When does it make sense to build a starship? Back in the late 1960s, Freeman Dyson went to work on the question of how much an interstellar probe might cost. Extrapolating from nuclear pulse propulsion and the state of the art in spacecraft design as then understood, Dyson arrived at an estimate of $100 billion to build the craft, which translates into roughly $650 billion today. Though stark, that figure is by no means as eye-popping as one of the estimates drawn up by the original Project Daedalus team: $100 trillion in 1978 dollars. These figures numb the senses, and you may recall the recent work by Ralph McNutt (Johns Hopkins Applied Physics Laboratory) and team, which pegged the cost of a series of human expeditions into the outer Solar System at $4 trillion. It's helpful to remember, though, that calculating when a project becomes fiscally feasible can be a useful undertaking in itself. Richard Obousy goes to work on these matters in a recent post in the Project Icarus blog,...