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.
In which I finally get around to writing about a paper published last August: Enceladus' plumes sometimes spout more and sometimes spout less, depending on where Enceladus is in its orbit. This discovery was enabled by Cassini's longevity at Saturn, and we'll be able to follow up on it, as long as Cassini is allowed to complete its mission.
Here's a lovely new view of Dione, one of the lovely mid-sized icy moons of Saturn, assembled by Daniel Macháček.
This week's PlanRad talks to one of the creators of a new effort to build support for the Europa Clipper, a spacecraft that would tell us far more about what's going on under that icy moon's surface.
It took months of work (and no wonder) but the wait was worth it: here is Cassini's spectacular view of Saturn, captured on July 19, 2013, as Cassini passed through Saturn's shadow. If you're a little confused by the image, I'm here to help: I've posted a video explainer.
Can features on Neptune be observed by amateur astronomers? For years, the Hubble Space Telescope and some professional terrestrial observatories have been revealing incomplete belts and spots on the surface of Neptune. Now, spots have been imaged by amateurs.
It’s been a long time since anyone paid Uranus a visit. The Uranus system is, however, fascinating, as evidenced by the wealth of topics covered by the diverse group of planetary scientists who gathered to discuss it last week at the Paris Observatory.
I try to be measured in my praise for spacecraft images. Not every photo can be the greatest space image ever. But this enormous mosaic showing the flattened globe of Saturn floating within the complete disk of its rings must surely be counted among the great images of the Cassini mission.