Τα φεγγάρια του Δία – The Moons of Jupiter

For six years, Jupiter IX has been the site of a secret project to develop an antigravity, or Agrav, space drive; but the Council of Science learns that information from the project is released to the enemy Sirians. A month after returning from Mercury, protagonists David “Lucky” Starr and John Bigman Jones are sent to Jupiter IX to investigate, bringing a V-frog to aid the investigation.