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.
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.
With the recent announcement by NASA that the 36 year-old spacecraft Voyager 1 has officially entered interstellar space at a distance from the sun about four times further than Neptune's orbit, and with Voyager 2 not far behind, it seems worthwhile to explore how humans managed to fling objects so far into space.
The European Space Agency will announce two major science missions this November, one of which is likely to be devoted to solar system exploration.
By now I hope that everyone has seen some of the spectacular images of the Saturn system (and especially Titan!) from the Cassini-Huygens mission. However, the measurements that often make my heart race are taken by instruments that reveal Titan in ways that our eyes cannot see.
Here it is: the view from Saturn of our Earthly home, one and a half billion kilometers away. We see Earth and the Moon through a thin veil of faintly blue ice crystals, the outskirts of Saturn's E ring. Earth is just a bright dot -- a bit brighter than the other stars in the image, but no brighter than any planet (like Saturn!) in our own sky.