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.It is the third in a projected five-book series of the parallel-Earth sequence The Long Earth.