Three basic principles govern the course we recommend. The first is the “flexible path” approach that the Augustine commission advocated and that President Obama and Congress accepted. This strategy replaces the old insistence on a fixed path from Earth to moon to Mars with an extensive selection of possible destinations. We would begin with nearby ones, such as the Lagrangian points (locations in space where an object’s motion is balanced by gravitational forces) and near-Earth asteroids.

The flexible path calls for new vehicle technologies, notably electric propulsion. We propose using Hall effect thrusters (a type of ion drive) powered by solar panels. A similar system propelled the Dawn spacecraft to the giant asteroid Vesta and will, by 2015, carry it onward to the dwarf planet Ceres [see “New Dawn for Electric Rockets,” by Edgar Y. Choueiri; Scientific American, February 2009]. Whereas traditional chemical rockets produce a powerful but brief blast of gas, electric engines fire a gentle but steady stream of particles. Electric power makes the engines more efficient, so they use less fuel. (Think space Prius.) Because the price of this greater efficiency is lower thrust, some missions can take longer. A common misperception is that electric propulsion is too slow for crewed spaceflight, but there are ways around that. The idea that emerged at our first brainstorming session was to use robotic electric propulsion tugs to place chemical boosters at key points in a trajectory like a trail of bread crumbs; once the trail is laid, astronauts can set out and pick up the boosters as they go along. In this way, missions get the fuel efficiency of electric propulsion while keeping the speed advantage of chemical propulsion.

Crucially, electric propulsion saves money. Because the ship does not need to lug around as much propellant, its total launch mass drops by 40 to 60 percent. To first order, the price tag of space missions scales linearly with the launch mass. Thus, slimming the mass by half could cut the cost by a similar fraction.

Many space enthusiasts wonder why we would bother visiting an asteroid when Mars is everyone’s favorite destination. Actually asteroids are the perfect targets for an incremental approach toward reaching Mars. Thousands are sprinkled through the gap between Earth and Mars, providing literal stepping-stones into deep space. Because asteroids’ gravity is so weak, landing on one takes less energy than reaching the surface of the moon or Mars. It is hard enough to mount a long interplanetary expedition—six to 18 months—without also having to develop elaborate vehicles to touch down and blast off again. Asteroid missions let us focus on what, in our estimation, is the most complex (and still unsolved) problem for humans ever to venture far from Earth: learning how to protect astronauts from the deleterious effects of zero gravity and space radiation [see “Shielding Space Travelers,” by Eugene N. Parker; Scientific American, March 2006]. As NASA gains experience dealing with the hazards of deep space, it will be in a better position to design vehicles for Mars surface missions.

Several scientifically interesting asteroids could be visited by astronauts with flight times ranging from six months to a year and a half using a 200-kilowatt (kW) electric propulsion system, which is a reasonable advance over our present capability; the ISS currently has 260 kW of solar arrays installed. Such a mission would break the deep-space barrier, while taking a crucial step toward the two- to three-year flight times and 600-kW systems that would be needed for Mars exploration.

The second governing principle of our plan is that NASA does not have to invent completely new systems for everything as it did in the 1960s. Some systems, most notably zero-g and deep-space radiation protection, will require new research. Everything else can derive from existing spacefaring assets. The deep-space vehicle can be assembled by combining a few specialized elements. For instance, the structure, solar arrays and life-support systems could be adapted from designs that have been implemented on the space station. And many private companies and other nations’ space agencies have expertise in these areas that NASA could tap.

The third principle is to design a program that can maintain forward momentum even if one component runs into problems or delays. This principle should be applied to the most debated component of the space policy adopted by Congress: the launch vehicle that will ferry the crew and exploration vehicles from the surface of Earth into orbit. Congress directed NASA to build a new heavy-lift rocket, the Space Launch System (SLS). As announced this past September, NASA plans to develop this vehicle in steps starting at roughly half the capacity of the Apollo Saturn V and working up to just beyond the full launch capability of that rocket. The first SLS launcher, plus the Orion capsule now in the works, could carry astronauts on three-week excursions to lunar orbit and the Lagrangian points but can take astronauts no farther without the development of a new system.

Fortunately, journeys to deep space do not need to wait for the SLS to be completed. Preparations could begin now with the development of the life-support and electric propulsion systems that will be needed for trips beyond the moon. By making these systems an early priority, even while the new rockets are still under development, NASA would be better able to refine details of the SLS design to make it better suited to deep-space missions. These components could even be designed to fit on commercial or international launchers and then assembled in orbit, just as the ISS and the Mir space station were. The use of existing rockets would generate momentum toward deep-space exploration. With the flexibility from a portfolio of options, NASA could fit more exploration into its increasingly limited budget.