List Of Figures

1. The principal stars in Cassiopeia

2. Temperature preferences of the world's population 7

3. Approximate human time-temperature tolerances, assuming optimum clothing 9

4. Human time tolerance: positive^ 11

5. Time required to complete a given act relative to the time required at 1 g 13

6. Breathable mixtures of oxygen and nitrogen as a function of barometric pressure 14

7. Mass-volume relationships of stars and planets 28

8. Density-radius relationship of terrestrial planets 30

9. Characteristics of terrestrial planets: mass, gravity, and escape velocity versus radius 31

10. Number of objects in the solar system in the mass interval

11. Mass-density relationship of the solar-system planets 34

12. Planetary atmospheres 37

13. Profiles of rotating bodies 42

14. Oblateness of rotating planets versus angular-velocity-density parameter 44

15. Relative geopotential (resultant of gravitational and centrifugal forces) at the surfaces of rotating bodies at selected geocentric latitudes 45

16. Rotational energy per unit mass versus mass (freely rotating planets of the solar system) 46

17. The changing composition of an accreting body after it becomes large enough to capture light gases (illustrative) 48

18. The spacing of planets in the solar system 51

19. Atmospheric pressure at planetary surface as a function of planetary mass 56

20. Atmospheric pressure as a function of altitude for large and small habitable planets compared with Earth (illustrative) 57

21. The extreme size range of habitable planets 59

22. Rotation of the Earth produced by impacting particles 61

23. Comparison of calculated mean temperatures and observed mean temperatures for the Earth (equatorial inclination, 23.5°) 65

24. Percentage of surface area that is habitable as a function of inclination of equator and intensity of radiation from primary 66

25. Some characteristics of the smaller main-sequence stars 69

26. The boundaries of ecospheres 72

27. Effect of tidal forces on a planet's habitability 74

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28. Hypothetical satellites of the Earth 75

29. Regions within which stable, direct, near-circular planetary orbits can exist, // = m2l(m1 -I- m2) = 0.1 77

30. Contours of constant total illumination E in the vicinity of two radiating sources, LJL^ — 0.04 and //, = 0.3 78

31. Regions in the vicinity of two radiating sources where planets can exist on stable, near-circular orbits and where total illumination varies less than 10 per cent 80

32. Absolute visual magnitude of main-sequence stars as a function of stellar mass 83

33. Mass as a function of spectral class (main-sequence stars) 83

34. Adopted curve for estimating the probability that the inclination of a planet's equator is less than a given value 88

35. Relative width of ecosphere as a function of inclination of planet's equator 89

36. The probability that at least one planet exists in the distance interval Z)? to D0 (based on solar-system data) 91

37. The probability that a planet chosen at random lies within the mass range from M1 to 2.35 times the mass of the Earth 93

38. Adopted curve for estimating the probability that the eccentricity of a planet's orbit is less than a given value 94

39. The probability of at least one habitable planet in orbit around an isolated star as a function of stellar mass 103

40. Positions of the nearest stars that have significant probabilities of possessing at least one habitable planet; coordinates of right ascension and declination 111

41. The orbit of component B of the system Alpha Centauri 112

42. Angular separation or size of objects as a function of distance from observer 118

43. Apparent magnitude of an object as a function of distance from observer 119

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