There are places in the American West where you can still see the tracks carved by pioneer wagon wheels more than a century ago. This year I stood on a path in England, still in daily use, that travelers have walked since the days of Stonehenge.

So strong is the human need to blaze trails that we stretch them out everywhere--extending them even to places where our own feet can't take us. You can follow our tracks, in fact, all the way to the deserts of Mars.

The first people to succeed in making their own trail on the planet were the members of the team behind the Mars Pathfinder mission in the 90's. They earned their mission name by deploying a small, relatively simple rover called Sojourner that opened the way for more sophisticated expeditions. Sojourner was named after another trailblazer, Sojourner Truth.