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.
Pluto's moons, formerly known as "P4" and "P5," are now named Kerberos and Styx; I thought I'd help place them into context with a little help from Cassini. Also, Neptune now has a 14th known moon.
Despite the fact that Voyager 2 returned relatively few high-resolution images from either Uranus or Neptune, there are many more photos in the archives than regularly make it to public view.
Nothing reflects the romance of deep space exploration more than the evocative names of places on the planets and moons.
A look at an older paper describing Galileo's possible sighting of individual ring particles orbiting Jupiter as companions to its inner moon Amalthea.
I've been out of town for a couple of days and am overwhelmed with work and an overflowing email box. So what do I do about that? I ignore what I'm supposed to be doing and play with Cassini raw image data, of course. Here is a "mutual event" of Mimas (the bigger moon) and Pandora (the outer shepherd of the F ring).
An easy image processing trick -- using lower-resolution color data to colorize a black-and-white photo -- is relied upon by many space missions to keep data volumes low. Here's how to do it.