Roy parks the Cepheus in orbit, and then uses a pod to travel to Lima. When he's preparing to return to Cepheus, he clearly expects it to be in the same relative location, meaning its orbit must be parallel to and the same speed as Lima's. He leaves Lima at the speed of the tip of the rotating antenna (ridiculously slow), but arrives at Cepheus as if falling to it from a great height in Earth's gravity. So, Cepheus' relative speed to Lima would make its location entirely unpredictable without a computer, and thousands of miles away.