A very average aliens' story. The story is set in sometime in 1990s (now we know, of course, that he was too optimistic).
An MP3 version of this story is available online (link via SF Signal & Free SF Reader).
Story summary (spoiler).
A local expedition off one of the permanent human bases on moon stumbles upon a machine on earth side of lunar surface that is found to be a beacon planted by ETs. Measurement of the depth of cosmic dust around the beacon dates it to a time before there was any kind of life on earth.
Not only has the machine stayed there all these eons, it appears to be still in perfectly working condition. And it is shielded in ways that frustrates examination by curious humans. So the foolish human adventurers blast through it with some kind of atomic fuse - only to discover the interiors totally beyond their grasp.
Story ends with this speculation: The ETs must have planted many such beacons across the galaxy near worlds that promise intelligent life in future. And ETs probably assumed that the curious explorers will do something foolish - switching off of beacon is probably a signal to them that it's time they come & have a look. So the smarter of the humans await the arrival in fear - for the ETs are an old race, & old are normally jealous of the young!
Note there is another Clarke story where ETs plant a beacon on moon to monitor evolution of intelligence on earth: the second of the four stories in "2001 A Space Odyssey".
Note (12 April 2007): According to this Wikipedia article, this story was 'written in 1948 for a BBC competition but first published in 1951 under the title "Sentinel of Eternity"'.
- "The Collected Stories of Arthur C Clarke"
- "Expedition to Earth"
- "Of Time and Stars"
- "Across the Sea of Stars"
- "The Sentinel (collection)"
- "More Than One Universe"
- "The Lost Worlds of 2001"
- "What Goes Up": An industrial accident on earth creates a field very similar to the shield protecting the alien beacon on moon in Sentinel.
The Sentinel, short story, review
Author: Arthur C Clarke
First published: 1951
Listed in Contento's Top Ten Most Reprinted Stories.