The Astronaut is a character of Zathura. He arrived at the house after Danny rolled the card "Rescue Stranded Astronaut", which is misread by Danny as "Rest On Standing Astroturf". The Astronaut gave Walter and Danny tips on surviving in the game. Examples are hiding the House from the Zorgons and helping Danny to retrieve the game when the Zorgons stole it. He also helped Walter and Danny become better brothers. In his last appearance, it is shown that he is actually a future version of Walter who decided to wish Danny out of existence when he played Zathura, leading him to travel the galaxy for fifteen years before passing through a Time Sphincter to return to the house and meet his past self.
When the Astronaut wished Danny away, the game wouldn't let him continue moving. So the Astronaut was stuck in space forever. He was glad that Walter didn't make the same mistake. During the second Shooting Star, Walter wished that the Astronaut had his brother back. Danny then appears, causing the truth to be revealed, before the Astronaut merges with Walter.