Agrav. It was the century's most important advance in space travel...and an experiment so revolutionary that only the men who huddled beneath the surface of Jupiter Nine were permitted to know its full meaning.
Yet someone else did know--knew everything, saw everything, head everything--and was diabolically sabotaging the top-secret mission.
Who or what the enemy was, Lucky Starr didn't know. but one thing was certain. The deadly force was not human...not even remotely human!