Working in the background, a relatively small group of U.S. spaceflight engineers has been figuring out just what it will take to get humans out of low Earth orbit to Mars, with stops along the way in cislunar space and perhaps on near-Earth asteroids. Spearheaded by William Gerstenmaier, associate NASA administrator for human exploration and operations, and managed in part by John Shannon, the last space shuttle program manager, the team is developing mission architectures that will guide the elected and appointed politicians who must fund and manage mankind's next steps into the Solar System. With budgets tight for the foreseeable future, a lot of attention is going into affordable technology.
One promising technology finding a big role in future exploration architectures is solar-electric propulsion (SEP). Work is underway at advancing the readiness levels of technologies that covert energy from the Sun into propulsion by using solar-generated electricity to force ions out of an engine at high speeds to produce thrust. It isn't a lot of thrust, but it can go a long way on relatively little fuel. A SEP system weighs a lot less than chemical propulsion. Given enough time it can move a lot of mass through space. That in-space advantage makes it particularly attractive for pre-positioning cargo—supplies, habitats and the like—to keep human explorers alive after they arrive on a faster vehicle to explore a distant location.
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