For some, the real lesson of robotic exploration might be that we should not send people at all. If NASA’s only goal was scientific discovery, robotic probes would certainly be cheaper and lower risk. Yet NASA is tasked with more than just science; science is only one aspect of a broader human impulse to explore. Space exploration has wide appeal because of a desire for ordinary people to experience it firsthand someday. Robotic probes are just the first wave of solar system exploration. Government-funded human missions will be the second wave, and the third will be private citizens seeking their fortune and adventure in space. NASA’s past investments developed the technology that is fueling today’s commercial space race, with capsules launching to the space station and space planes jetting over the Mojave Desert [see “Blastoffs on a Budget,” by Joan C. Horvath; Scientific American, April 2004]. NASA can now develop the technology that we will need to push deeper into the beyond.
Flexibility Is the Watchword
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.