Robert and Summer fell asleep in the back of Rick's Space Cruiser the night before Rick and Morty went out to Planet MWA739. Summer woke up while they were still on the way so she woke up Robert to complain that they fell asleep. While confused about where they were, Rick ejected Robert out into space in while in suspended animation to keep him alive.
Robert was picked up by Rick and Morty on their way back to Earth. He was still dazed and didn't even react when Summer and Jerry suddenly appeared out of thin air in the backseat with him. He did not speak a word on the way home and his relationship with Summer presumably ended.