Some early "portal" appearances in science fiction include A. E. van Vogt's novella "Secret Unattainable" (July 1942, "Astounding"), a radio episode of "Space Patrol" that aired October 25, 1952 (in which it was called a "cycloplex" or a "hole in space"), and Robert A. Heinlein's "Tunnel in the Sky" (1955) and its "Ramsbotham jump". In "", Arthur C. Clarke uses the term "Star Gate" for the large monolith "sentinel" TMA-2, which is a classic stargate portal to another part of the universe.