Viewpoint: Yes, a manned mission to Mars is the next logical step for space exploration.

Viewpoint: No, a manned mission to Mars would be an enormously expensive enterprise with insufficient return to justify it.

Mars has long been an object of peculiar fascination. As one of the nearest worlds beyond Earth, it is naturally one of the most accessible for visitation by robot or manned spacecraft. Numerous unmanned missions have targeted the so-called Red Planet, from the Mariner and Viking spacecraft of the 1960s and 1970s to the more recent Mars Pathfinder, whose robot explorer Sojourner generated widespread public interest in 1997.

For more than a century, Mars also has been regarded as a likely—or at least possible—location of extraterrestrial life. In 1877 the Italian astronomer Giovanni Schiaparelli identified features on the Martian surface...