Beyond the task of developing a realistic and workable propulsion system that would make interstellar travel possible and practical, there is the prior challenge of identifying an extrasolar planet that would be suitable for long-term human habitation. Any planet that is a candidate for human colonization has to satisfy a surprisingly large number of requirements stemming from the fact that human biology has evolved on Earth and nowhere else, and is therefore fit to survive only in an environment that is substantially similar to our own. As Daniel Deudney has said in his book Dark Skies, “Humans are sprung from the Earth, have never lived anywhere but on Earth, and the features of this planet have shaped every aspect of human life .… Life is not on Earth, it is of Earth.” And for that reason, a planet fit for human colonization elsewhere must be earthlike in several important respects.