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125 An exploring machine might not even need to decelerate into the target system. The British Interplanetary Society’s study of an interstellar probe assumed that the main ship would fly by Barnard’s Star without slowing down, launching much smaller subprobes to do more thorough investigation.’ That would cut the energy requirements again. We can lower those requirements still more by extending the time for the mission, reducing the velocity of the spacecraft to a small percentage of the speed of light. That is easier to do when we send machines; people impose much greater burdens. Even for human crews, there are several proposed solutions. Our descendants may come up with better ones. Might the solutions for interstellar flight rest on physical principles that we have not yet discovered? Some SETI scientists have shown themselves willing to consider that possibility when they discuss interstellar commu- nication. Yet, most of those scientists resist extending that flexibility of mind to interstellar travel. This reminds us of how Arthur Clarke described stages in the way the scientific establishment reacts to a new big idea. First, it is impossible. Second, it may be possible, but it’s not worth doing. Third, I said it was a good idea all along. Interstellar flight by machines has entered the second stage. The demonstration that no possible combination of known substances, known forms of machinery and known forms of force can be united in a practicable machine by which men shall fly long distances through the air, seems to the writer to be as complete as it is possible for the demonstration aL al Lone en bn Journeys to the stars are a daunting technological challenge for a civili- zation with our present level of technology. The distances involved, and by implication the time, energy, and machine reliability required, are orders of magnitude greater than anything we have attempted. Yet, no known law of physics or engineering tells us that interstellar flight is impossible. Although there is a huge jump in scale from solar system travel to interstel- lar travel, Clarke reminded us that we children of the Space Age can no longer remember how enormous the solar system seemed only a lifetime ago.'® Leslie Shepherd, Eugen Sanger, Ernst Stuhlinger, Robert Bussard, and others began studying the physics and engineering of interstellar flight as long ago as the 1950s, addressing the central issue of propulsion with pro- posals for fission, fusion, and other systems. These pioneers found that energy requirements decrease dramatically if we extend the allowable time Starships of the Mind Starships of the Mind of any physical fact to be. —Astronomer Simon Newcomb, 1893