Lucky Starr and the Moons of Jupiter (Lucky Starr #5) (Signet SF, T3975)

The sixth novel in the extra-terrestrial odyssey of David Starr. Sabotage of a revolutionary advance in space travel means every life is at hazard. Only a handful of highly trusted men were supposed to know the secret. But, someone else knew! For David Starr it was probably the most important and dangerous mission he had ever faced. He had to find the mysterious saboteur before the damage became irreparable. But the unknown enemy didn't seem human. Not even remotely alien, either! This is the final volume in the star-blasting series of David 'Lucky' Starr, Space Ranger.