Juno's Solar Cells Ready to Light Up Jupiter Mission

NASA's Juno spacecraft is going to Jupiter powered by an electrical source seldom deployed in deep space: solar arrays. Commonly used by satellites orbiting Earth and working in the inner solar system, solar arrays are typically set aside for missions beyond the asteroid belt in favor of generators powered by radioactive isotopes.

For Juno, however, three solar array wings, the largest ever deployed on a planetary probe, will play an integral role in stabilizing the spacecraft and generating electricity.

In order to operate five-and-a-half times farther away from its power source than Earth-observing satellites, Juno is equipped with more than 18,000 solar cells. Russ Gehling, the solar array subsystem's lead engineer with Lockheed Martin, said using the sun to generate power is an old-school, yet proven technology.

"In general, once we’re out at Jupiter, we need 405 watts, which is not really enough to even run your hair dryer," Gehling said. "Of those 405 watts, about half of them go toward keeping the spacecraft warm. So, the other half, somewhere in the 250 range, is to run all of the instruments and all of the avionics."

The thousands of reddish-blue solar cells are located on 11 panels, four on each on two of the spacecraft's 250-pound wings. The third wing has three panels and is outfitted with a boom at the end that carries the spacecraft's magnetometer.

At this point, Juno is fueled and ready to embark on its five-year journey to Jupiter where it will spend at least a year investigating the gas giant's origins, structure, atmosphere and magnetosphere. Two years into the journey, it will fly back by Earth for a gravity assist and then spin through the frigid cold as it approaches its destination deep in our solar system.

After NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif., and the magnetometer team at Goddard Space Flight Center in Greenbelt, Md., agreed to an array design that met all of the mission's requirements, processing of these massive wings began at Lockheed Martin's main plant outside of Denver in 2006. Then, the solar cells and their miles of electrical wiring were installed to the panels in California at Spectrolab Inc., which is a division of The Boeing Company. After that, they were sent back to Denver for installation, integration, inspections, cleaning and launch acoustic testing. Gehling said one of the main processing challenges came in the fact that the wings are so large, they can't support their own weight in gravity.

"The entire solar arrays combined are almost 750 pounds," Gehling said. "They’re a little more massive than typical solar arrays because of all these various requirements of stiffness and pointing and carrying the magnetometer."

The reason the wings have to be so stiff and strong is because Juno will be a spinning spacecraft -- another retro-aspect of this mission. "The wings dominate how true it spins on its axis," Gehling said. "Our goal is to make it spin about the direction of our high gain antenna boresight."

NASA's last mission to Jupiter was Galileo, another spin-stabilized spacecraft, and launched aboard space shuttle Atlantis on the STS-34 mission in 1989. Galileo operated on nuclear power, though.

Juno's wings arrived at Astrotech in Titusville, Fla., in March for additional checks and tests ahead of launch. About a month later, the spacecraft itself arrived and the wings were installed onto it before the entire package will be integrated into the United Launch Alliance Atlas V rocket. Juno is targeted to liftoff Aug. 5 at 11:39 a.m. from Cape Canaveral Air Force Station. Gehling said the rocket's Centaur upper stage will help get the spacecraft spinning in orbit at about 1.4 revolutions per minute.

"After the spacecraft separates from the upper stage and starts transmitting data, then the separation nuts release and the wings deploy," Gehling said. "It only takes them on the order of a minute to deploy."

After the wings span out to 34 feet each, Gehling said actuators will help balance the spacecraft, a few degrees at most, to make sure it spins perfectly. He described the deployment much like a figure skater spinning on the ice, and once the wings deploy, the spacecraft will slow to a graceful twirl about three times slower than when it began. Initially, only two of the three inner panels will be needed to generate power, but as the spacecraft travels farther away from the sun, the remaining panels will come alive.

It will take the spacecraft half a decade to reach its destination. Then, from a very elliptical polar orbit, Juno's instruments, including a color camera that will capture images of the planet's poles, a six-wavelength microwave radiometer for atmospheric sounding and composition, plasma and energetic particle detectors, and ultraviolet and infrared imagers and spectrometers, will begin sending data back to Earth.

"The layout and size of the panels are oriented in that nice symmetrical hexagon so the instruments will have an unconstrained field of view," Gehling said.

While the Juno science team will have to wait for its gas giant data, Gehling and his team will know about an hour after launch if all their work paid off.

"In real time, we’ll immediately start to see power generated, we’ll see temperatures increasing on the panels, and we’ll see the vehicle respond to the fact that wings deployed," Gehling said. "We’ll get all that data in. That’s how we’ll assess that the wings are out and the spacecraft is safe."