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I previously asked about a rogue solar system, ejected from the Milky Way and set on a course for Andromeda. After millions of years, arrival is imminent - now, the inhabitants who had been in hibernation for a fraction of that time wake up, and have to quickly find a way to stop their system in the galaxy, or risk being cast into the void forever.

How would my characters accomplish this exactly? This is obviously super speculative, super theoretical kind of stuff. My idea is that the civilization that set their course millions of years ago targeted several celestial bodies in Andromeda that by encountering them will hopefully slow the system's velocity in order to get captured in Andromeda's gravity.

In this case I am planning the conflict to be that they need to find a way to correct the course to make sure they don't miss - in which case I need some kind of technology capable of moving a whole system - something that has been dormant that the newly awakened characters just need to figure out how to activate.

So, will I need to go full sci fi hand waving, or is there some sort of technology I can use to slow them down? I'm not sure if my understanding of orbital mechanics is right, that you can do a "reverse slingshot" maneuver.

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    $\begingroup$ You make this sound urgent or dire if they "miss"? But it really isn't. Even at 0.1c it'll take them tens of thousands of years to pass through the galactic disc, if for whatever reason they really do need their home star and planets to stay. This is enough time to Dyson-sphere the local sun, dismantle asteroids for raw materials, and just fly everyone and their valued bits of homeworld away on conventional fusion-powered spacecraft or what have you. In what context are characters thinking about how to do this? Whole civilizations can rise and fall in the time they have. $\endgroup$
    – parasoup
    Commented Sep 3 at 22:19
  • $\begingroup$ Yes, stars accidentally get accelerated and ejected from galaxies, and since trajectories can be reversed, slingshot capture is in principle possible. But even for slingshot maneuvers within the Solar system you need maneuverability at critical points; the inaccuracies of intergalactic travel would make that need substantial. You need substantial maneuverability anyway. Simply come up with something like space bending tech and use it to accelerate and slow down. $\endgroup$
    – Peter - Reinstate Monica
    Commented Sep 4 at 7:51
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    $\begingroup$ In your other question, were you given a solution on how your civilization managed to eject their solar system from the Milky Way and aim themselves at Andromeda in the first place? Because I imagine that any civilization with the technology to accomplish that would be able to just do the same thing in reverse. $\endgroup$
    – Abion47
    Commented Sep 4 at 15:30
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    $\begingroup$ BY the way, are you aware of the speeds and distances? The fastest known rogue star moves at 5E6 mph, or 0.007 c; Andromeda is 2E6 light years away, which makes the journey 285 million years long. You cannot sleep that long. And it is very very likely that an intelligent species would cease to exist during that time frame -- or, rather unlikely, come up with something inconceivable (e.g., transcend physical existence and become god-like etc.). $\endgroup$
    – Peter - Reinstate Monica
    Commented Sep 5 at 6:40
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    $\begingroup$ Galaxies are big. They'll be spending 1-10 million years in Andromeda at least - possibly more if they deflect around the core. That's a lot of time. Why waste all that beautiful kinetic energy to save one little planet when you could use it to colonize the galaxy? All your interstellar space craft get a .007c boost (relative to the galaxy) in whatever direction you want around the nearest star to your trajectory. All you have to pay for in rocket fuel is escape velocity from your own star system and few little nudges to make sure you slingshot in the right direction. Awesome! $\endgroup$
    – g s
    Commented Sep 6 at 18:34

4 Answers 4

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Stellar Tug

This is a real idea. You park a massive engine - not in orbit - on the side of the star you want to accelerate towards. Then you aim it so the exhaust just misses the star, and turn it on, adjusting the thrust so that it floats. It's like a skycrane, but bigger and uses gravity instead of ropes to hold the payload.

Not very fast, as the acceleration is based on the acceleration of the sun into the engine. This is on the order of $10^{-9}m/s^2$. With a moon as your thruster (as the mass of the thruster determines the acceleration). At a distance of 0.4 AU from the star. The proposals also envision supplying this engine with fuel and remass by using a Dyson swarm in order to perform stellar lifting. So, not simple at all.

A civilization doing this is going to need to be able to move planetary bodies around already. Having the engineering to perform stellar lifting is likely going to something already developed.

(And yeah, the acceleration is slow, but it's literally a thousand times more effective than something like a Shkadov thruster, which is completely passive. For a Shkadov thruster, you float a big mirror ahead of the star using radiation pressure, and that imbalance leads to the star system accelerating.)

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  • $\begingroup$ It is as real of an idea as a Dyson Sphere is... in fact it is still science-fiction. I read the parts available from that paper. I cannot see how such an engine can have any significant effect of acceleration without also depleting the sun's mass. You don't even need a Dyson swarm you can just burn the sun's hydrogen. This induces drag in one direction while also covering the whole system in the exhaust products (whatever this is in space without oxygen). In the end all forces alleviate each other. There will be no "steering" effect. Not even in GY scale (i.e. the assumed age of the universe) $\endgroup$
    – Antares
    Commented Sep 5 at 13:04
  • $\begingroup$ But in the end it is a handwaving concept of course! $\endgroup$
    – Antares
    Commented Sep 5 at 13:09
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Build a Shkadov_thruster. The most basic version of which is simply a huge mirror that has it's reflectance tuned so that the outward light pressure on it matches the inward tug of gravity from the star. The mirror creates an emission asymmetry and turns the solar winds of the star into an exhaust plume. If they're in a hurry a rotating magnetic ring around the elliptic can be used to compress stellar matter and turn the star into an enormous fusion torch drive.

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Real Physics part:

Yes, you can do slingshots in both modes to either accelerate or decelerate (with or without "curving" of the final trajectory).

With that out of the way, now to the fun part:

Technical solution: "A Black hole generator"

Tech-Babbel: A generator for "temporary artificially induced spacetime warpfield fluctuations" aka a black hole "thrower" (and those holes dissolve after a while again).

I won't go into details here, you will need to figure out how it looks like, how it is powered etc. (maybe that is worth a follow-up question, if you should be stuck). But since you have a whole solar system with you (including a sun I mean), energy is maybe not too much of a problem. See Dyson Swarm if you need an idea.

In principle my idea is to have a device which is able to induce "very high gravity effects" on a designated position somewhere on the course of the planetary system. The effect is similar to a gravity well that a black hole would cause and therefore can influence the course of the planetary system plausibly. The trajectories would be like a "slalom" shaped orbital path around those black holes (like in "snake lines") if fine grained course corrections would be needed.

Strength/size of the black hole as well as distance need to be programmed into that machine. Also the time until the black hole effect is dissolving needs to be adjustable. Either because this "black hole" radiates away "quickly" or because the machine is able to control the "temporary" aspect very accurately. Like in: cut the energy and the effect is gone. With this you can explain all sorts of flight paths and maneuvers (course corrections, speed ups/downs, quasi standstill).

Also possible would be to create this black hole exactly right for the sun to enter a stable orbit around it. This would maybe be the "better" solution to stay at a certain spot instead of decelerating to "stand still" (which is not possible to a perfect accuracy).

Because of the gravitational properties in the scale of a black hole (with appropriately high mass), this can plausibly influence the course of a sun and therefore all their planets as well if it is configured exactly right so that the planets will not be ripped out of their orbits around the sun and into an uncontrolled orbit around that black hole thing.

But your scientists have certainly mastered that bit. Right? ;)

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Use a Bussard ramjet.

The Bussard ramjet has a huge magnetic scoop to collect interstellar matter. In the original proposal, this was used to drive a fusion engine. The scoop will effectively act as a giant brake as it scoop up matter. In the original design it was doubtful whether the Fusion Drive could provide enough thrust to offset this braking effect.

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    $\begingroup$ A ramjet can stop a space ship, but I doubt it can be scaled up to stop a solar system. $\endgroup$
    – Paŭlo Ebermann
    Commented Sep 4 at 22:34
  • $\begingroup$ A magnetic scoop for interstellar matter, perhaps based on a magnetar, might slow things down. The braking from the scoop is the important bit, not the ramjet. $\endgroup$
    – Richard Kirk
    Commented Sep 6 at 8:31

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