The problem of probes from an outside civilization - Radio echoes and the Bracewell hypothesis

The problem of the presence of an extraterrestrial space probe in the solar system (the Bracewell hypothesis) is discussed with particular reference to the nature of long delayed radio echoes. Advantages of the use of an interstellar probe with a vast memory and a high level of artificial intelligence over radio signals in the search for and establishment of contact with extraterrestrial civilizations are pointed out, and the retranslation and delay of radio signals is considered as the most likely means for an extraterrestrial probe to attract attention to itself. Experience in the 1920s with very long delays in radio signal propagation during the opening of the shortwave frequency band is then discussed as possible evidence for the presence of such an extraterrestrial space probe at a libration point in the lunar orbit.