A key part of the proposal is to use slow but highly efficient solar electric propulsion (SEP) systems for cargo and faster but a lot less efficient high thrust chemical (e.g. LOX/LH2) propulsion for crews.
Neil deGrasse Tyson decries the lack of action in dealing with the threat of asteroids and comets despite the overwhelming evidence, even before the Chelyabinsk event, of the devastation the impacts of such objects can have to life on earth: