How to explore the galaxy

What to expect from an interstellar probe

Imagine you are an interstellar probe, out in the dark between stars. You detect signals near an approaching star. At distance, determining the exact source is not possible. To more closely observe, you want to pass through the "Goldilocks zone" (were life is most likely). If an inner planet proves interesting, you want the option to change course. Your best choice is a gravity-assist maneuver around the largest planet.

So you adjust course to pass through the inner planets, and close to the planet Jupiter.

Once near the inner planets, you can now tell the signal is clearly from Earth. As you pass nearest the Sun, you adjust course for the maneuver around Jupiter.

The following sections sketch why the above might be somewhat likely.

So far, the object 3I/ATLAS fits the above exactly. Could just be a rock. But curious.

Preamble

Imagine that the humanity wants to explore the galaxy. Assume we have not found any way to travel faster than light. Also assume we have become a long-lived stable civilization. We can wait for probes to travel the long distance between stars. But we want to minimize the time to detect an emerging civilization.

Caution

We do not know what other intelligent life might be like. Other life might fall generally into three categories:

Good	-- Folk we might very much want to meet.
Bad	-- Folk we might very much NOT want to meet.
Weird	-- Creatures we might never understand. Might be harmless ... or not, in time.

We have no notion of the probability of each.

Exploration calls for a degree of caution. Given that some encounters might prove hostile, and some probes would likely get captured, we would want to build probes with the least-advanced viable technology. We do not want to give away our more-advanced knowledge.

Pieces of the puzzle

Could we solve the above problem using current technology, or near?

Propulsion (low-tech)

We already (nearly) have a solution for propulsion. The Project Orion folk figured out how build an interstellar craft with 1960s technology. (The main problems are political, not technological.) Travel time is decades, or centuries, or millenium - with various design trade-offs - and entirely managable for a long-lived civilization.

This is a perfect fit for low-tech probes.

Probes

Self-replicating probes as originally suggested by John von Neumann (and thus oft-called von Neumann probes) would travel the galaxy, making more probes far distant from the home system. Given due caution, and given the ability to replicate is somewhat more advanced, we want to separate the roles of probe and factory.

Building the probe factories is a fair ways from what we can do at present, but possible.

Seed factories through the galaxy and manufacture of thousands of probes becomes possible.
Given time, we might make millions of probes. Or billions.

Non-stop

When thinking about interstellar probes, we calculate the cost of acceleration, at the start, and deceleration at the destination. Building a probe that can decelerate is hundreds of times more expensive.

What if we never slow down?

Provide the main acceleration to probes at launch, then make small adjustments at low energy cost, so to use gravity assist for larger changes.

In sum

Factories replicate, and make billions of probes. Probes use crude nuclear fission/fusion explosives for initial boost. Velocity is kept low enough so gravity assist works. Solar power and ion thrusters can handle the fine changes. Probes fly from one star system to another, never slowing. Given enough probes, time is minimized to detect an emerging threat/civilization.

Depths of time

Over the age of the galaxy, with billions of worlds on which intelligence may have evolved, thousands or millions of civilizations might have reached the point of building interstellar probes. Given even few civilizations capable, in our present galaxy, artificial probes might well be a very common sort of interstellar visitor.

In present

We have only recently built instruments more capable of detecting interstellar visitors.

The first detected interstellar visitor is a flickering pixel - that might be a tumbling cylinder, of the right size for an Orion interstellar probe. This is a highly unusual shape. Maybe it is ... just a rock (if very odd).

In present, the third interstellar visitor is much larger (or at least brighter), and will pass within the orbit of the Earth, close to Mars, and then Jupiter. This trajectory is improbable - and also perfect for a probe that might want to alter course after a close look.

In truth, we have no experience with interstellar objects. They might all be boring rocks.

On the other hand, we have no experience with interstellar objects. They might all be probes from long-extinct (or not) civilizations. We have no basis for making any sort of prediction.

Simple test - if an incoming object alters course...

Possible this is time when we discover that we are not alone in the universe.

2025.11.03 - Update

At present, 3I/ATLAS has just passed nearest the Sun. There is no report as yet on whether it has changed course. (May not be easy to determine, as yet.) If 3I/ATLAS is in fact an interstellar probe, we might expect a gravity-assist maneuver at Jupiter in March.

Found a site that allows us to follow the present course. Even on the known-present course, the object passes very near Jupiter. Only a small adjustment is needed.

We shall see between now and then.