A World of Difference is a 1990 science fiction novel by American writer Harry Turtledove. The book begins with a space voyage that departed Earth in an alternate 1989. In the universe of the book, the fourth planet from the Sun, in the orbit occupied by Mars in our reality, is named Minerva, which is similar in size and makeup to Earth. The book contrasts Earth's culture during the ongoing Cold War with Minerva's feudal culture which resembles the European culture of the Late Middle Ages. Mercantilism is just emerging in Minerva during the events of the book. Source: Wikipedia (en)