Alien Civilizations III: Travel

An alien civilization is likely to have developed billions of years ago. They would have swiftly progressed through the technological invention stage, and long ago reached the point where they could do all the things they wanted, bound only by the laws of physics (they couldn’t, for example, create matter and energy out of nothing).

So what would they do for all those billions of years? Most likely, they’ll begin to spread out. Staying on one planet, or even in one planetary system, is more or less suicide. There are innumerable catastrophies that can befall such a multi-planet but single-system species: supernovae, gamma ray bursts, war, and eventually the death of their star itself. A species that has been able to survive the evolutionary process would have survival instincts and desires hard-wired into their brains (or whatever thinking organ they have… if they have organs…). And, they are very very smart, even smarter than us. It seems overwhelmingly likely that they would take to the stars.

Interstellar travel is slow and it is hard. Even at the speed of light–the absolute fastest it is possible to go–travel across the galaxy takes tens of thousands of years. And getting even close to that speed takes enormous amounts of energy.

But wait…tens of thousands of years? The species we’re dealing with is billions of years old. From that perspective, interstellar travel is actually pretty fast. In fact, even using concievable methods of starship propulsion, the time to cross the entire galaxy is measured in only millions of years. Here is a table detailing various engine types both real and theoretical (though all physcially possible) and their performances:


H2 – O2



H3–D Fusion


Exhaust Velocity (km/s)






Max. Speed (km/s)*






Time to Alpha Centauri (4.36 ly)

65,350 y

7,140 y

2,140 y

84.3 y

12.6 y

100 ly

1.50 mil y

164,000 y

49,100 y

1,930 y

288 y

1000 ly

150 mil y

1.64 mil y

491,000 y

19,300 y

2,880 y

Galactic Core (26,000 ly)

390 mil y

42.6 mil y

12.8 mil y

503,000 y

74,900 y

Across Galaxy (100,000 ly)

1.50 bil y

164 mil y

49.1 mil y

1.93 mil y

288,000 y

Andromeda Galaxy (2,400,000 ly)

36.0 bil y

3.93 bil y

1.18 bil y

46.4 mil y

6.92 mil y

*Max. Speed is figured by assuming a staged rocket with an overall mass ratio of 64, and assuming the probe will stop at its destination. If the mission is just a fly-by, the maximum speed would be twice as high.

**Because hydrogen-oxygen rockets tend to be very slow, any probe would likely also utilize gravitational boosts by flying near the outer planets, increasing its speed while not using any propellant. Other engines could also utilize this, but the amount added is very small compared to the overall speed.

***It is likely impossible to have an staged antimatter rocket with a mass ratio of 64, since antimatter tends to annihilate when it touches normal matter. An antimatter fuel tank would have to be arranged to keep the antimatter suspended completely away from the rest of the normal-matter rocket This figure is based on a mass ratio of 8 (that is, three stages, each consisting of a mass ratio of 2).

As you can see, even today’s hydrogen-oxygen rocket technology is more than sufficient for crossing (and thus colonizing) the galaxy in the given. Keeping a ship working for a billion years is definitely a challenge, but if aliens have billions of years to work on the problem, they’d probably be using antimatter rockets and know how to keep them operational for the several hundred thousand years necessary.

But why would they come here? Our solar system is just one in two hundred billion. There must be many interesting locations to explore. The odds that they would choose ours are literally billions to one.

But it is more useful to see the galaxy from the alien’s perspective (which is actually not very different from our own). We live in one tiny patch of space and want to know as much about the rest of the galaxy as possible. Sending probes to stars one at a time is very inefficient. The mathematician John von Neumann showed that by using self-replicating robotic probes, one can create a vast number of interstellar probes for very little cost (you only have to pay for the first one). These probes would utilize the resources found in other solar systems to make copies of themselves, then send the copies off to the next star system. The aliens wouldn’t just colonize a few stars, they would colonize all the stars.

More likely, it would be desirable to make all the probes in your own star system using the resources available then launch them all at once. This is better because A) you avoid two robotic probes heading to the same star system B) you avoid the problem of a probe coming upon a system that has no resources and so can’t make a copy of itself and C) you can better control the self-replication process, avoid any accidents.

Such robots would then explore their respective targets and likely continue observing over a period of millions of years (assuming advanced self-repair capabilities). Perhaps they might even set up colonies if any of their creators wished to relocate there, or even begin terraforming any suitable planets.

The implication is obvious, if aliens have evolved at some point, they’re probes should already be here in our solar system. We have never found such alien artifacts, but the solar system is a largely unexplored place. The most likely place to find such artifacts are in a place where resources are plentiful and easy to get at. That is not the Earth, or the Moon, or any planet at all. They would likely reside in the asteroid belt, where there are vast resources, out of deep gravity wells, and close enough to the sun to provide a constant power source. The Kuiper Belt, farther out, has even more resources, and the probes might use on-board fusion plants for power instead of the Sun.

Maybe they’re out there right now. If so, there’s no use trying to signal them. Assuming they’re still active, they know all about us and aren’t talking anyway. There are hundreds of millions of asteroids out there and the only way to find them, perhaps, would be to send out our own von Neumann probes to scour the solar system.

Next Chapter: Survival


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