Related Clues:

• Place a trap in the earth?
• Mercury, perhaps — first of elements used in factory
• Engineer works outside with mercury
• 15-Across, once
• Pantera cover "___ Caravan"
• Craft needs a little time to become orbiter
• Alien after map to find Earth, perhaps?
• Earth perhaps flat — first of theories
• Way to load part-emptied revolver?
• Krypton or Vulcan, in sci-fi
• Pluto, before a 2006 downgrade
• Many a "Star Trek" stopover
• Mercury, for instance, showing temperature with level
• Earth, say, where flower perhaps stores energy
• Theme of this puzzle, repeated in some clues