in turn was coated with a nanometer 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 influence the results from sensitive magnetic and plasma instruments aboard the Juice vehicle.
The Azur Space 3G28 solar cells — made with substrate panels from Airborne in the Netherlands, laid down by Leonardo in Italy, and integrated by Airbus Defense and Space in the Netherlands — ended up becoming the best solar cells for LILT conditions. Accordingly, NASA’s Europa Clipper mission to Jupiter made the decision to use exactly the same solar cells, which was 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 by 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.”
Space exploration often leads to improvements in technology used on Earth. The technology that allows specialized solar panels to harvest energy more than 440 million miles away from the sun could lead to solar panels that can function well during the long winter nights in Arctic regions. For more about the ESA Juice mission, which launched successfully on April 14, 2023, please see the video below.
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