“I thank you because you were the first one, and practically the only one, to have complete faith in my assertions.” – Galileo In this public letter, Kepler expressed support for Galileo’s telescopic discoveries. He also considered whether the Moon and Jupiter are inhabited, and he supposed that they are. He speculated that explorers from the Earth might be able to visit them. This is the earliest work by a modern astronomer to entertain the possibility of space travel. These questions are conspicuously absent in Galileo’s works. This book therefore explains why the telescope used to search for terrestrial planets is named after Kepler.