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The chanee that Martians, under such widely divergent condi- tions, would have a civilization resembling our ow is ex= tremely remote. It is particularly unlikely that their Civilization would be within a half cmtury of our ow state of advancement. Yet in the last 50 years we have just started to use aireraft and in the next 50 years we will almost certainly start exploring space. thus it appears that space travel from another point within the Solar system is possible but very unlikely. Odds are at least a thousand-toeone against it This: leaves the totality of plan iets of other stars in the Galaxy as possible sources. J/any mocern astronomers believe that planets are fairly normal and logical affairs in the life history of a star (rather than cataclysmic oddities) so that many planets can be expected to exist in aioe 2 To narrow the field a little, some loose specifications can be written for the star about which the home base planet would revolve. let us say that the star should bear a family re~ semblance to the Sun, which is a member of the so-called "mnain- sequence" of stars, 1.e., we eliminate white dwarfs, red giants and supergiants. For a description of these types, see refer~ ence 2, chapter 5. There is no specific reason for making this 4iscussion: we pre still consider~= + assumption except to simplify ¢ ing the majority of stars. Next, true variable stars can be eliminated, since conditions on a planet attached to a variable star cue ratte too wildly to permit life. The number of stars deleted here is negligibly suall. Reference 3, pages 76 and &5 indicate that the most common types are too bright to be in 1 nearby space unnoticed. Lastly, we shall omit binary or multiple stars, since the condi- tions for stable planet orbits are obscure in such cases. About a third of the stars are eliminated by this restriction. As our best known sample of space we can take a volume with the Sun at the center anda radius of 16 light years. A compilation of the |? know stars, including the Sun, within this volume is given in reference ely pages 52 to 57. *liminating according to the above discussion: Three are white dwarfs, eight binaries account for 16 stars and two trinaries account for 6 more. The remainder, 22 stars, can be considered as eligible for habitable planets. 30 Declassification Authority: NND 57565