Space missions have a long timeframe. Back in 2001 or thereabouts, I was at a dinner associated with the LPSC and I was sitting at a table with Sean Solomon, the PI of Messenger. We had an interesting discussion about some of the design tradeoffs of Messenger - for instance, it will be in a highly oblique orbit so that it has time to cool off again after getting mildly roasted as it swoops over Mercury's sunlit side. There is a heat shield to keep the sunshine off the craft itself, but the reflected heat from mercury is not trivial. A circular orbit would have fried it (and would cost more propellant to acheive - a fortunate compromise).

Each flyby adds incremental information but when we finally get that baby into orbit, it'll be a whole new era in planetary surfaces...