Juno Is the First Solar-Powered Spacecraft in the Outer Solar System

NASA/JPL-Caltech / NASA/JPL-Caltech

Juno arrived at Jupiter last night, July 4, and safely entered into the gas giant's orbit. The successful maneuver had NASA scientists and engineers cheering; after traveling for nearly five years, the spacecraft was just one second off schedule, and when the 35-minute engine burn that slowed the craft enough to be snagged into orbit by Jupiter's gravity was complete, Juno successfully turned to face the Sun.

That was essential to the mission, because Juno is solar-powered—a feat once thought impossible for spacecraft operating far beyond the asteroid belt, where the Sun's rays are only scarcely felt. (Jupiter receives 1/25th the light of Earth.)

Juno will spend the next three months in a "capture orbit" phase, during which time its instruments will be calibrated and systems tested. Scientists will also use this time to get real-world (well, real-otherworld) practice with the science payload. Juno will then undergo a period reduction maneuver, in which its orbit is again changed in preparation for the science mission. Juno's job is to collect data on Jupiter's mysterious interior and study its gravity and magnetic fields. That begins on October 19.

From an engineering standpoint, Juno's July 4 arrival at Jupiter is fitting, as the endeavor is a sort of declaration of independence from the required use of nuclear power in missions to outer planets. Before Juno, such missions were previously required to pack under the hood what are called multi-mission radioisotope thermoelectric generators [PDF]—costly power sources fueled by NASA's diminishing supply of plutonium-238. Advances in solar panels, however, coupled with the clever designs by NASA's engineers and associates, have proven not only that solar power is possible for Juno, but also for NASA's coming flagship mission to the Jovian moon Europa.


Though they contain nuclear material, radioisotope thermoelectric generators (RTGs) are not nuclear reactors. The electricity generated by an RTG is derived from heat produced from its plutonium package. The heat is converted to electricity by way of thermocouples. (This isn't wild technology—your refrigerator uses thermocouples to turn its compressor on and off in order to regulate its temperature.) In short, the thermocouples of RTGs involve two dissimilar electricity-conducting metals, with each metal existing at a different temperature: one hot (heated by the naturally decaying plutonium) and one cold (chilled by the natural coldness of space). The temperature difference produces electricity in what is known as the Seebeck effect.

RTGs, while not particularly efficient power sources, are totally reliable, with a 0 percent failure rate of thermocouples in NASA spacecraft. They operate on the laws of physics; the decay rate of their radioactive packages is predictable for engineers, and because the generators have no moving parts, the uncertainty of wear-and-tear is removed from the equation.


RTGs are not without shortcomings. For one, NASA doesn't exactly have a warehouse filled with pellets of plutonium. In fact, the United States only has enough fuel for two more such generators beyond the Mars 2020 rover. Moreover, launching a plutonium-bearing power source into space requires enormous additional safety precautions on the part of NASA; extensive environmental impact planning involving the Environmental Protection Agency and the Department of Energy; and approval from the director of the Office of Science and Technology Policy. Local agencies are also brought to the table in the event of an accident or explosion. (It should be noted, however, that the risk is minimal. RTGs are designed to either burn up in the atmosphere in the event of a catastrophic launch or to survive a crash intact, the RTG keeping the plutonium secure. These situations actually happened in 1964, 1968, and 1970.)

Juno is fueled by sunlight that is collected by three 9-foot-by-29-foot solar panels. At Jupiter, these panels produce enough electricity to light five standard light bulbs. That doesn't sound like much, but it is ample for the spacecraft's science instrument payload. It took about one minute for the panels to expand after launch, and the full wingspan of Juno is about the size of the exterior set of the Millennium Falcon built for The Empire Strikes Back. (Mynocks would love chewing on Juno's solar-powered cables.) The spacecraft is oriented to keep the panels in continuous sunlight, and will continue to do so through completion of the mission. As noted by NASA, solar power in the outer planets has been made possible by a 50 percent increase in solar cell efficiency and radiation tolerance.

The scientists and engineers behind NASA's next flagship endeavor—the ambitious multiple-flyby mission to Europa, which doesn't have a launch date yet—tested Juno's solar panels and found that the technology would work for their mission as well. Accordingly, the Europa team abandoned RTGs and embraced the much less-expensive solar panels. (Less expensive both in terms of hardware and in the requisite environmental impact planning for nuclear power sources.) To that end, Juno began paying science dividends before it was even finished being built. The discoveries it makes in October will be a delightful bonus.