Extraterrestrial space archaeology is engaged with the search for relics of other technological civilizations. It resembles a survey for plastic bottles in the ocean as they keep accumulating over time. The senders may not be alive when we find the relics. These circumstances are different from those encountered by the famous Drake equation, which quantifies the likelihood of detecting radio signals from extraterrestrials. That case resembles a phone conversation in which the counterpart must be active when we listen. Not so in extraterrestrial archaeology. What would be the substitute to Drake’s equation for space archeology? If our instruments survey a...