The Long Mars is a science fiction novel by Terry Pratchett and Stephen Baxter. In it, several characters from the previous books in this trilogy travel to Mars and find that it too has co-existing alternate worlds accessible to their technology. While many are lifeless and possess atmospheres as thin as those of Mars within our universe, others possess oceans, life forms and intelligent life. None of them correspond to other fictional alternate Marses, such as those of The War of the Worlds or The Martian Chronicles.
It is the third in a projected five-book series of the award-winningparallel-Earth sequence The Long Earth. It was originally entitled The Long Childhood, but changed to The Long Mars, and was published on 19 June 2014. The paperback edition is due to be published by Harper on 7 August 2014.