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126 for the journey; the curve flattens considerably below half the speed of light (0.5c). By 1963, D.F. Spencer and L.D. Jaffe of the Jet Propulsion Labora- tory were arguing that interstellar travel was feasible using staged nuclear systems.” Hughes Research Laboratories physicist Robert Forward, one of the leaders in the emerging interstellar lobby, outlined an array of propulsion choices in 1975. One option is to propel the spacecraft with energy beamed from our solar system, eliminating the need to carry an energy source and fuel on board. Forward proposed a national program for interstellar explo- ration beginning with automated probes, leading to the launch of a human mission to Alpha Centauri in 2025." Dyson found no lack of propulsion systems available to any creatures who possess some technical competence and a desire to travel around the galaxy. Outlining several alternatives in a 1979 paper, he concluded that systems that are probably feasible but require very demanding new tech- nology should be capable of velocities of 0.05c, or one-twentieth of the speed of light.’” A mission at that average velocity would take 86 years to reach the distance of the Alpha Centauri system, the nearest stars to our eee One own Sun. The technical literature on this subject has grown rapidly since those early days; a bibliography of publications on starflight expanded to include hundreds of entries. By 1989, when science writer Eugene Mallove and astronomer Gregory Matloff outlined the basic propulsion choices in their Starflight Handbook, even the skeptical mathematician John Casti admit- ted that studies had shown that no new physical principles are involved in allowing a ship to travel at one-tenth the speed of light. However, he cau- tioned that the engineering hurdles are enormous.” Starwisp In 1983, Forward proposed a lightweight, high-speed interstellar fly-by probe pushed by microwaves beamed from satellite solar power stations in our solar system. A delicate wire mesh sail a kilometer across with microcircuits at each intersection, Starwisp could be accelerated to one- fifth the speed of light. As it passed through the Alpha Centauri system 21 years later, this spiderweb craft would receive beamed energy to power its detectors and logic circuits. The sail would become a phased- array microwave antenna, sending high-resolution imagery back to Humankind.” The interstellar idea is taking shape in a great, ongoing conversation. —Paul Gilster, 2004” Direct Contact Human Probes