HOW DO WE POWER A 6-YEAR JOURNEY? 

Jupiter’s orbit is five times farther from the sun than Earth’s location, so the giant planet receives about 25 times less sunlight than Earth. Juno is the first solar-powered spacecraft designed to operate at such a great distance from the sun, thus its solar panels are quite large to generate sufficient power.

The Juno orbit and spacecraft orientation have been carefully designed so that Juno’s solar panels face the Sun most of the time (except during engine burns). Therefore, the solar panels are illuminated by the sun for the entire mission. This means the spacecraft has continuous access to its power source. Juno only passes into eclipse once after launch, for a few minutes following the Earth flyby in 2013.

HOW DO SOLAR CELLS WORK? 

A solar cell converts sunlight into electricity. Materials within the cell have special properties that cause electrons to flow from them when struck by light. Flowing electrons are what we call electricity. Not all of the energy of the sunlight is converted to electricity – some of the light is reflected and some is turned into heat. The more light a solar cell converts into electricity, the more efficient the cell. Juno’s very efficient solar arrays benefit from many advances in solar cell technology achieved over the past 20 years.    

Three large solar arrays provide the power needed to operate everything. Because the sunlight intensity at Jupiter is only 1/25th that at Earth, the arrays are “super-sized” to ensure that enough power is produced to operate the spacecraft. The solar arrays have 18,000 solar cells that convert the sun’s energy into electricity through the photovoltaic effect. Jupiter’s extremely cold temperatures (-300° Fahrenheit) and intense radiation fields (equivalent to 20 million chest x-rays) required specialized testing and design. At Earth, the arrays produce over 2,000 watts of power (enough to power a large microwave oven), but at Jupiter they produce only 420 watts. This is a small amount of power, to be sure, but Juno is designed to be extremely energy efficient.

The panels extend outward from Juno’s hexagonal body giving the overall spacecraft a span of more than 20 meters (78 feet). One of the arrays is modified to hold the magnetometer experiment. The magnetometer needs to be far away from the spacecraft to ensure that it measures Jupiter’s magnetic field and not Juno’s magnetic field. Before launch, the solar panels will be folded into hinged segments so the spacecraft can fit within the fairing (or nose cone) of the Atlas V rocket.

Juno’s solar arrays are also used to help balance the spacecraft’s rotation. Following a maneuver to adjust Juno’s course or the angle of its tilt, the spacecraft may wobble a bit, like a top. The arrays can be tilted up or down a small amount to reduce this wobble, similar to the way that an ice skater raises his or her arms in order to spin fast and perfectly straight. This use of the solar arrays allows mission controllers on Earth to adjust the precise pointing of the spacecraft’s saucer-shaped main antenna.