I don't think anybody can argue that new knowledge of our universe and our solar system is a bad thing (unless of course you consider new knowledge vastly unimportant), and space science has helped humanity vastly through earth science. I mean, it wasn't that long ago that satellites weren't overhead giving us a better view of forest fires and hurricanes. Thus its easy to argue whether we do need to go back to the moon or to Mars (manned mission), because frankly what happens on earth is vastly more important to everyone who lives there. Which is again why we should leave it to the robots, they are cheaper and last longer in these hostile environments than we would ever be capable of doing.