The Apollo 11 astronaut Buzz Aldrin proposes in the Washington Post:
Let the lunar surface be the ultimate global commons while we focus on more distant and sustainable goals to revitalize our space program. Our next generation must think boldly in terms of a goal for the space program: Mars for America's future. I am not suggesting a few visits to plant flags and do photo ops but a journey to make the first homestead in space: an American colony on a new world.
There's no doubt about the excitement of Mars exploration. And it's certainly possible that astronauts would be able to conduct investigations and perform experiments that robots could not match. Nor does the word "colony" bother me; contrary to centuries of speculation there were no Native Martians to be dispossessed by us (or to invade Earth), in fact no unambiguous sign of present Martian surface life, though who knows what aquatic organisms might exist in the planet's interior.
One expression is unnerving: homestead. And it's worrisome precisely because it summarizes what are otherwise attractive attributes: courage, perseverance, willingness to take risks. There is also the belief that self-sustaining Mars colonies could perpetuate human society if the earth became uninhabitable, an idea endorsed at the very highest levels of science:
Homesteading Mars is based on the assumption that humanity can modify extraterrestrial environments as self-sustaining producers of the means of life. But the Dust Bowl of the 1930s was the result of the boundless technological optimism that encouraged the settling of the Great Plains, as explored in the classic work of the environmental historian Donald Worster, whose roots are in the region. First, in the nineteenth century came the idea that human activity would modify natural patterns: "Rain follows the plow." Then the First World War created an agricultural boom that produced a wave of speculative farming in the 1920s.
There's no truly rational way to budget for Mars exploration. If we knew what we were going to find, it wouldn't be necessary. Much of the benefit of exploring is surprise, images and data that upset our assumptions. So yes, let's send people to Mars if that's the best way to advance knowledge. But remember that the Martian soil turns out to be laced with the oxidizing salt perchlorate, an ingredient of rocket fuel, among other things. Let's balance Aldrin's appeal and Hawking's injunction with a comment that another giant, the Nobel Laureate physicist Edward Purcell, made to me twenty years ago: With the energy it takes to get a person out of the earth's gravitational well, you can feed them for a lifetime.