WHERE ARE THE ALIENS? (CONT’D):

Depending on motive the space probes will have different capabilities and missions.

If the motive of aliens is defensive then an artificial intelligence might be sitting out in the asteroid belt monitoring Earth for a hundred million years watching for signs of intelligence. The AI might have algorithms for rating intelligences by aggressivity and hostility. It might have a means to sterilize this planet if it decides we are a threat. Such an AI would not have been sent to leave a clear record of its presence.

Scientists have actually looked for alien probes in likely spots in the Solar System, but an advanced alien technology could presumably avoid notice. This piece on extraterrestrials by Gregg Easterbrook is worth reading, for those interested.