It's as big as a basketball court, in all, but its heart is a 400lb/181kg titanium vault that will temporarily protect the computers and avionics from the high radiation that surrounds Jupiter.
"Temporarily" because the electronics still will slowly fry, literally bit by bit. NASA has planned the mission so that --- they hope --- damage will accrue slowly enough for Juno to finish its tasks.
But no human craft has ever approaches Jupiter as closely as Juno will, so there are many unknowns.
I was there, a guest of NASA, at the launch.
my 60-sec launch video:
(Note: The launch was in 2011; my still camera was decent, but by video cam was not meant for today's large screens, alas.)
my pix, as slideshow: http://s225.photobucket.com/user/flanga_bucket/slideshow/Juno%20to%20Jupiter
my pix, as photobucket "story":
How Juno Will Survive Its Death-Defying Mission to Jupiter
Why Juno Has Just One Dangerous Chance to Enter Jupiter's Orbit