How the galaxy is explored

No visitors

I do not consider living extraterrestrial visitors to be at all likely. Travel between stars takes too long. At the end of that long voyage, most likely all you are going to find is a bunch of rocks, and maybe some primitive life. While the first such trip might seem exciting, after a few thousand stars, this would get a bit boring.

Also a visitor from an extraterrestrial civilization would find the humans of Earth to be extremely primitive. Any long-lived civilization is likely to be millions of years more advanced.

For a long time, I had stopped at this point. When I first considered this notion, the terms to the Drake Equation were largely unknown. At that time, we simply did not know if in our galaxy there were any other planets that could support life.

Over the last fifty years we have detected planets around other stars, some of which might support life. The chance of other intelligent life now looks far more likely.

One visitor

More recently our third observed interstellar visitor arrived on a somewhat improbable path. An extraterrestrial probe might follow a path similar to 3I/ATLAS, so wrote up what I would expect of an interstellar probe.

What I did not describe was what this suggested in turn.

Many visitors

There are hundreds of billions of stars in just our local galaxy. Most are yet-another collection of boring rocks. Too expensive and too boring to send folk on a visit.

Automated probes are far more practical. A single long-lived extraterrestrial civilization might over time send out billions of probes. Over time, millions of extraterrestrial civilizations might have evolved. Not every civilization would adopt the same strategy, so the types of probe might vary a lot.

Taken together, our solar system might be littered with old probes (most long-dead).

I do not pretend to be the first to have thought about this question. Reading the short story The Sentinel first got me thinking in the early 1970s.

What I do want to emphasize is where this takes us.

We lack the tools

Finding signs of extraterrestial life out in the galaxy is hard. Distance is huge, and we do not know how to look.

Looking for probes passing through or left in our solar system is easier, but not easy. Our most practical approach to proving the existance of extraterrestrial life might be to search our own solar system.

Given that our solar system might contain many alien probes, how do we find them?

For probes passing through our system, we need better "eyes". We have only recently become capable of spotting interstellar visitors on an automated basis. But we can only barely see at that great distance. We need to build better instruments to determine in any of these objects is artificial.

For probes left in our system, we need to look in likely places. Long-dead probes in the asteriod belt might be buried in rubble, and very hard to find. Clarke's notion of an intentionally buried probe on our Moon (as in the story, and the movie "2001") is still entirely valid. A careful survey of the Moon is very much in-order.

We need to look.