By MARCIA DUNN
AP Aerospace Writer
CAPE CANAVERAL, Fla. - NASA's upcoming mission to Jupiter can't get much greener than this: a solar-powered, windmill-shaped spacecraft.
The robotic explorer Juno is set to become the most distant probe ever powered by the sun.
Juno is equipped with three tractor-trailer-size solar panels for its 2 billion-mile journey into the outer solar system. It will be launched Friday morning aboard an unmanned Atlas V rocket - barely two weeks after NASA's final space shuttle flight.
The shuttle's demise is giving extra oomph to the $1.1 billion voyage to the largest and probably oldest planet in the solar system. It's the first of three high-profile astronomy missions coming up for NASA in the next four months.
Jupiter - a planet several NASA spacecraft have studied before - is so vast it could hold everything else in the solar system, minus the sun. Scientists hope to learn more about planetary origins through Juno's exploration of the giant gas-filled planet, a body far different from rocky Earth and Mars.
"Look at it this way - it is a new era," said Jim Green, NASA's director of planetary science. "Humans plan to go beyond low-Earth orbit. When we do that, it's not like 'Star Trek.' It's not 'go where no man has gone before."'
Plunging deeper into space will require robotic scouts first, he said.
Southwest Research Institute astrophysicist Scott Bolton, Juno's principal investigator, said it's also important for people to realize "NASA's not going out of business."
"If we're going to learn who we are and where we came from, and how the Earth works, we've got to keep doing these science missions, not just Juno," Bolton said.
NASA's long-range blueprint would have astronauts reach an asteroid by 2025 and Earth's next-door neighbor Mars a decade later, although there's still uncertainty surrounding the rockets needed for the job. A Juno success would be a good sign for future solar-powered missions of all types.
Jupiter may be just two planets over, but it's far enough away to be considered the outer solar system.
It will take Juno five years to reach its target, five times farther from the sun than Earth. No spacecraft has ever ventured so far, powered by solar wings. Europe's solar-powered, comet-chasing Rosetta probe made it as far as the asteroid belt between the orbits of Mars and Jupiter.
Each of Juno's three wings is 29 feet long and 9 feet wide, necessary given that Jupiter receives 25 times less sunlight than Earth. The panels - folded for launch - emanate from the spacecraft much like the blades of a windmill.
At Jupiter, nearly 500 million miles from the sun, Juno's panels will provide 400 watts of power. In orbit around Earth, these panels would generate 35 times as much power.
The choice of solar was a practical one, Bolton said. No plutonium-powered generators were available to him and his San Antonio-based team nearly a decade ago, so they opted for solar panels rather than develop a new nuclear source. They wanted to avoid ballooning costs and possible delays connected with developing new technologies.
"It's nice to be green, but it wasn't because we were afraid of the plutonium," Bolton explained.
Indeed, NASA's six-wheeled, Jeep-size Mars rover named Curiosity, due to launch in late November, will be powered by more than 10 pounds of plutonium. Despite safety efforts, there's always the question of public safety if an explosion occurred.
NASA's Grail mission - twin spacecraft to be launched next month to Earth's moon - employs solar panels.
Eight robotic craft already have flown to or near Jupiter and its many moons, as far back as the 1970s: NASA's Voyagers and Pioneers, Galileo, Ulysses, Cassini and, most recently in 2007, the Pluto-bound New Horizons.
Juno - named after the cloud-piercing wife of Jupiter, the Roman god - will go into an oval-shaped orbit around Jupiter's poles in July 2016, after traveling 1.74 billion miles.
The craft will fly within 3,100 miles of the dense cloud tops, closer than any previous spacecraft. Any closer, and Juno would feel the tug of the planet's atmosphere, which in turn would alter the spacecraft's orbiting path and hamper its gravity experiment.
The spinning spacecraft will circle the planet for at least a year, beaming back data that should help explain the composition of its mysterious insides. Each orbit will last 11 days, for a total of 33 orbits covering 348 million miles.
Nine instruments are on board, including JunoCam, a wide-angle color camera, which will beam back images.
Juno's most sensitive electronics are inside a titanium vault to protect against the incredibly harsh radiation surrounding the planet. The radiation exposure will worsen toward the end of the mission. "We're basically an armored tank going to Jupiter," Bolton said.
Scientists believe Jupiter was formed from most of the leftovers of the sun's creation. That's why it's so intriguing; by identifying the planet's contents, besides hydrogen and helium, astronomers can better explain how the solar system came to be.
"We want to know that ingredient list" for Jupiter, Bolton said. "What we're really after is discovering the recipe for making planets."
For these answers, Juno will study Jupiter's gravity and magnetic fields, and turbulent, cloud-socked atmosphere, which can spawn 300 mph wind and hurricanes double the size of Earth. The experiments will investigate the abundance of water, and oxygen, in Jupiter's atmosphere and help determine whether the planet's core is solid or gaseous.
Once its work is done in 2017, Juno will make a kamikaze dive into Jupiter. NASA doesn't want the spacecraft hanging around and crashing into Europa or other moons, possibly contaminating them for future generations of explorers.
Southwest Research Institute: http://missionjuno.swri.edu