“I propose a version of the Drake Equation for Lurkers on near-Earth objects. By using it, one can compare a Search for Extraterrestrial Artifacts (SETA) strategy of exploring for artifacts to the conventional listening-to-stars SETI strategy, which has thus far found no artificial signals of technological origin. In contrast, SETA offers a new perspective, a new opportunity: discovering past and present visits to the near-Earth vicinity by ET space probes.”—Paul Gilster, “A Drake Equation for Alien Artifacts.” Centauri-Dreams.org. April 20, 2021.
Imagine an alien civilization finding the Voyager space probes a billion years in the future. Over the span of cosmic time, how many other civilizations managed the same? What is the typical civilizational life span of those civilizations capable of doing it?
Then, there is the question of how many would have survived and developed far enough to place probes in nearby stars with environments conducive to life?