Charles Wohlforth and Amanda R. Hendrix. Beyond Earth: Our Path to a New Home in the Planets. Pantheon Books, New York, 2016

Most scientific speculation about the possibility of establishing settlements on other planets usually involves the consideration of Mars as the most likely, but the authors have examined these ideas and have concluded that the proposed methods of making Mars habitable are not practicable and remark: "These are fun ideas to think about, but the time and cost are too colossal to take seriously."

Chapter 2 is particularly interesting, as it deals with some of NASA's problems with the way in which it was organised, and its tendency to concentrate on the inner solar system, rather than any possibilities offered by the outer planets, in particular the result of taking a short-term approach, where colonies would have to be reliant on constant assistance and supplies from Earth.

The most spectacular and tragic result of bad management in NASA was the crash of the Space Shuttle Challenger on 28 January 1986, killing its seven occupants. The Shuttle was launched despite the fact that five engineers had warned their superiors that the weather at the launch site was too cold and that this would probably cause the rubber O-rings on the boosters to fail, which is what happened.

Most of the book is devoted to considering the possibility of establishing a permanent self-maintaining colony on Titan. The authors remark, optimistically perhaps: "There's plenty we don't currently know about Titan. But we do know that, if you could get there, we could live there."

Although it at first seems an unpromising place to establish a colony, many scientists have imagined what it would be like to live there. It would be possible to survive without space suits and walk around in warm clothing and oxygen masks, even though the temperature is about -180 C. The atmospheric pressure is about 50% greater than on Earth, and the air is four times denser, and gravity is only 14% of the gravity on Earth.This would make it possible to fly by flapping wings attached to one's arms or by using an electrically powered propeller.

There are plenty of interesting ideas on the possible organisation of colonies on Titan,which are developed by imagining what might happen in the near or distant future. Many readers will probably try to imagine what it would be like to live in colonies on Titan but they must be aware that it will not be possible for them to return to Earth, having become accustomed to low gravity.

Not content with devising ingenious methods for colonising Titan, the authors finally turn their attention to what perhaps might be the possibility of going further. "Beyond our solar system's outermost planets, the next stop is a long way off ." Indeed it is. This would be accomplished by a version of the space warp, familiar to readers of science fiction. Few scientists will consider it possible, but at least some readers will find it interesting, and many readers will at least be entertained by the more credible ideas discussed. The authors have succeeded in presenting their arguments without lapsing into crankiness. Recommended. -- John Harney

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