Iossif Chklovski

I think Shklovskii and I can be described as cautious optimists on this question. If it seemed likely that technical civilizations existed on planets only 10 or 20 light years away, or civilizations greatly in advance of our own, at larger distances, a serious effort to establish contact might be justified. On the other hand, if we can only reasonably expect civilizations at about our level of technical advance thousands of light years away, attempts at communication would not seem profitable, at least at the present time.
The number of extant civilizations substantially in advance of our own in the Galaxy today appears to be perhaps between 50 thousand and one million. The average distance between technical civilizations is between a few hundred light years and about 1,000 light years.
Because of the real possibility that such interstellar probes actually exist ... it is very important for us to investigate carefully all radio signals of cosmic origin.