Elements of Intergalactic Travel

6 min readApr 15, 2017

To summarize the engineering of an intergalactic trip, it feels desperate.

This is a response piece to the Galaxyship post on Crowlspace, because I have a thesis to offer here. The mathematics in the reference there are extremely characteristic of the “old” rocket technology thinking — disposable rockets, use higher specific impulse to fight the rocket equation.

The first comment of the post about sums up my own feelings:

This concept sounds like it has too many zeroes… just everywhere. What does having a 10 billion ton dry mass spaceship accomplish?

Whatever high-performance propulsion technologies we dream of, they still result in parameters that are so poor that it’s downright depressing. The immense absurdity of the task forces us to assume use of all the options with the highest theoretical values.

Components of Departure

Relativistic speeds are necessary for intergalactic travel. Because of that, I have two mental reference points that I can compare to reflecting propulsive and launch assist technologies respectively:

  • If accelerating via on-board propellant, then you’re looking at a delivering a significant fraction of its rest mass directly as forward momentum p=αmc (forward momentum equals efficiency factor time differential unit of mass times speed of light). The ship itself could, thus, be called a slow-burn form of matter decomposition, most easily imagined as an anti-matter bomb. Interstellar travel would be akin to a slow-burn nuclear bomb.
  • If accelerating with stationary equipment, then the process is most similar to a particle accelerator, in terms of mass ratio, energy economics, and everything else.

It’s the latter scenario that I find the most plausible.

Stationary Launch Assist

The best form of propulsion is the kind that you don’t take with you. It would make the most sense for the “1st stage” to be a series of stationary equipment that will accelerate the craft (along with propellant for latter stages) to tremendous speeds. Since any humans will need to be cryogenically frozen, it makes sense to both crank up the g-forces to extreme levels and make the contraption span possibly solar-system level scale with many co-linear acceleration stations along the way. I imagine that either photonic or magnetic propulsion to be used in these systems.

Intermediate Economical Stage

Antimatter is comparatively expensive and I find this unlikely to change no matter what the overall level of technological capability is. Also, the most efficient profile of specific impulse is one that starts out with lower values and then increases as you go one (although still higher values than what the launch assist system will provide). Because of that, it might make the most sense for some conventional fusion engines to act through an intermediary acceleration stage. Even recovery and reuse for subsequent intergalactic launches might make sense at sufficient scale.

This is what I imagine as the “2nd stage”. I think it needs mention, but I don’t particularly favor the idea because it strikes me as hard to compete with simply making the accelerator larger, and re-usability would work on such a dramatically long time frame that it hardly seems worth it.

Max-Tech Main Burn

Antimatter is clearly one of the best options, and even things like fusion would be insufficient for generating the majority of the thrust for the journey, making this the “3rd stage”.

Passive Dark Energy Slowdown

Even if you don’t do anything to slow down, over the coming billions of years the craft will slow down naturally because the destination galaxy is acceleration away from the departure point because of dark energy. It’s unclear to me exactly how significant this might be, and it seems like it would butt heads with a timing imperative. If you traveled for long enough for this effect to matter, maybe the evolution of the cosmos will make life more difficult once you get there. Habitability isn’t guaranteed far into the future. It also seems hard to justify if you’re not starting at nearly the speed of light. If not, then you’ll be waiting billions of years for the acceleration to matter.

I also want to point out that this method seems like it would conflict with use of on-board propellant because the need for speed would make that mass difficult to justify. You would send as small of a thing as you can as fast as you can, and it would remain in stasis for a long time, as experienced on the ship. This requires something like a true Von Neumann Machine — amost something like post-singularity tech in the payload.

Components of Arrival

I’ve seen most of those components of departure discussed to some degree in other forums (apologies for not actually compiling a bibliography). However, I feel like the role of black holes and other things the local astronomical neighborhood haven’t often been tackled.

Black Hole Oberth Effect

The extremal case for selecting a large body to use for the powered deceleration is actually relatively easy to pinpoint, because you would obviously use a black hole. Googling the subject, I found a hilarious Reddit comment that shows the depth of popular misunderstanding of the subject. This comments makes 4 points against the proposition, each of which are almost the exact opposite of the truth. However, the bullet points offer a relatively good semantic organization, which I will borrow:

  • Basically every galaxy has a supermassive black hole which can be identified with even today’s technology
  • As long as there is no object actively falling into the black hole, the instability of the knife-edge orbits gives good assurances of a pristine, usable, low-radiation, space environment on close-approach
  • The sheer size of supermassive black holes gives ample time to use low-thrust engines and negligible tidal forces
  • This is all ideal of accelerating or decelerating to near light-speed

This isn’t to say that rotating or charged black holes aren’t useful, but that gets into very different territory.

Using a Black Hole’s Parameters

If a black hole is rotating at a significant speed, you can actually gain energy from it with certain ballistic approaches. This could be extremely useful to take care of almost all of your incoming speed without spending any of your mass.

The Problem of Incoming Astronomy

These tantalizing possibilities for “free” deceleration raise the engineering challenge of selecting the destination object. You might use a stellar black hole to slow down, but it’s almost certain that you won’t be able to resolve that object from the galaxy that you depart from. That means that if you wish to make use of any of these objects, you would have to add an astronomy and science complement to your spacecraft. This has some mass disadvantage, but also imposes a great amount of risk, should not acceptable objects be found, leaving the craft stranded in space.

Stellar Capture

After that exotic journey, whatever mass makes it to the destination galaxy going at “normal” speeds relative to the local bubble of interstellar dust and star systems would still have to cope with the “conventional” challenges of attaining a Heliocentric orbit about the star and then docking or landing on the body that it will then make its home.

The best way to do this is to make maximum use of the local solar Oberth Effect by coming in a close-approach to the sun and firing boosters at that point (given the need for high thrust and high impulse, this would be an appropriate place for Orion-like propulsion systems). This would get you captured, but even after that you’d need some extra burn to circularize.

After circularizing, the only remaining challenge would probably be to dock with an asteroid or something which you could make use of to build out your infrastructure in order to colonize this new galaxy.




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