The human journey to Mars will be a great adventure, the greatest of our age. No destination is more exciting, and none more difficult. Mars is very far away, much further than the Moon. A round trip takes years—years in which you have to supply food, water, and oxygen for astronauts, all while minimizing radiation damage, health issues, and even boredom.

But difficult is not the same as impossible.

We have most of the technical knowhow and ability to pursue this adventure if we so choose. The central challenge bedeviling human exploration of the Red Planet is far more prosaic: cost. Various concepts for sending humans to Mars have ranged into the hundreds of billions of dollars—far beyond what NASA, which currently