In their commentary “Let’s send the DOE to Alpha Centauri” (Physics Today, September 2013, page 8), Edwin Kite and Andrew Howard propose that NASA and the US Department of Energy work together to develop an interstellar rocket that would explore the possibility of life on “an ocean-bearing planet orbiting a nearby star.” They consider, among other things, a controlled fusion propulsion system to achieve large terminal rocket velocities.Other scientists and engineers who have previously considered the nature of the target planet and a propulsion system to get there reached conclusions different from those of Kite and Howard. In addition, the authors omit some critical steps that would surely precede construction of any mission rocket.A NASA-sponsored workshop, “Robotic Interstellar Exploration in the 21st Century,” was held at Caltech in 1998 to explore the possibility of a rocket voyage to a nearby star. I was one of dozens of scientists and engineers who attended. During a brainstorming session, we unanimously agreed that the only way the citizens of planet Earth might be willing to fund such a voyage would be if the target was a living planet—that is, one on which the existence of life was deemed likely. That requirement goes well beyond finding an ocean-bearing planet.Identification of a nearby living planet could be achieved by NASA’s proposed space-based terrestrial planet finder (TPF) or by a similar proposed European satellite system. The idea would be to find a rocky planet with an atmosphere that, like Earth, contains water, oxygen, ozone, and methane or some other mixture of gases that differs significantly from the carbon dioxide–dominated atmosphere of nonliving planets, such as Venus and Mars, and that could not plausibly be a consequence of nonbiological processes.Discovery by TPF of an exoplanet with an atmosphere indicative of life could then motivate the development of a larger, more expensive space satellite. That second-generation TPF could, for example, spatially resolve the living world into oceans and continents and track seasonal changes. Such a satellite would not be cheap, but it would be far less costly than an interstellar spaceship able to travel at a speed a few tenths of a percent that of light. It is inconceivable that construction of such a spaceship would begin before a TPF had laid the groundwork.11. B. Zuckerman, Mercury 31(5), 14 (2002), http://www.astrosociety.org/pubs/mercury/31_05/zuckerman.html.Many interstellar propulsion systems have been considered.22. F. Dyson, in Extraterrestrials: Where Are They? 2nd ed., B. Zuckerman, M. H. Hart, eds., Cambridge U. Press, New York (1995), p. 45. 978-0521448031,33. I. A. Crawford, in ref. 2, p. 50. Controlled nuclear fusion, if realizable at all, may require an enormous superstructure ill-suited for acceleration to speeds greater than 10−3c. If fusion power is to be employed, the Orion nuclear-bomb rocket proposed half a century ago by Freeman Dyson (Physics Today, October 1968, page 41) and others seems a much more plausible choice.Other, very different propulsion systems have been suggested—for example, Clifford Singer’s pellet-stream model44. C. Singer, in ref. 2, p. 70. in which an electromagnetic mass driver located in the solar system launches a stream of small pellets that can accelerate a spaceship to high velocities. That system vitiates the requirement that the spaceship carry a massive reactor and massive quantities of fuel.As much as many people would love to see the construction of a TPF, discovery of a nearby living world, and construction of a spaceship able to get there, our technological society is now going in just the opposite direction. The TPF project is on hold. What is more, our vastly overpopulous, overconsuming species is rapidly driving the biosphere into realms it has not experienced for many millions of years. The time scale for those changes—catastrophic for countless humans and other species—will surely be much shorter than that required for construction and launch of a spaceship to a living world.REFERENCESSection:ChooseTop of pageREFERENCES <<1. B. Zuckerman, Mercury 31(5), 14 (2002), http://www.astrosociety.org/pubs/mercury/31_05/zuckerman.html. Google Scholar2. F. Dyson, in Extraterrestrials: Where Are They? 2nd ed., B. Zuckerman, M. H. Hart, eds., Cambridge U. Press, New York (1995), p. 45. 978-0521448031, Google ScholarCrossref3. I. A. Crawford, in ref. 2, p. 50. Google Scholar4. C. Singer, in ref. 2, p. 70. Google Scholar© 2014 American Institute of Physics.
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