In 1975 I found a book with an Isaac Asimov story about a robot spacecraft sent from earth to fight "space things". On the return trip the spacecraft pass through a time portal that send it to earth 300 years(?) before it left. The spacecraft has a thinking computer that runs things and a calculating computer that navigates etc. It returns to earth sometime in the 1960's as it mentions picking up Walter Cronkite on the new. It is a very entertaining story as the robot tries to understand what has happened to the earth it left. Would any one know the name of this story? and where I can find a copy?