31Jul/090
Fermi Paradox Points to Fewer Than 10 Extraterrestrial Civilizations (or maybe 1,000… pick your multiplier)
They say that if each probe has a life span of 50 million years, and if evidence of their solar-system visits lasts about a million years, there can be no more than about 1,000 advanced civilizations out there now. If, instead, these probes can leave longer-lasting evidence of a visit--evidence that remains for 100 million years--then there can be no more than about 10 civilizations out there.
Full artlcle here.