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.
Huygens may have landed on Titan over a decade ago, but a group of researchers from York University were able to make a new and unexpected discovery with this older dataset.
Image processing enthusiast Ian Regan produced a pretty view of Titan's lake-filled north pole, now visible to Cassini's cameras in the summer sun.
January 24 was the 30th anniversary of the Voyager flyby of Uranus. Uranian moons have been on my mind ever since New Horizons sent us close-up images of Charon. On the occasion of the anniversary, Ted Stryk produced latest-and-greatest versions of the Voyager views of these worlds.
Tomorrow, Cassini will fly by Titan, picking up a gravity assist that will tilt its orbit slightly up and out of the ring plane. That will end what has been a wonderful year of frequent encounters with Saturnian moons.