Earthlings are fragile, demanding, and germy, not obviously suited to life elsewhere. Illustration by Script & Seal

On March 27th, an American astronaut named Scott Kelly blasted off from Earth and, six hours later, clambered onto the International Space Station. He’s been there ever since. Each day, the I.S.S. orbits the planet fifteen and a half times, which means that after a month Kelly had completed more than four hundred and fifty circuits. By now, he’s made nearly a thousand.

Kelly, who is fifty-one, is short—five feet seven—and stocky, with a round face and a thin smile. If all goes well, he will not return to sea level until March, 2016. At that point, he will have set an endurance record for an American in space.

Even in brief bursts, space is tough on the human body. Changes in intracranial pressure can lead to eye problems. Weightlessness induces vertigo. Fluids collect in places they shouldn’t. Muscles atrophy and bones grow brittle. Astronauts’ internal organs drift upward and their spines extend. It is expected that by the time Kelly finally descends he will have stretched to five feet nine.

NASA has dubbed Kelly’s circular odyssey the One-Year Mission. As he spins around the Earth, scientists at the agency are tracking his physical and emotional deterioration, monitoring, among other things, his sleep patterns, his heart rate, his immune response, his fine motor skills, his metabolism, and his gut bacteria. Kelly has an identical twin, Mark, who was also an astronaut. (Mark Kelly is perhaps best known as the husband of Gabrielle Giffords, the former Arizona congresswoman.) In the course of the year, Mark will submit to many of the same cognitive and physiological tests as Scott, though without leaving Earth. This will provide a glimpse into the effects of space travel down to the molecular level.

Kelly’s One-Year Mission represents a kind of dress rehearsal for a longer, straighter, and even more punishing voyage. In NASA’s Buzz Lightyear-esque formulation, it’s “a stepping stone” to “Mars and beyond.” At its closest, Mars is thirty-five million miles from Earth, and, under the most plausible scenario, getting there takes nine months. Owing to the relative motion of the planets, any astronauts who make it to Mars will have to cool their heels on the red planet for three more months before rocketing back home. What NASA learns about Kelly—at least, so the theory goes—will help it anticipate and overcome the challenges of interplanetary travel.

But even as NASA rehearses for “Mars and beyond” its actual reach has been shrinking. The last time an American made it as far as the moon was in 1972. In fact, since the Nixon Administration, no American has got past what’s known as low Earth orbit, or LEO. (The International Space Station, which circles the globe in LEO, maintains an average altitude of two hundred and twenty miles.) And nowadays even this is farther than NASA can manage.

Since the retirement of the Space Shuttle, in 2011, the agency has lacked the wherewithal to get astronauts into LEO. And so, before Kelly could embark on the One-Year Mission, he first had to fly to Baikonur, on the steppes of central Kazakhstan. There he spent a few nights at the Cosmonaut Hotel before hitching a ride with two Russians on a Soyuz rocket.

It’s true that even a journey of thirty-five million miles has to start somewhere. Still, a reasonable person might ask: Where are we headed? Is it really to Mars? Or is it just to Kazakhstan?

Several recent books take up these questions, some head on, others more elliptically. Chris Impey is an astronomer at the University of Arizona who studies the structure and the evolution of the universe. In “Beyond: Our Future in Space” (Norton), he foresees a bright “off-Earth” future. Within twenty years, he predicts, there will be a vibrant space-tourism industry, complete with “zero-gravity sex motels.” In thirty years, he expects “small but viable colonies” on both Mars and the moon. And within a century these colonies will have produced a generation of space-bred babies. In 2115, he writes, a cohort will come of age “who were born off-Earth and who have never been home.”

Impey acknowledges NASA’s current difficulties. Prominently featured in “Beyond” is a graph showing how the agency’s budget has changed over time. From the late nineteen-fifties through the late sixties, it shot up, until, a year or two before the first moon landing, in 1969, it represented almost five per cent of all federal spending. Then, like a piece of space debris hurtling toward Earth, it plummeted. Today, NASA appropriations make up less than 0.5 per cent of federal spending.

“No bucks, no Buck Rogers,” Impey observes. And he’s frank about the failures of the Space Shuttle program, which resulted in two disasters—the loss of the Challenger and the Columbia orbiters and, with these, the lives of fourteen astronauts. Even when its vehicles weren’t blowing up, the shuttle, Impey notes, never functioned as advertised: “the launch rate ended up ten times lower than originally planned and the cost per launch twenty times higher.”

But NASA is no longer the only game in town. Impey is excited by the rash of privately owned firms that are getting into the space business. He cites the “audacious” plans of a Dutch entrepreneur named Bas Lansdorp, who’s been marketing one-way trips to Mars on the Web. Lansdorp, he says, “plans to finance his venture by turning it into a reality TV epic—think Survivor meets The Truman Show meets The Martian Chronicles.” Other commercial ventures include Jeff Bezos’s Blue Origin, Richard Branson’s Virgin Galactic, and Eric Anderson’s Space Adventures. Space Adventures has already carved a niche for itself by negotiating visits to the I.S.S. for well-heeled amateurs. (Most recently, the company “arranged” a visit for the British soprano Sarah Brightman, at a cost of fifty-two million dollars; the singer has now postponed her trip, however, and it seems that a Japanese entrepreneur, Satoshi Takamatsu, will go in her stead.) “After years in the doldrums, space is heating up,” Impey writes.

Stephen L. Petranek, the author of the forthcoming “How We’ll Live on Mars” (Simon & Schuster/TED), is, if anything, even more boosterish. By his timetable, the first people should be showing up on Mars just a little more than a decade from now. Petranek is a journalist who served as the editor-in-chief of the magazine This Old House before moving to Discover, a career path that perhaps explains his book’s focus on issues like bringing the right tools to Martian construction projects. “Someone drilling for water cannot discover halfway through the process that they have failed to anticipate a specific problem—a mineral deposit that requires a special drill bit, for instance,” he points out.

Petranek envisions a multistage settlement program. The first pioneers on Mars, not unlike the American frontiersmen, will have to struggle to survive. Just to have drinking water, they’ll need to plow up the planet’s soil—known as regolith—melt down its ice, and distill the results. To breathe, they’ll have to separate the water into hydrogen and oxygen, then mix the oxygen with an inert gas—argon, perhaps—which they’ll get from, well, somewhere. Eventually, Petranek imagines a shift in the balance. Instead of adjusting to life on Mars, humans will adjust Mars to their needs. They will reëngineer the atmosphere and warm the planet. As the regolith thaws, ancient streams will flow again and life will flourish along their ruddy banks. More and more people will be drawn to Mars, until there will be whole cities of them.