I want to discuss what I learned from this exercise of ranking great explorations in history. I really think NASA should do an exercise like this to better understand how to perform its mission well.
Basically I think that all of the missions listed as Tier 1 in my previous post are great because of a record or legacy that they left behind. There have been great explorations throughout history, but the ones that have the most impact leave behind an inspiring legacy.
Darwin is a very good example of this. His voyages on the Beagle would not be remembered if it wasn't for his book "Origin of Species" and future work laying the foundation for evolutionary biology. If he never writes the book or if no-one reads the book then the mission is meaningless.
The Hubble Telescope and Voyager missions returned incredible pictures that by themselves inspired the world and gave us new insight into the nature of the cosmos. These missions were designed for this purpose and their images are so great because they were the first to show in breathtaking color things that we had never before seen or imagined.
Alexander and Columbus did not produce any well known direct record of their explorations, but the nature of their exploration was so fundamental that it was recorded and transmitted by others. An interesting comparison here is with Leif Ericson who apparently preceded Columbus to North America by nearly 500 years. This is not nearly as well remembered because it was simply not recorded or transmitted. Imagine if Leif's story had somehow been transmitted throughout England and Europe? It may have changed the course of history.
Now, imagine that the Apollo missions did not benefit from the wealth of news coverage, magazine articles, tv shows, and movies that the exploration spawned. Would this have still have the same stature and influence as it does today? Walking on the Moon captured everyone's imagination back in 1969 but today would merit probably only a single news cycle.
Imagine the cultural tools we have available to record, document, capture, and imagine space exploration -- books, paintings, poems, movies, blogs, twitter, photography, music, etc... Now go and try and learn about NASA's space exploration - more often than not you have to rely on classroom activities, press releases, and press conferences. I've got nothing against scientists, but storytelling and inspiration aren't really our strong suits.
So if we want to inspire the next generation, or any generation for that matter, it seems NASA should utilize the tools that have always worked for inspiration: art. Without art to tell the story, space exploration rapidly becomes a bland, boring, and inaccessible for the regular person. You don't need some fancy web developer to make NASA more appealing to the younger generation, you need artists who can design content that appeals to that generation - video games, music, movies, books, magazine articles, and stories.
So I suggest all future NASA missions include artists as part of their EPO program. Not only that, but I think artists should have involvement in mission operations, inspiring photos to take etc...
This isn't a new idea, and it has been utilized on and off in several missions I know about. The Mars Phoenix mission allowed an author to have direct access to nearly every meeting and discussion and he has produced a book about it called "Martian Summer". Furthermore MSL involved James Cameron in the development of a camera for filming IMAX type footage on Mars. Unfortunately I think that was dropped for budgetary reasons. I think this type of artistic involvement could be developed to a much greater extent without increasing mission costs or compromising scientific results.