CHAPTER 3
SPACE EXPLORATION: THE INTERSTELLAR GOAL AND TITAN DEMONSTRATION
3.1 Introduction
The small Pioneer 10 spacecraft, launched from Earth on March 2, 1972, represents mankind's first physical extension into interstellar space. Having traversed the Asteroid Belt and given scientists their first good look at Jupiter and its satellites, the vehicle now rushes toward the edge of the Solar System at a speed of about 3 AU/yr. The exact moment of penetration into extrasolar space is unpredictable because the boundary of our System is not precisely known, and because the spacecraft's ability to transmit useful data will likely degrade by the time of passage (circa 1986) that it will be unable to report transit of the heliosphere when this occurs.
Several other unmanned vehicles will also eventually exit the Solar System. However, as Pioneer 10 none of these were designed specifically as interstellar probes, and comparatively little work has yet been accomplished with the aim of developing such craft. Still less effort has been directed toward the ultimate goal of manned interstellar exploration. 3.1.1 Automated Interstellar Space Exploration
The most extensive study of interstellar space exploration to date has been Project Daedalus, an analysis conducted by a team of 13 people working in their spare time under the auspices of the British Interplanetary Society from 1973 to 1978 (Martin, 1978). The focus was a feasibility study of a simple interstellar mission using only present technology and reasonable extrapolation of foreseeable near-future capabilities.
The proposed Daedalus starship structure, communications systems, and much of the payload were designed entirely within today's capabilities. Other components, including the machine intelligence controller and adaptive repair systems, require a technology which Project members expected would become available within the next several decades. For example, the propulsion system was designed as a nuclear-powered, pulse-fusion rocket engine burning an exotic deuterium/helium-3 fuel mixture, able to propel the vessel to velocities in excess of 12% of the speed of light. Planetary exploration and nonterrestrial materials utilization were viewed as prerequisites to the Daedalus mission, to acquire useful experience and because the best source of helium-3 propellant is the atmosphere of the gas giant Jupiter (to be mined using floating balloon "aerostat" extraction facilities). This ambitious interstellar flyby was thought possible by the end of the next century, when a solar-system-wide human culture might be wealthy enough to afford such an undertaking. The target selected for the first flight was Barnard's star, a red dwarf (M5) sun 5.9 light years away in the constellation Ophiuchus.
The central conclusions of the Project Daedalus study may be summarized roughly as follows: (1) Exploration missions to other stars are technologically feasible; (2) a great deal could be learned about the origin, extent, and physics of the Galaxy, as well as the formation and evolution of stellar and planetary systems, by missions of this kind; (3) the necessary prerequisite achievements in interplanetary exploration and the accomplishments of the first interstellar missions would contribute significantly to the search for extraterrestrial intelligence (SETI); (4) a funding commitment over 75-80 years is required, including 20 years for vehicle design, manufacture and checkout, 30 years of flight time, and 6-9 years for transmitting useful information back to Earth; and (5) the prospects for manned interstellar flight are not very promising using current or immediately foreseeable human technology.
A more recent study (Cassenti, 1980) concludes on a more optimistic note: "We are like 19th Century individuals trying to imagine how to get to the Moon. Travel to the stars is extremely difficult and definitely expensive, but we did get to the Moon and we can get to the stars." Cassenti supports the Project Daedalus judgment that only vehicles capable of achieving more than 10% of the speed of light should be examined and that the preferred propulsion system now is "a version of the nuclear pulse rocket for unmanned exploration and combinations of the nuclear pulse rocket and the laser-powered ramjet for propelling manned interstellar vehicles."
Even more imaginative and longer-range interstellar missions of galactic exploration have been considered by Robert A. Freitas Jr., a participant in the present study (Freitas, 1980a, 1980b; Valdes and Freitas, 1980). He concludes that self-reproducing interstellar probes are the preferred method of exploration, even given assumptions of a generation time of about 1000 years and a 10-fold improvement in current human space manufacturing technology. He envisions "active programs lasting about 10,
