Sarakka crater is a real location, located near Callisto's equator. It is 56 kilometers in diameter, and located almost due south of the Valhalla impact basin, the largest crater (ring feature) on the moon. The region surrounding the Valhalla impact basin seems to have many missing craters and strange unexplained smooth regions; this is why this location was chosen for the story. It is also on the Jupiter-side of the moon, so the scenery isn't too bad, either.