It's a good question that I had to think about a little. First, I think you may not know how many missions there currently are. There has been a huge increase

It's a good question that I had to think about a little. First, I think you may not know how many missions there currently are. There has been a huge increase over the past years. Here's a list: http://www.nasa.gov/missions/current/index.html http://www.nasa.gov/missions/current/index.html Second, the outer planets missions are not numerous. They tend to be big and all-encompassing, with multiple science objectives and specialized imagers. (Not a quick flyby like Voyager.) Because they need to show science impact, they can't just pack a simple imager and some particle detectors. Right now, the big outer planets mission is Cassini, which has been orbiting Saturn and visiting its moons, which are a varied bunch. Before that, there was Galileo, which visited Jupiter. For context, I believe it's accurate to say that the era of large "flagship" missions was wound down years ago, and replaced by what, at the time, was called "faster, better, cheaper". (Engineers replied, "sure, pick any two".) Despite grumbling, this doctrine pretty much won out for planetary missions. For inner planets, of course, most attention has gone to Mars, using the "follow the water" strategy.