The Sentinel

"Before there were men on Earth, that signal-sending pyramid had stood alone on a lifeless moon. What would happen now that its alarm was silenced?"

- Opening line

"The Sentinel" is a short story by British author Arthur C. Clarke, written in 1948 and first published in 1951 as "Sentinel of Eternity", which was used as a starting point for the novel and film 2001: A Space Odyssey.

Plot
The story deals with the discovery of an artifact on Earth's moon left behind eons ago by ancient aliens. The object is made of a polished mineral, is tetrahedral in shape (pyramidal), and is surrounded by a spherical forcefield. The narrator speculates at one point that the mysterious aliens who left this structure on the Moon may have used mechanisms belonging "to a technology that lies beyond our horizons, perhaps to the technology of para-physical forces." The narrator speculates that for millions of years (evidenced by dust buildup around its forcefield) the artifact has been transmitting signals into deep space, but it ceases to transmit when, sometime later, it is destroyed "with the savage might of atomic power". The narrator hypothesizes that this "sentinel" was left on the moon as a "warning beacon" for possible intelligent and spacefaring species that might develop on Earth.

2001: Space Odyssey
The object of the story, The Sentinel, also appears in the 2001: A Space Odyssey and in the film daption as TMA-1. The operation of the sentinel is activated when sunlight touches it for the first time after it is dug up.

Read online

 * Sentinel of Eternity, 1951