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The fifth novel in the Lucky Starr series, six juvenile science fiction novels by Isaac Asimov that originally appeared under the pseudonym Paul French.

For six years, Jupiter IX has been the site of a secret project to develop an antigravity, or Agrav, space drive. However, the Council of Science learns that information from the project is being leaked to the Sirians. A month after returning from Mercury, David "Lucky" Starr and John Bigman Jones are sent to Jupiter IX to investigate. Starr brings along a V-frog from Venus to aid in his investigation.