While down here there’s room for debate about the suitability of 3D printing for anything more serious than rapid prototyping, few would say the same once you’ve slipped the surly bonds of Earth. With 3D printing, astronauts would have the ability to produce objects and tools on-demand from a supply of inert raw building materials. Instead of trying to pack every conceivable spare part for a mission to Mars, replacements (assuming a little forward thinking on the part of the spacecraft designers) can be made to order out of the stock of raw plastic or metal kept on-board. The implications of such technology for deep space travel or off-world settlement simply cannot be overstated.
In the more immediate future, 3D printing can be used to rapidly develop and deploy unmanned spacecraft. Tiny satellites (referred to as CubeSats) could be printed, assembled, and deployed by astronauts already in orbit. Innovations such as these could allow science missions to be planned and executed in months instead of years, and at a vastly reduced cost.