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.
Last year, NASA’s managers invited the European Space Agency to propose a small spacecraft to explore the Jovian system. Van Kane describes the recently-posted results of ESA's concept studies for two possible missions.
A couple of days ago, Cassini flew past Enceladus for its 20th targeted encounter. Cassini has seen and photographed quite a lot of Enceladus before, but there's still new terrain for it to cover.
There are no spacecraft at Uranus or Neptune, and there haven't been for 30 and 25 years, respectively. So we depend on Earth-based astronomers to monitor them, including Damian Peach.
People often ask me to produce one of my scale-comparison montages featuring the small moons of the outer solar system. I'd love to do that, but Galileo's best images of Jupiter's ringmoons lack detail compared to Cassini's images from Saturn.
Damian Peach's photo-documentation of Jupiter helps us monitor the giant planet's ever-changing patterns of belts, zones, storms, and barges, during a time when no orbiting missions are there to take pictures.
Today's press briefing at the Applied Physics Laboratory in California was preceded by hours of New Horizons team members cryptically dropping hints on Twitter at astonishing details in the seven images downlinked since the flyby. The images are, in fact, astonishing, as well as beautiful, surprising, and puzzling.