ST. LOUIS, Mo.—More than just about anything, Robert Zubrin would like to see humans visit and then settle on Mars during his lifetime. The aerospace engineer has made a living of identifying technologies needed to get astronauts to the Red Planet and trying to build a public consensus that Mars is humanity’s next great leap.

Zubrin also likes to knock down hurdles and roadblocks that he sees standing between humans and Mars. Concerned about radiation? Don’t be, Zubrin says, because the in-flight dose won’t be appreciably greater than some US and Russian astronauts have accumulated during long-duration missions to the International Space Station. And what about the cost? If NASA were to buy services directly from industry and bypass the cost-plus method of contracting, humans could walk on Mars for tens of billions of dollars, he says.

Of late, Zubrin has been bothered by another potential difficulty between humans and the exploration and settlement of Mars—planetary protection. This is the prime-directive-style notion that humans should not contaminate other worlds with Earth-based microbes and, on the flip side, that humans should not introduce any potentially dangerous pathogens to Earth.