1. C. S. Lewis, Perelandra: Voyage to Venus. HarperCollins Publisher (1943). Perelandra is the second book in the science-fiction trilogy written by Lewis. The first book, Out of the Silent Planet, concerns a voyage to Mars (or Malacandra, as Lewis called it), while the final book, That Hideous Strength, is set upon Earth.
2. In his Science review paper of March 1961 (volume 133, 849-858) Carl Sagan argued with some passion that the term Venusian was entirely incorrect. ''We do not say Sunian or Moonian, or Earthian,'' he noted, and indeed, this is so. Sagan suggested, and used throughout his review, the adjective Cytherian, since this was the Ionian island upon which the mighty Aphrodite is said to have emerged. In spite of Sagan's well-reasoned argument, the term Cytherian has not caught on, rightly or wrongly, in the general literature, so we have kept to the word Venusian.
3. Tellurium is a silver-white metal commonly used in semiconductor devices and has a melting point of 722.66 K. It is often written that the surface temperature of Venus is greater than that of the melting point of lead. This statement, while true, is not particularly useful, since lead actually melts at a temperature of 600.61 K, a temperature some 136c cooler than the surface temperature of the planet.
4. In a highly influential paper published in the journal Icarus [Runaway and moist greenhouse atmospheres and the evolution of Earth and Venus. 74, 472-494] in 1988 James Kasting (Pennsylvania State University) developed one of the first detailed models describing the characteristics of the moist runaway greenhouse effect. He suggested that the initial oceans on Venus might have survived for a minimum time [my italics] of about 600 million years. Most people who have read Kasting's paper since, however, have forgotten that he derived a minimum time for the oceans to evaporate. The time estimate is a minimum since Kasting intentionally left out from his model the cooling effects of clouds. If one allows for clouds to reflect more of the Sun's radiant energy back into space, then the oceans of Venus might reasonably survive for several billion years. Likewise, the often-projected demise of the Earth's oceans in about 1 billion years from the present is a minimum time, and it might not occur for perhaps 2 or 3 billion years even if nothing is done to cool the Earth down via the use of sunshades and/or the many other geoengi-neering options described in the Prolog and Chapter 4.
5. When it comes to terraforming, esthetics should not dominate our thinking. At the end of the day, humanity will have to get down and dirty. You cannot change a planetary atmosphere without making a great deal of mess first. The only proviso is, of course, that the initially induced chaotic mess will ultimately end in a beautiful result.
6. First described by the Italian-French mathematician Joseph-Louis Lagrange, the five Lagrange points identify the locations where a small mass moving under gravity alone can remain stationary with respect to two larger objects. They represent special-case solutions to the three-body gravitational problem.
7. Roger Angel, Feasibility of cooling the Earth with a cloud of small spacecraft near the inner Lagrange point (L1). Proceedings of the National Academy of Science 103(46): 17184-17189 (2006).
8. Key to determining the effects of an impact is the angular momentum transfer. The angular momentum of the impactor is given by the expression MIVId, where MI is its mass, VI is its impact velocity, and d is the offset distance of the impact location relative to the center of the target. The spin angular momentum of the planet (treated as a constant density sphere) will be (2/5) Mp R2p o, where MP is the mass of the planet, RP is the planet's radius, and o = 2p/P is the planet's angular velocity, with P being the planet's spin period. If we assume that there is no appreciable change in the size of the planet after the impact and that the mass of the impactor is much less than that of the planet, then after the impact the planet's new spin rate will be onew = oold + (5/2)[Mj / MP] Vi [d / R2p], assuming that the direction of impact is in the same sense as that in which the planet is rotating. The largest possible offset distance for the impactor is d « RP, and if we take Mj / MP = 10~6 (that is, the impactor is a million times less massive than the target planet), then with an impact velocity of, say, 25 km/s we have in the case of Venus that onew = ®old + 2.05 x 10~9, which translates into an increase of about 1.4 days in the planet's spin period. To produce an appreciable spin-up effect on Venus, therefore, a KBO impactor would need to be about 170 km in diameter. There is clearly a lot of room for maneuvering in such calculations. The impact velocity could certainly be twice as high and the impactor mass ten times larger than assumed in our earlier calculation, and accordingly the spin period might be increased by about 88 days (a 40% reduction of the initial spin period). There is certainly a good supply of appropriately sized KBOs in the outer Solar System so that our descendants may reasonably try to increase the spin rate of Venus to a value of perhaps 50 Earth days (which would require five large KBO-grazing impacts).
9. The idea behind the Dyson sphere is that a sufficiently advanced civilization will have extremely high energy demands, and that one way of acquiring this energy is to tap more of the reserves from its parent star. The ultimate energy trap would, of course, be a sphere placed around the star. Russian radio astronomer Nikolai Kardashev suggested in the mid-1960s that a civilization might be classified according to how much energy it can tap and use. A civilization capable of generating and using the energy equivalent of its parent star was accordingly classified as a Kardashev type ii civilization. A civilization capable of using the energy equivalent to that of its host galaxy is called a Kardashev type iii civilization. We, that is, all humanity, are currently classified as a Kardashev type i civilization, in that we nearly tap all of the energy that the planet (Earth) can provide. Searchers for Dyson spheres, at infrared wavelengths where they will radiate most strongly, have been made, but no good candidates have been found. James Annis (Fermi National Accelerator Laboratory, in Batavia) has also performed a study of several hundred nearby galaxies to see if Kardashev type III civilizations might exist; none has so far been detected.
10. Dyson describes the detailed physics behind his planetary spin motor in his article The Search for Extraterrestrial Technology, which was one of a series of essays honoring Professor Hans Bethe published in Perspectives in Modern Physics, B. E. Marshak and J. W. Blacker (Eds.), Interscience Publishers, New York (1966).
11. Stephen Gillett, Diamond ether, nanotechnology, and Venus. Analog Nov. 1999, pp. 38-46.
12. Stephen Gillett remarked in his article Second planet—second Earth, published in Analog Science-Fiction/Science Fact [104, 64-78 (1984)] that he thought terraforming would be ''a pointless exercise'' unless the end result provided a long-lived, stable, and self-regulating environment. Although the thinking is laudable, it is also probably wishful thinking, and Gillett is destined to be disappointed with the results of future terraforming, although hopefully he would still conclude that, ''It seems to be worth it.'' External control of terraformed environments will always be required, but this, fortunately (yes, fortunately) places great demands on humanity's future development; humankind must either learn how to look after its future worlds using good stewardship or it will perish, and if the latter result occurs, well, perhaps it is good riddance.
13. Alexander Smith, Terraforming Venus by induced turnover. Journal of the British Interplanetary Society 42, 571-576 (1989).
14. Freitas' ideas are described in NASA Conference Publication 2255. These proceedings can found at http://en.wikisource.org/wiki/ Advanced_Automation_for_Space_Missions.
15. Deborah Cadbury's Space Race (Harper Collins, New York, 2006) is an exceptionally well written and very readable account of the struggle and rivalry between the United States and the former Soviet Union to land a man upon the Moon in the 1960 s (although the origins of the rivalry essentially stretch back to the end of World War II). Marina Benjamin's Rocket Dreams (Free Press, New York, 2003) is also a very informative read that looks into how the space age has shaped our everyday lives.
16. Details of the Moon, Mars and Beyond vision of NASA can be found at http://www.nasa.gov/mission_pages/exploration/mmb/index.html.
17. Haym Benaroya and Leonhard Bernold, Engineering of lunar bases. Acta Astronautica 62, 277-299 (2008).
18. The current NASA timeline calls for a manned 30-month roundtrip mission to Mars beginning in 2031. The supporting cargo and habitation systems will be launched ahead of the astronauts during 2028/29, and it is anticipated that the Mars exploration phase will last about 16 months.
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