Lucky Starr and The Rings of Saturn

the long version

Earth officials were hard on the heels of the mysterious Sirian spy, Agent X, when he blasted off in a stolen spaceship. But before they could catch him, the master spy jettisoned the capsule that held his report into the icy rings of Saturn. In a flash, Lucky Starr and Bigman Jones found themselves in a race with the Sirian war fleet to recover it. When the Sirians couldn't find the capsule, they kidnapped Lucky and Bigman, bringing them to their secret military base on Titan. There the arrogant Sirian commander offered Lucky a terrible choice: turn traitor to Earth—or Bigman would die! It was not an idle threat.