I've been intrigued by the political aspects of two space science missions: the Mars Exploration Rovers trundling around the Red Planet, and the Cassini mission to Saturn. Both missions arrived around early 2004, long ago completed their initial mission timelines, and have been granted funding for multiple mission extensions.
Several months ago, the Mars Opportunity rover controllers announced they were sending it to a massive crater 12 kilometers from its current position. That travel distance is equal to the total distance it's traveled in five years, and they expect moving quickly it will still take two years. After it arrives, many more months or years would be needed to analyze the crater's geology.
More recently, the Cassini mission announced plans for an extended mission all the way to 2017, with all the exciting things they plan to do in that time.
The trick in both cases though is that Congress hasn't authorized the budgets that would allow the missions to achieve their self-appointed goals, and Congress authorizes extensions only for significantly shorter periods. The mission managers acknowledge they need approval, but they leave unsaid how they've also put Congress in a bind.
If Congress cuts off the funding in the near future, they finish in the middle of a mission instead of at a logical conclusion of a mission extension. The Opportunity rover in particular would be seen as the equivalent of being commanded to cross half-way through a river, and then abandoned. Why send it all then?
I'm not criticizing. This will result in good science. But it's even better politics, and noone's paid much attention to it.