The conspicuous absence of extraterrestrial intelligence (ETI) known as the Great Silence (or the Fermi paradox) raises the question: where are they? A civilization capable of interstellar travel could colonize the galaxy in 1 to 100 million years. The galactic thin disk is ~10 billion years old, so enough time has elapsed for an advanced civilization to have colonized the Milky Way several times over by now. The fact that we have not been visited leads some people to conclude that intelligent life is unique to Earth.
The rarity of intelligence or life in the galaxy is one explanation for the silence in the skies, but many other resolutions to the Great Silence have been suggested. ETI may exist but have no desire to communicate, or perhaps no one else is around beacause intelligent civilizations inevitably destroy themselves. Then again, maybe they're stealthily watching and waiting for us to cross a technological threshold before making contact!
Humans will likely develop the technological capability to launch interstellar probes within ~100 years. Exploration by remote probes provides an in situ follow-up to missions that aim to detect extrasolar planets (such as Kepler) and characterize their atmospheres as possibly indicative of life (such as the Terrestrial Planet Finder). Interstellar travel to these newly discovered worlds might be a distant possibility, but in the near-term we are apt to explore our region of the galaxy with remote unmanned probes.
Probes can also serve as a backup device to archive human cultural achievements. A catastrophic event, such as a pandemic or large asteroid impact, could conceivably lead to irreversible information loss: the burning of the library of Alexandria is one such example. But unlike Alexandria, our modern digital storage is easily duplicated and transported. Probes may be useful as a means of protecting our accumulated knowledge from catastrophic risk on Earth.
Most SETI (Search for Extraterrestrial Intelligence) efforts aim to detect intentional ETI broadcasts at radio or optical wavelengths. However, broadcasting our own presence to potential ETI listeners will require unprecedented intergenerational cooperation and foresight, so exploration of the galaxy via interstellar probes may be well underway before we even begin our own active SETI broadcasting program.
Probes in the Solar System
Technological ETI have not yet colonized the galaxy, but enough time has elapsed for their probes to have reached the Solar System. We can only speculate on the motives of technological ETI, but exploration by small probes is within the realm of possibility for future humans. If ETI choose to remotely explore the galaxy, then their use of interstellar probes is at least a plausibility.
Defunct Probes: Relics of past exploration, a defunct probe no longer communicates with its home world. Defunct probes may originate from an extinct civilization that collapsed long ago or from an extant civilization that has lost the ability or interest to maintain the probe. Our own Pioneer and Voyager spacecraft will soon be defunct drifters in interstellar space.
Autonomous Probes: An active exploratory probe will periodically communicate with its home world to transmit information and receive new instructions. Autonomous probes are operational and must originate from an extant civilization.
Intelligent Probes: Self-replicating probes (also known as von Neumann machines) are a candidate for rapid exploration of the galaxy because they can use raw materials (such as asteroids) to construct additional exploratory probes. Intelligent probes may have the ability to observe, learn, adapt, hide, and even converse.
Long-lived interstellar probes would have a limiting size of ~1 to 10 meters, which makes discovering an ETI probe in the Solar System analogous to finding a needle in 2000 tons of hay. Even so, probe activity and communication may be detectable, especially in known stable orbits.
Lagrange Points: Low-maintenance orbital positions are favorable for long-lived probes, especially the five Lagrange points between any two massive bodies. Searches of the stable Earth-Moon L4/L5 libration points have found no artifacts to date and comprise most of the Solar System SETI effort thus far. ETI probes could conceivably reside at any stable Lagrange point, though, and the outer Solar System (such as Jupiter-Sun L4/L5) might be a cleaner orbit and a better vantage point.
Asteroid Belt: Small probes have a plethora of hiding places in the asteroid belt, while intelligent probes also find the raw materials necessary for self-replication. Searches for infrared excesses in the asteroid belt may reveal the activity of ETI probes. Additionally, intermittent communication between a probe and its home world might be detected as anomalous microwave phenomenon using radio telescope arrays.
For Solar System SETI, piggybacking with other observing programs is an ideal strategy to complement directed searches of known stable orbits. Scanning the Solar System for probes is a colossal task laden with null results, but the discovery of a single ETI artifact would resolve the Great Silence once and for all. Maybe they're already here, and we just haven't looked.
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