The Juno spacecraft that just entered orbit around Jupiter passed another milestone last January when it became the furthest-flung spacecraft ever to use solar power. That achievement was made possible by the spacecraft’s large (60-square-meter) solar panels containing nearly 19,000 advanced solar cells, in addition to mission engineers designing Juno’s instruments to draw as little power as possible.
Two previous missions—the Dawn spacecraft now orbiting the dwarf planet Ceres, and Europe’s Rosetta comet mission—“were the pioneers in using large, efficient solar arrays in space,” says Dawn project scientist Christopher Russell. Juno’s big contribution is demonstrating that these advanced arrays can be used so far from the Sun, where the amount of solar energy is only four percent of what it is on Earth.
Does that mean nuclear-powered spacecraft are a thing of the past?
Not quite yet. Spacecraft bound for Jupiter and beyond—including the two Pioneers, Voyagers 1 and 2, and Cassini—have traditionally used plutonium-powered batteries to produce electricity for their instruments. But the United States currently faces a plutonium shortage, after it voluntarily chose to import and produce less of the radioactive material. Plutonium is expensive and poses a slight risk if the spacecraft were to crash to Earth during launch or a flyby.
Powering a large spacecraft in the “low intensity, low temperature” (LILT) conditions of the outer solar system using solar arrays alone used to be considered impossible, and is still a challenge. Most solar cells are designed for sun-friendly Earth environments, and they can have very different results in a LILT environment. And, like a cheap string of Christmas tree lights, one malfunctioning solar cell can affect other healthy cells in the same string.
To avoid this, Juno’s cells were specially tested for any problems due to LILT, and the bad ones were discarded. This increased the complication and cost of the solar array, says Michael Piszczor, photovoltaics branch chief at NASA’s Glenn Research Center in Ohio. Lab results have shown that spacecraft could theoretically operate as far away as Uranus on solar power, but it’s not known yet whether that actually is possible.
“The photovoltaics will work out there,” Piszczor says. “Is it feasible or not? That becomes an issue, because you would need a very big array.”
Going past Jupiter on solar panels alone would be a doubtful proposition because the science return would be so low, says Nicolas Altobelli, a European Space Agency scientist who is working on the solar-powered JUICE (JUpiter ICy moons Explorer) expected to launch in 2022. “You could take just one full battery with you and operate it during a single flyby of an icy moon around Saturn or the ice giants,” he says. “But it would take ages to recharge, during which time you would not be able to do any science and possibly would have difficulties downloading the data to Earth.”
Advances in spacecraft efficiency might be able to push missions out further, Piszczor says. One newer solar panel technique involves layering solar cells, with different layers sensitive to different wavelengths of light, making them more robust. NASA is also examining using solar electric propulsion to cut down on the use of fuel, which would make missions cheaper.
NASA plans to use solar power for its proposed mission to Europa, but that will require yet another push in technology, Piszczor says. At Earth, solar panels degrade faster when they are exposed to the planet’s radiation belts. Jupiter’s radiation environment is 20,000 times more intense than Earth’s. Either the cells will need to be shielded in some way, or the mission could be restricted, as Cassini is, from spending too much time in high-radiation zones. A NASA contract will be going out shortly, says Piszczor, to look more closely at the LILT effect and help reduce the cost of solar arrays.