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127 MacGowan and Ordway, writing in 1966, drew a sharp distinction between manned and unmanned interstellar flight. They thought that the short human life span, limited acceleration tolerance, vast distances, and the cosmic-ray barrier combine to make human interstellar travel unfeasi- ble even in the quite distant future. However, unmanned interstellar probes may prove practicable because of their smaller payloads. Instrumented probes and intelligent automata could be designed with very long, even unlimited life expectancies; such machines could undertake interstellar journeys of at least moderate distances.” We launched our first machines toward the stars during the next decade: Pioneers 10 and 11, and Voyagers I and 2. Those spacecraft now are far beyond the traditional nine planets of our solar system. Although they will cease operating long before they reach the distance of the nearest stars thousands of years from now, they hint at what might be possible. We could take our next steps with missions that reach part of the way into interstellar space. Bernard Oliver, deeply skeptical about interstellar flight, acknowledged that voyages into our outer solar system are within our near-future capability.* The Jet Propulsion Laboratory began studying an interstellar precursor mission using known space technology in 1976. A modified version, which came to be known as the Thousand Astronomical Unit mission, would journey out 25 times the distance of Neptune, completing its voyage in 50 years. That may sound like a long time, but we still are receiving signals from robotic missions that were launched more than 30 years ago. Controllers may be able to detect faint telemetry from Voyager J until 2020, 42 years after it left the Earth.” A 1977 NASA report on SETI grudgingly admitted that unmanned probes to a few nearby stars for the purpose of scientific exploration might be a worthwhile endeavor; in all likelihood, such an attempt will be made at some future date. However, that report ridiculed the idea of sending probes to all Sun-like stars within 1000 light-years because of the astro- nomical cost.” The British Interplanetary Society’s Project Daedalus study, published a year later, concluded that an interstellar probe could be built by a civilization whose technology was only slightly more advanced than ours. A massive spacecraft powered by inertial fusion might reach Bar- nard’s Star (8 light-years away) within 50 years using foreseeable technology.”’ Interest in interstellar probes was revived in the 1990s, when NASA Administrator Daniel Goldin commissioned studies. The space agency already has launched a spacecraft on a voyage to Pluto that may be extended into the Kuiper Belt from 2016 to 2020.** Missions like that one could be interim steps toward the stars. There are obvious advantages to sending robotic scouts. Humans can live in only a tiny percentage of the universe without elaborate physical Human Probes