They Also Serve by Donald E. Westlake, Science Fiction, Adventure, Space Opera (Paperback) (ISBN-13: 9781606645918)

He led the way into his quarters, motioned Ebor to a perch, and rang for his orderly. "It was just a little remote-controlled apparatus, of course," he said. "The fledgling attempt, you know. But it circled this Moon here, busily taking pictures, and went right back to the planet again, giving us all a terrible fright. There hadn't been the slightest indication they were planning anything that spectacular." "None?" asked Ebor. "Not a hint?"