NASA Is Pushing for Solar Power, But It Can't Get Us Past Jupiter

Juno, NASA's biggest outer-planet spacecraft venture since New Horizons, has proven that solar power can match nuclear power. But not in deep space.

"If you want to go beyond the sphere defined by the radius of Jupiter's orbit, solar power is going to severely constrain what you can do," said Kevin Rudolph, the lead systems engineer at Lockheed Martin, NASA's go-to spacecraft developer.

As we've recently learned from Philae Comet lander, which the European Space Agency was forced to shut down, solar powered spacecrafts quickly lose power without the Sun's light. Juno is also the first solar-powered spacecraft to explore the outer planets, which raises the question: why solar power?

Like most things, the answer has to do with money. NASA has made significant investment in Earth-bound solar panels, but beyond the political impetus to go green is the economic bind NASA faces when it asks Congress for funding. "Juno is sponsored by NASA's New Frontiers program—which has a cost cap. If we designed a spacecraft that was too expensive, they wouldn't approve it," Rudolph said.

Under NASA's New Frontiers program, designed for medium-grade exploration of our solar system, Juno's cost cap was $700 million when Lockheed Martin started to build the craft in 2003. Since solar power doesn't require buying expensive plutonium from the US Department of Energy, the Juno team was hoping solar panels would power the mission.

So in the early 2000s, NASA assigned scientists to design instruments—from a specialized radio system that measures gravitational changes to JunoCam, a camera ready to snap visceral close-ups—with the proviso to minimize power expenditure. They wanted to "get the data they needed, but without compromising the science," Rudolph explained.

The disparate instruments confect a total power rating of less than 500 watts, making Juno's total power usage the equivalent to about eight ordinary fluorescent light bulbs.

"The current power usage is about 450 watts, and we need 480 to 490 watts to run the spacecraft when we begin the science investigations," Rudolph said. "More than half of that power is just thermostat controlled heaters, needed to keep the electronics warm, and keep the propellant from freezing."

Solar vs. Nuclear

Although Juno was designed to be thermally efficient, its heaters were not designed to convert energy into heat with maximal efficiency. This is because the engineering team found it "easier to expand the solar array than to scale down the power requirements for heaters," Rudolph said.

The last major NASA space probe to explore the outer planets, New Horizons, relied on a singular radioisotope thermoelectric generator (RTG, i.e. nuclear power) for its Pluto flyby. In other words, its main source of energy for carrying out science was not dependent on any light source, which is drastically minimal amidst the outer planets, compared with Earth-orbit.

But with its active solar cell array expanded to about 50 square meters, Juno surprisingly generates roughly the same power as the spacecraft Cassini, with its three nuclear generators. With Cassini now close to the end of its mission, nuclear decay has reduced its power output—by the end of mission each RTG will only generate about 211 watts, for a total output of 633 watts. Juno, meanwhile, has no nuclear decay.

This is an amazing achievement because it means solar power—a nascent technology compared to nuclear power—can rub shoulders with the technology we've been relying on for deep space exploration.

Beyond Jupiter

The problem is that solar power doesn't work so well past Jupiter. Despite Juno's advanced gallium-arsenide solar cells—which convert light into energy at three times the efficiency of earlier solar power arrays—Rudolph said it would generate substantially less energy when it reaches Jupiter, and the planets beyond.

This is what begins to happen to solar power as one travels from Jupiter orbit—where there is 1/25 the light experienced at Earth orbit—to Saturn, about twice the distance of Jupiter (888.2 to 483.8 million miles from the Sun, respectively). Despite Juno's state-of-the-art solar array the spacecraft wouldn't generate enough energy after Saturn, where light intensity drops to one-third that of Jupiter. So in the race between solar and nuclear power generators within Jovian space, solar is already hitting a snag.

Europa, the renowned ice moon of Jupiter, is suspected to house a vast subterranean watery abyss—one that, if its core warms enough, could potentially be hospitable to basic forms of life. But in order for us to get there and find out, we'd need more than the passive instruments Juno relies on for its scientific studies; we'd need to power radar to map the surface for a suitable landing site—somewhere solid, yet thin enough to dig—then we'd need to power sonar to limn it's rumored murky depths. Both of these "active" instruments require a substantially greater power source.

"NASA talks about Europa as a target for future missions," Rudolph said. "If they desired radar to map the surface of Europa, there would be a serious problem. The Magellan spacecraft of the 1970s had active radar that measured the surface of Venus," and was solar powered, he said. But this was only possible because of the amorous planet's extreme proximity to the Sun, Rudolph explained.

Solar power may still make passive exploration of the Jovian system a reality as Juno begins its dive-bomb science orbits in the coming months. And we'll probably get some really interesting data from probing Jupiter. Unfortunately, Juno won't make it any farther into that dark frontier.