As you report, America’s space agency, NASA, is in trouble in these difficult economic times, and human spaceflight is likely to be the first victim of sharp cuts in its program (“Over the Moon?” July 18, 2009). Fair enough. But you fail to mention the only reason why human spaceflight is of any lasting interest. Take away space colonization, and exploration of space is best left to machines. In addition, space colonization makes long-term sense only if it stretches beyond the solar system. This is the only way for the human species to survive past the sun’s demise. Of course, space colonization is unimaginable without terraforming—large-scale engineering that secures adequate atmospheric and other planetary conditions for the human species’ habitation anywhere in space. Is this within the species’ capabilities? Perhaps. But the state of our own planet is now such that climate change may lead to a point where terraforming will be needed on earth. Peter Ward thus argues in The Medea Hypothesis: Is Life on Earth Ultimately Self-Destructive? (Princeton: Princeton University Press, 2009), where he attempts to show that we cannot count with our planet’s long-term habitability. Ensuring that earth continues being habitable would be such a costly feat of engineering that space colonization would remain practically unimaginable. Human spaceflight thus ought to be axed from NASA’s program as unadulterated folly.