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A Bracewell probe is a hypothetical concept for an autonomous interstellar space probe dispatched for the express purpose of communication with one or more alien civilizations. It was proposed by Ronald N. Bracewell in a 1960 paper, as an alternative to interstellar radio communication between widely separated civilizations. (Bracewell, 1960)



A Bracewell probe would be constructed as an autonomous robotic interstellar space probe with a high level of artificial intelligence, and all relevant information that its home civilization might wish to communicate to another culture. It would seek out technological civilizations — or alternatively monitor worlds where there is a likelihood of technological civilizations arising — and communicate over "short" distances (compared to the interstellar distances between inhabited worlds) once it discovered a civilization that meets its contact criteria. It would make its presence known, carry out a dialogue with the contacted culture, and presumably communicate the results of its encounter to its place of origin. In essence, such probes would act as an autonomous local representative of their home civilization and would act as the point of contact between the cultures.

In the movie 2001: A Space Odyssey and the short story The Sentinel by Arthur C. Clarke, the Monolith appears to be a Bracewell probe. The authors placed the probe on the moon to ensure that only a civilization capable of spaceflight would be able to discover it.

Since a Bracewell probe can communicate much faster, over shorter distances, and over large spans of time, it can communicate with alien cultures more efficiently than radio message exchange might. The disadvantage to this approach is that such probes cannot communicate anything not in their data storage, nor can their contact criteria or policies for communication be quickly updated by their "base of operations".

While a Bracewell probe need not be a Von Neumann probe as well, the two concepts are compatible, and a self-replicating device as proposed by Von Neumann would greatly speed up a Bracewell probe's search for alien civilizations.

It is also possible that such a probe (or system of probes if launched as a Von Neumann probe) may outlive the civilization which created and launched it.

There have been some efforts under the SETA and SETV projects to detect evidence for the visitation of our solar system by such probes, and to signal or activate such a probe that may be lying dormant in local space. Variations in the echo delay times of radio transmissions, known as long delayed echoes, or LDEs, have also been interpreted as evidence for such probes.

1991 VG, a peculiar near-earth object, is a hypothetical candidate for a Bracewell probe from an alien civilization.[1]

Fictional examples

  • In Arthur C. Clarke's novel The Fountains of Paradise the extraterrestrial Starglider probe is an example of a Bracewell probe.
  • Alien Planet is a 94-minute special which aired on the Discovery Channel in 2005 about two internationally-built robot probes, and their mothership, searching for alien life on the fictional planet Darwin IV.
  • In the Star Trek: The Next Generation episode The Inner Light, a Bracewell probe transmits details of an extinct civilization to Captain Picard.
  • An alien probe contacts the space station Babylon 5 in the season 3 episode A Day in the Strife. The probe is a Bracewell probe, asking a series of questions and offering new technologies, medicine and science in return for answers to said questions. However, if the probe receives correct answers -thus proving that whatever civilization the probe has come into contact with is advanced - the probe detonates with a yield of 500 megatons, removing said civilization's homeworld or facility. This makes it more of a berserker probe.
  • The Slylandro, an alien race in the game Star Control II, are offered a Bracewell probe with which to contact other civilizations, since they cannot leave their gas giant home planet.


  1. ^ Steel, D. (1995). "SETA and 1991 VG". The Observatory 115: 78–83. Bibcode1995Obs...115...78S.  
  • Bracewell, R. N. (1960). "Communications from Superior Galactic Communities". Nature 186: 670–671. doi:10.1038/186670a0.   Reprinted in A. G. Cameron, ed (1963). Interstellar Communication. New York: W. A. Benjamin. pp. 243–248.  
  • Bracewell, R. N. (1973). "The Opening Message from an Extraterrestrial Probe". Astronautics & Aeronautics 11: 58–60.  
  • McCollum, M. (1983). LifeProbe. New York: Ballantine Books.  
  • Freitas Jr., R.A.; Valdes, F. (1985). "The Search for Extraterrestrial Artifacts". Acta Astronautica 12 (12): 1027–1034. doi:10.1016/0094-5765(85)90031-1.  

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