A considerable amount of storage space on flights goes to odds and ends that can break or go missing (which means replacement parts need to be sent up on the next trip). A 3-D printer would let astronauts only print the stuff they need, when they need it. What kind of stuff? Well, anything, really. The agency explains that an on-board 3-D printer could be loaded with blueprints at takeoff, but could also have plans beamed from Earth to the Space Station.