Interplanetary flight is the next giant leap for humans in space. Yet consensus on even the smallest steps forward has proven elusive. In June, a US National Research Council report1 illuminated many options but offered no recommendations. Return to the Moon? Head straight to Mars? Pluck a boulder off an asteroid and tug it to lunar orbit, just so that idle astronauts have somewhere to go and something to do? NASA must decide which path to follow before President Barack Obama’s budget announcement in January 2015. Some options are better than others. The cost and complexity of human space exploration demands that each element be measured by its value towards the ultimate goal: Mars.