The primacy of solar panels wasn’t always clear. In the 1980s and ’90s, solar cells were only about 10 to 15 percent efficient, meaning that if they captured 100 watts of sunlight, they delivered just 10 to 15 watts of electricity. And at Jupiter — five times farther from the sun than Earth — sunlight is 25 times less energetic. “When you got out as far away as Jupiter, the thinking in those days was that you couldn’t use solar power because it was just too hard,” says Rudolph.
By the time the team started building Juno, in the early 2000s, solar cells could deliver the same power for around one-third the panel size.
The engineers decided to give Juno three solar arrays, each 9 feet wide and 30 feet long. The solar cells weren’t wired identically, with different sections designed to take advantage of different conditions. Juno needed just 75 square feet of cells to be turned on when it was near Earth, where the light is bright; another 150 square feet helped power Juno through the asteroid belt; and the rest was optimized to work near Jupiter, where radiation is high.
“We get a whole different kind of electrical output from a cell at that condition,” says Rudolph.
While Juno was close to home, only the Earth-tuned cells were turned on. After it passed into the middle distance and then went beyond, all three arrays spooled up. That combination yields some 550 watts of power — about 50 more than Juno generally needs. And so far, that’s worked well.
Now, it’s time to go farther, better. Juno’s solar cells sit on heavy aluminum-graphite panels. In newer designs, engineers affix cells to a flexible, lightweight sheet that’s easier to launch. Although the next planet (reminder: Saturn) is twice as far from the sun as Jupiter, Rudolph believes that someday sun-catching technology could power that trip without plutonium.
But those next-next-gen solar technologies aren’t yet ready for space. And NASA’s Game Changing Development Program is on it. According to LaNetra Tate, the program’s executive, “We go after high-payoff, high-reward technology.” That includes solar.
In March 2016, the program gave four teams up to $400,000 each to develop solar cells designed to work in low-temperature, high-radiation environments. This month, officials plan to choose the best two to continue, with up to $1.25 million more for development, before naming the final team to potentially create a solar array that could be used on a future mission.
Tate says the inspiration came from several missions, including Juno. Engineers discovered that they had to take customization to the extreme and essentially screen each cell of the solar array, using only the best of a batch, to squeeze more power out. NASA hopes to make perfect cells in large batches soon — without having one great cookie and lots of wonky ones.
If they can pull it off, such arrays could soon extend solar’s reach to the outer planets, freeing up precious plutonium for more energy-intensive NASA missions. That means better data cheaper — and lots more victory dances.