A command antenna was a piece of advanced communications technology aboard NASA's Mariner 2 space probe when it was launched in the mid-20th century.
A schematic of the Mariner 2 spacecraft depicting the location of the command antenna was stored in the USS Enterprise's library computer database. This database was accessed by the Talosians in 2254. (TOS: "The Cage")
The schematic of the Mariner 2 spacecraft was from the archives of the National Aeronautics and Space Administration - Jet Propulsion Laboratory.