<i>At first the idea seems cool but flying all the way to Mars to only get an hour of flight? Why not send a ballon based craft that could float around for a month+ ?</i><br /><br />This is a common question. While a Mars balloon could produce some good science, there are advantages with going with an airplane.<br /><br />First, any Mars UAV has the capability for a larger science payload than any balloon. In fact, most Mars balloon concepts I've seen are extremely constrained by mass. Secondly, a Mars UAV would be <i>targetable</i>, or at least would have a more predicatable ground track, much more so than a balloon, which is at the mercy of the prevailing air currents.<br /><br />Its presumed longer lifetime notwithstanding, a Mars balloon is not as attractive as a UAV to most scientists I know, most of whom have specific targets on the Martian surface in mind (<i>e.g</i>., Valles Marineris).