One story resurfaces every few years: the tale of the Mercury 13–13 women whom NASA recruited for the space program, then ditched with nary a launch or an explanation. Or so the story goes. According to Promised the Moon, the fine new book by Stephanie Nolen, a foreign correspondent for Canada’s Globe and Mail, the whole truth went deeper.

Gravity-assist trajectories between Earth and Mars would reduce the cost of shuttling human crews and their equipment. A gravity-assist maneuver can be likened to a rubber ball bouncing off a wall. In this analogy, the spacecraft is like the rubber ball, and the planet is like the wall. As the ball bounces off the wall, the bounce-off velocity will be higher or lower if the wall is moving toward or away from the ball as they meet. The mathematical relation is described by a fundamental principle of Newtonian physics: conservation of momentum. The change in the ball’s momentum is balanced by an inverse change in the wall’s momentum.

The basic advantage of astronauts is that they can explore Mars in real time, free of communications delays and capable of following up interesting results with new experiments. Robots, even after decades of research to make them completely autonomous, cannot manage without people in the loop. But the question arises: Where should the astronauts be? The obvious answer–on the surface of Mars–is not necessarily the most efficient. At the first “Case for Mars” conference in 1981, one of the more provocative conclusions was that the Martian moons, Phobos and Deimos, could serve as comparatively inexpensive beachheads.

The goal of human exploration and settlement of Mars is not beyond our reach. No giant spaceship built with exotic equipment is required. Indeed, all the technologies needed for sending humans to Mars are available today. We can reach the Red Planet with relatively small spacecraft launched directly to Mars by booster rockets embodying the same technology that carried astronauts to the moon more than a quarter of a century ago. The key to success lies with the same strategy that served the earliest explorers of our own planet: travel light and live off the land. The first piloted mission to Mars could reach the planet within a decade. Here is how the proposed plan– called the Mars Direct project–would work.

Today Mars looms as humanity’s next great terra incognita. And with dubious prospects for a short-term financial return, with the cold war a rapidly receding memory and amid a growing emphasis on international cooperation in large space ventures, it is clear that imperatives other than profits or nationalism will have to compel human beings to leave their tracks on the planet’s ruddy surface. Could it be that science, which has long been a bit player in exploration, is at last destined to take a leading role? The question naturally invites a couple of others: Are there experiments that only humans could do on Mars? Could those experiments provide insights profound enough to justify the expense of sending people across interplanetary space?

Even when humans aren’t doing it, they are likely to be thinking about it. The representatives of the National Aeronautics and Space Administration, however, might be among the few who don’t like to think about sex, at least not officially. But as permanent space habitation nears reality, there are signs that NASA may finally, albeit reluctantly, confront the issue of sexual behavior in space.