Possible Clues:

• Many a "Star Trek" stopover
• What a probe may observe
• Pluto, before a 2006 downgrade
• Krypton or Vulcan, in sci-fi
• Alien after map to find Earth, perhaps?
• Saturn or Mars, eg
• Craft needs a little time to become orbiter
• 15-Across, once
• Mercury, perhaps — first of elements used in factory
• Place a trap in the earth?