NBC chose this episode
to air first because they thought it was an 'appropriate
science-fiction' episode with an 'appropriate science-fiction
monster' scaring folks.
Michael Zazlow, who
plays Crewman Darnell, would later go on to play Eddy the 21st
Century bartender in
"Star Trek: First
Contact".
Mr. Spock claims that Vulcan has no moons, but later
in "Star Trek:
The Motion Picture", when Mr. Spock undergoes the Kohlinar
ritual, there are moons visible in the sky. (This has been fixed for
the Director's Edition.)