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Assumptions 123 around the universe in space suits,” he declared, “belongs back where it came from, on the cereal box.”? Most SETI scientists have tended to agree. A NASA report on SETI declared that manned interstellar flight is out of the question not only for the present but for an indefinitely long time in the future. The SETI advo- cates who wrote that report admitted that making such journeys is not a physical impossibility, but argued that it is an economic impossibility at the present time. Some unforseen breakthroughs must be made before humans can travel to other stars.* Even if interstellar travel were technically feasible, SETI researchers argue, the costs involved would be so enormous that other civilizations would not expend the necessary resources. If the purpose is to gather information, interstellar communication would be far more economical. “It is always easier to transmit photons,” five scientists wrote, “than to transmit matter.”* SETI conventional wisdom assumes that each technological society stays close to the star of its birth. By virtually ruling out interstellar travel, the SETI paradigm depicts what Brin called “isolated motes of intelligence separated by sterile tracts of space.” Drake confirmed this when he wrote that the distances between the stars create an interstellar quarantine.° An interesting exception was Carl Sagan, who argued in 1962 that inter- stellar travel not only might be possible, but that other civilizations eons more advanced than ours must today be plying the spaces between the stars. Very advanced societies, he wrote later, should surely be capable of interstellar flight; there are no fundamental physical objections to it. Sagan extended that capability to future humans in the book he co-authored with Shklovskii, declaring that “efficient interstellar spaceflight to the farthest reaches of our galaxy is a feasible objective for humanity.”” This question has profound implications for the consequences of contact. If direct contact is possible, our hopes and fears acquire a new intensity. meaning. The argument that interstellar travel is impossible or too difficult to be worth doing has rested on several challengeable assumptions. The first, made by Von Hoerner and others, is that in order to cover interstellar dis- tances within “reasonable times,” we ought to fly as close as possible to the velocity of light. Because that would require titanic amounts of energy, interstellar flight would be extremely difficult. (Von Hoerner implicitly argued the other side of this issue when he stated that the only goal that Assumptions Mere distance is nothing; only the time that is needed to span it has any —Arthur C. Clarke, 1968°