Juno is the first use of solar panels for a probe going that far out. You're right that they want to err on the side of caution using well-known technology to minimize risk. But you're missing the fact that RTGs are that well-known risk-minimized technology.
If solar panels saved weight, they would have been used on past missions like Galileo. Better solar panel technology will eventually shift the balance, but we're not there yet. Juno's panels weigh 340kg. Galileo's two RTGs, which produced about the same amount of power at Jupiter, weighed 57kg each.
I was referring to the new stirling engine RTG. According to wikipedia, NASA doesn't see it being used in a mission for another decade or two due to further testing and certification.
I wasn't aware solar was so heavy. Thanks for the info.
If solar panels saved weight, they would have been used on past missions like Galileo. Better solar panel technology will eventually shift the balance, but we're not there yet. Juno's panels weigh 340kg. Galileo's two RTGs, which produced about the same amount of power at Jupiter, weighed 57kg each.