The international space station is scheduled to de-orbit and expire in a few years. Supporters of maintaining the space station beyond this time argue that one should take into account the money that has already been spent on it. As Senator Bill Nelson said, “If we’ve spent a hundred billion dollars, I don’t think we want to shut it down in 2015” [Achenbach, 2009]. Comment on the senator’s reasoning. How would you determine whether the space station should be discontinued?