 What is the Jupiter weapon?
50 THE JUPITER WEAPON By CHARLES L. FONTENAY It was a squalid saloon in the rougher section of Jupiter's View, the terrestrial dome-colony on Ganymede. Perhaps she could call one of Motwick's friends; she had no one on Ganymede she could call a real friend herself. The gravity of Ganymede was hardly more than that of Earth's moon, but the way the man picked up the limp Motwick with one hand and tossed him over a shoulder was startling: as though he lifted a feather pillow. The transparent dome of Jupiter's View was faintly visible in the reflected night lights of the colonial city, but the lights were overwhelmed by the giant, vari-colored disc of Jupiter itself, riding high in the sky. “No one has ever landed on Jupiter. But his ship was drawn into Jupiter and lost.” “It was drawn into Jupiter, but he landed it successfully,” said Quest. “He and my mother lived on Jupiter until the oxygen equipment wore out at last. I was born and brought up there, and I was finally able to build a small rocket with a powerful enough drive to clear the planet.” “If Dr. Mansard succeeded in landing on Jupiter, why didn't anyone ever hear from him again?” she demanded. For some reason she decided against telling him that the assignment on which she had come to the Jupiter system was to gather his own father's notebooks and take them back to Earth.
 The Jupiter weapon is Quest himself.
 Asrange is the Jupiter weapon.  The surgiscope is the Jupiter weapon.  No one knows what the Jupiter weapon is, but the plans are in Dr. Mansard's notes.  The Jupiter weapon is Quest himself.