Tuesday, January 17, 2012
SETI and space probes
The Search for Extraterrestrial Intelligence (SETI) seems to be largely concerned these days with attempting to detect radio signals from distant star systems. See, for example, the web site of the SETI Institute, which says of possible civilisations elsewhere in the galaxy, that
. . such a civilization could be detected across interstellar distances, and may actually offer the best opportunity for discovering extraterrestrial life in the near future.
However, it has long been realised that this would be very unlikely to yield results. In a book by I.S. Shklovskii and Carl Sagan (Intelligent Life in the Universe, Dell Publishing Co., New York, 1968) chapter 31 is devoted to consideration of the possibility of interstellar contact by alien space probes, based on the ideas developed by Ronald Bracewell. The authors were not optimistic about receiving messages from other planets. They commented:
The detection of artificial signals, even in the simplest situations, is a difficult and intricate task at our present level of advance, provided that we are listening to a vastly more advanced civilization. But it would become incommensurately more difficult if, over many centuries or millennia, we must direct beams of electromagnetic radiation with great precision at tens of thousands of stars, while patiently waiting, perhaps in vain, for a reply.
Those who consider interstellar space probes to be impractical should take note of the Voyager space probes, launched in 1977. Their progress can be followed on NASA's Voyager - The Interstellar Mission web site.
Apparently, Bracewell believed that an alien probe arriving in the solar system would listen for artificial radio signals. If it detected any it would record them and play them back to their source. Then it would attempt to establish two-way radio contact.
However, anyone who has enjoyed the interstellar adventures of the characters in the Star Trek series will know that this would be a violation of the Prime Directive, which forbids outside interference in the development of planetary civilisations. Surely any open intervention in human affairs, even if entirely benign, would effect profound changes in the development of our society.
Thus I speculate that the two main possibilities regarding space probes in our solar system are:
- There are no space probes in the solar system.
- There are alien space probes in the solar system, but they merely gather information for those who sent them and avoid contact.