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'.

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! 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. Story ends with this speculation: The ETs must have planted many such beacons across the galaxy near worlds that promise intelligent life in future. So the foolish human adventurers blast through it with some kind of atomic fuse - only to discover the interiors totally beyond their grasp. And it is shielded in ways that frustrates examination by curious humans. Not only has the machine stayed there all these eons, it appears to be still in perfectly working condition. Measurement of the depth of cosmic dust around the beacon dates it to a time before there was any kind of life on earth.

The story is set in sometime in 1990s (now we know, of course, that he was too optimistic).Īn MP3 version of this story is available online (link via SF Signal & Free SF Reader).Ī 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.

The sentinel by arthur c clarke