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124 may be important enough to justify the immense effort needed for inter- stellar space travel appears to be the search for other intelligent beings.’) Other key assumptions are that interstellar spacecraft are inhabited by human beings or their alien biological analogs; that voyages must be round- trips requiring acceleration and deceleration for both the outward and inward legs of the trip; that such journeys must be completed within a human working lifetime (typically 40-50 years); and that an interstellar vehicle must carry its entire energy supply and reaction mass on board. “We cannot plan on building a spacecraft that could travel anywhere close to the speed of light with our present technology,” wrote Goldsmith and Owen. “Yet only by traveling at nearly the speed of light can we hope to cover interstellar distances in tens of years.” They then proposed an extremely unlikely scenario that exaggerated the timescale of interstellar exploration. “If a choice had to be made between spending money to search for extraterrestrial signals now and spending money to send out a space crew to return after, say, 200,000 years, then an immense prejudice in favor of human journeys would be needed to choose the second alternative.”'° Exaggerating obstacles to interstellar flight is comparable to exaggerat- ing the difficulty of finding evidence of alien intelligence through astro- nomical observations. In both cases, some critics are trying to discredit ideas that they do not like. Bracewell observed that calculations by opponents of starflight produce astronomical sums. “Then you'll find that they’ve thrown in a factor of a thousand, which they introduced themselves. When you point this out to them, it’s not very long before they come up with another irrational objection.”"! There are abundant precedents for this behavior. Before the Space Age, some scientists presented analyses showing that interplanetary flight was impossible. The President of the Royal Astronomical Society of Canada, for example, calculated that a spaceship would need an initial weight of about 1 trillion tons to make a round trip to the Moon.” What makes interstellar travel different is not just the distances between stars but also our own impatience, due to the short lifetimes of human Laies. We aes te a Le beings. We are in a hurry. Voyages to the stars begin to look more feasible if we remove humans from the equation and let machines do our exploring on one-way voyages at lower velocities over longer periods of time. We almost certainly would not send humans in the early stages of interstellar exploration; crewless probes would be less expensive, easier to propel, and more expendable. In any case, the assumption of a round-trip makes no sense. Unmanned spacecraft need not return, any more than the machines we send to Mars; they could tell us what they found through electromagnetic signals. We would not send humans to another star system unless we were sure that they could survive there and would not need to come back.'* That would cut the energy requirements in half. Direct Contact