The various reasons offered for sending humans to Mars, at a cost of billions if not hundreds of billions of dollars — “but less expensive than the war in Iraq!” insisted Andrew Rader, a Mars One candidate and expert in human spaceflight with a doctorate from M.I.T. — include elements both practical and profound, optimistic and dystopian. Ellen Stofan, NASA’s chief scientist, said that for all the success of robots like Curiosity, sending humans to the surface “may be the only way to prove life evolved on Mars and what the nature of it is.” And demonstrating that some form of life arose at least twice in our solar system would lend ballast to the argument that the universe teems with life. Humans will soon need more space and more resources than Earth can offer, Dr. Shostak said, adding, “If you want to have Homo sapiens for the long run, you have to move out somewhere.” Whatever hardships the Mars homesteaders endure, Mr. Lansdorp argued, may well improve life for those back on Earth. “We’re a species that explores and pushes boundaries,” he said. “By exploring our own planet, we’ve developed technology to make our life more comfortable. Mars is the next logical step, the boundary to push, to make us more developed still.”

Scientists agree that of all the places in the solar system where a few expatriate earthlings might settle, Mars is the least hostile. It’s roughly one-sixth the size of Earth, but given its lack of oceans, it nearly matches us in landmass. It rotates on an Earthlike tilt of 24 degrees and so has seasons, the length of its day is similar to ours, and its soil is about 2 percent frozen water, which in theory could be melted out and put to use. Its gravity is about 40 percent that of Earth — enough to keep inhabitants from the severe bone and muscle loss caused by long-term stints in outer space, but still of sufficient levity, said Norbert Kraft, chief medical officer for Mars One, “that maybe your knees won’t hurt and your wrinkles will go away.”