Chemical rockets have served humankind well in its first, tentative steps into space. Having ridden atop them to the moon and back, one of us (Aldrin) can vouch for the technology's merits. Nevertheless, for trips beyond our nearest neighbor in space, chemical rockets alone leave much to be desired.

Even Mars, the next logical destination in space, would be a stretch for chemical rockets. To deliver a human crew to the planet would require so much fuel that essentially all scenarios for such a voyage involve producing, on the planet's surface, large amounts of fuel for the return trip. That requirement adds another element of risk and complexity to the proposed mission. Much more powerful plasma rockets, on the other hand, are still probably a decade away from use on a human-piloted spacecraft.

This article was originally published with the title A Bus Between the Planets.

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