Jupiter. Saturn. Uranus. Neptune. Each of these giant planets is the center of its own miniature solar system. Each is spectacularly beautiful and scientifically fascinating, which are reasons enough to explore them. But by studying the giant planets and their rings and moons, we can also learn about the forces that operated during the formation of our own solar system, as well as the origins of the hundreds of new extrasolar planetary systems that we discover every year.
And their moons are worlds in their own right. There are at least 16 outer planetary moons that would be called dwarf planets if they orbited the Sun rather than a planet. Two (Jupiter's Ganymede and Saturn's Titan) are larger than the planet Mercury, and one (Triton) is probably a captured Kuiper belt object.
But it is challenging and expensive to explore the outer planets, and missions to the outer planets take a very long time to develop, fly, and operate. Cassini will be orbiting Saturn until 2017, and Juno will operate at Jupiter from 2016 to 2017. After that, it's not clear if anyone will be sending a followup mission to Saturn or Jupiter or its moons, or an orbiter to survey the Uranus or Neptune systems. And there is a critical shortage of the isotope of plutonium that is needed to generate power for outer planetary missions.
It is not easy to observe Jupiter’s moons as more than points of light with Juno, because Juno will never get very close to any of the moons, but as its orbit shifts there will be opportunities to collect data on some of the moons.
NASA announced this afternoon that Juno passed through its first perijove since entering orbit successfully, with science instruments operating all the way. This is a huge relief, given all the unknowns about the effects of Jupiter's nasty radiation environment on its brand-new orbiter.
As it approached Jupiter from June 12 to 29, JunoCam captured an animation of the major moons orbiting the planet. The mission released a processed version of the animation on the day of orbit insertion, but took a few weeks to release the raw image data. I've prepared a page hosting all the raw data, and share a few processed versions.
When are the solstices and equinoxes on the giant planets, and when are they best positioned for view from Earth? I ask these questions a lot as I write about Earth photos of giant planets, and I finally decided to gather the answers to those questions in a single post.
We're now just about 12 hours away from Juno's Jupiter orbit insertion. As anticipation ramps up, NASA has released this sneak peek at JunoCam's approach movie, made of views of Jupiter and its largest moons shot during the final approach, up until about five days ago.
Jupiter is growing in Juno's forward view as the spacecraft approaches for its orbit insertion July 5 (July 4 in the Americas). The mission has released images from JunoCam and sonifications of data from the plasma waves instrument as Juno begins to sense Jupiter.