Turning an interstellar ship around before reaching coasting speed?

[Strato Incendus]

This is a follow-up question emerging from another thread in the Sci-Fi Writing and World Building forum. Specifically, @DaveC426913 had criticised another book in which the plot is set in motion by a plan to turn an interstellar colony ship around and return back home. In my setting, a similar thing is part of the backstory / potential prequel book 2.

So my general question is: Is this possible at all?

The main difference in my case, through which I hope to make this believable, is that the attempt to turn the ship around is performed before the ship reaches its full interstellar coasting speed. Accelerating in the opposite direction from coasting speed, for all I know, would merely bring the ship to a complete halt, causing it to get stranded between the stars forever as a result.

(Unless it has an inexplicable excess amount of fuel, which it could then use to accelerate even further, back towards the home planet. Since the only reason it has fuel to accelerate in the opposite direction in the first place is merely so that it can come to a halt once it reaches its intended destination, once the decades-long coasting phase is supposed to end.)

I’ll quickly insert my description of the events in the second prequel from the other thread here:

In my case, it’s a little more plausible, because the rebels from Generation One are trying to turn the ship around before it has reached its full coasting speed. Meaning, not even the fuel for accelerating the ship towards Teegarden has been fully used up yet: There is still remaining fuel in the tank to not only bring the ship to a halt (and get stranded between the stars as a result), but to accelerate in the opposite direction again (=back to Earth).

How long they’d actually need to get there, and whether they’d still have enough fuel to stop when reaching Earth, that is yet another question. I haven’t done the math on that yet.

In short, with our current calculation, the ship accelerates with at 0.048 m/s2 in the beginning of the journey, and does so for the first 25 years. The rebels in Generation One are in their late teens or early twenties, so they have a few years before the ship reaches full coasting speed. By the time it does, it has covered a distance of 1.56 light years, meaning it is still within the Oort Cloud.

The rebels are ultimately stopped by the commanding officers; upon realising that they won’t be able to return to Earth, the rebel leader attempts to destroy the ship by pushing it off course and into an asteroid within the Oort Cloud. He does this with the aim of sparing future generations (most notably, Generation Three) the fate of having to spend their entire lifetime in the void between Earth and Teegarden b.

It’s hard for me to calculate how far the ship would continue to travel towards the destination star as the rebels try to slow it down, at an acceleration of 0.048 m/s2. Also, on a related note, how fast the ship would already be after, say, 22 years of acceleration, if it starts from 0 and the coasting speed it is trying to reach is 0.125 c, over the course of an acceleration phase of 25 years at 0.048 m/s2.

It might well be that 22 years into the journey is already too late, because the ship is already so fast (=so close to its intended coasting speed) that accelerating back towards Earth would basically only make it come to a halt — with a tiny amount of extra momentum towards Earth, but so little that it would take decades or even centuries until at arrives back home again.

 

[phinds]

If it's not going TOO fast you could see if there's a star or planet near enough to try a slingshot, but that seems unlikely.

 

[DaveC426913]

If it's not going TOO fast you could see if there's a star or planet near enough to try a slingshot, but that seems unlikely.

That exact idea has already been addressed in this other thread:
https://www.physicsforums.com/threads/emergency-braking-in-space.1015593/post-6917875

Short answer: no can do.

 

[Algr]

A third problem: Even if you can reverse the ship, you still have to decelerate again once you reach Earth's solar system, or you'll shoot past it into interstellar space again.

 

[anuttarasammyak]

The main difference in my case, through which I hope to make this believable, is that the attempt to turn the ship around is performed before the ship reaches its full interstellar coasting speed. Accelerating in the opposite direction from coasting speed, for all I know, would merely bring the ship to a complete halt, causing it to get stranded between the stars forever as a result.

It is obvious that soon after landing the pilot should make ship go back to spaceport when some troubles take place. If a trouble takes place in mid journey, the pilot should search spaceports around. When a journey almost ends, the pilot would decide to land on the destinated spaceport as planned.
If hijackers order the pilot to go to some other destination, the pilot should persuade them it is impossible, explaining the distance, current relative speed to it, remaining fuels with relativity and reducing rocket mass by injection of fuel considered and some other important issues.

 

[DaveC426913]

It is obvious that soon after landing the pilot should make ship go back to spaceport when some troubles take place. If a trouble takes place in mid journey, the pilot should search spaceports around. When a journey almost ends, the pilot would decide to land on the destinated spaceport as planned.
If hijackers order the pilot to go to some other destination, the pilot should persuade them it is impossible, explaining the distance, current relative speed to it, remaining fuels with relativity and reducing rocket mass by injection of fuel considered and some other important issues.

There are no spaceports in this story. It is a journey into new territory, far from human civilization.

 

[Algr]

If you children don't stop arguing I will turn this ship around!

 

[Strato Incendus]

At long last, I’m coming back to this thread again. Thanks once more for your ideas! I think I may at least have found a solution for this part of the problem:

A third problem: Even if you can reverse the ship, you still have to decelerate again once you reach Earth's solar system, or you'll shoot past it into interstellar space again.

Since we’re talking about returning to Earth here, rather than landing on an uncolonised planet, one option would be to simply allow the ship to fly off into interstellar space — and merely escape from it using the drop ships once it passes through the solar system. With all the infrastructure on Earth already present, there’s less of a need to preserve anything from on board the ship than when landing on a new planet, where the mothership would be the only source of technology.

That still doesn’t solve the issue of getting back to the solar system in the first place, though.

 

[Algr]

one option would be to simply allow the ship to fly off into interstellar space — and merely escape from it using the drop ships once it passes through the solar system.

And then everyone on Earth would ask why they came back to a disaster area, having just jettisoned everything of value they owned.

Edit: I may have gotten this mixed up, this is the story where there is a disaster back on Earth and they loose contact, right? They would have to have gotten through less than half of their acceleration fuel to turn around and come home. (Unless the destination start is rapidly moving away from Earth.) So they might still be in the solar system.

 

[DaveC426913]

...one option would be to simply allow the ship to fly off into interstellar space — and merely escape from it using the drop ships once it passes through the solar system.

If the drop ships have enough fuel to take the entire complement of the crew and decelerate them to rest wrt of the solar system, then that's also an amount of fuel that could be employed to declerate the entire ship.

That's one of the mistakes I keep seeing in space stories.
"We don't have any fuel left to steer the ship! We're gonna die out here!"
"Maybe we can survive if we abandon it and take all the landers down to that planet over there..."
"Wait. What? You mean the landers are full of ... fuel??"

Not that that's a mistake you are making, it's just a word of caution that your crew will definitely think of it and do the calcs to see if they have enough fuel to bring the ship to a stop.

 

[Rive]

...your crew will definitely think of it and do the calcs...

...that crew does not seem to be particularly mindful of calcs...

 

[Algr]

A ship like this failing and having to return to Earth can only be a dark and grim ending for all involved. But total armageddon could lead to a happy ending: What if there is no disaster on the ship? Instead:

The ship leaves. Back on Earth, the AI becomes sentient and kills everyone, including itself. (Or some other suitable doomsday scenario.) The empty Earth is now far more habitable than anything the colonists could expect to find on the new world. So they turn around because Earth is now the best & closest destination.

Of course that eliminates basically everything Strato has written so far, and kills lots of dramatic conflict. Oops, Sorry Strato.

 

[Strato Incendus]

Thanks a lot once more for your many ideas!

And then everyone on Earth would ask why they came back to a disaster area, having just jettisoned everything of value they owned.

Edit: I may have gotten this mixed up, this is the story where there is a disaster back on Earth and they loose contact, right? They would have to have gotten through less than half of their acceleration fuel to turn around and come home. (Unless the destination start is rapidly moving away from Earth.) So they might still be in the solar system.

It’s the same story, but different time periods:
- The crew that loses contact to Earth is Generation Five.
- The rebels trying to turn the ship around are from Generation One, i.e. live 75 years earlier.

Generation Five indeed lives at a time when half the fuel has been used up, so the other half that’s left can only be used for braking (from it’s coasting speed of 0.125 c). Even using the “extra fuel from the landers” solution (discussed below), at that point it should not get the ship to a sufficiently high speed to make it back to Earth within a human life span, as the ship is alreayd 10 light years away from Earth at that point.

Hence, the plot about turning the ship around, if at all, can only happen with Generation One, who grow up during the acceleration phase; the ship is still within the Oort cloud by the time it reaches full coasting speed.

If the drop ships have enough fuel to take the entire complement of the crew and decelerate them to rest wrt of the solar system, then that's also an amount of fuel that could be employed to declerate the entire ship.

…and with that simple solution, you may have just blown my mind.

Coincidentally, for the final book, I need to get rid of a bunch of the landers, so that the crew is forced to use the “dismantling function” of the ring habitats (the rings break down into smaller sections that can land on the surface like overly long space shuttles; this also acts as an explanation for why the rings don’t have more decks — so that their overall height remains comparable to that of a space shuttle). I could have the antagonist of that story destroy all the landing ships.
But given how much the youth rebel organisation from Generation One already has destroyed (the sperm bank, the artificial wombs etc.), it might be more consistent with their motivations if they were also responsible for fewer shuttles being available (or fewer shuttles having fuel; they might still be within the hangar, but without any fuel, they’d be useless; and then, the reason for that fuel being lost could easily be that the rebels from Generation One burned it all in the attempt to turn the ship around).

This would even work with the intended structure of the various books, as by now, rather than doing the Star Wars structure of “three main books, three prequels”, I’m aiming for an alternating structure (Main Book 1, Prequel 1; Main Book 2, Prequel 2; Main Book 3, Prequel 3). The rebels from Generation One star in Prequel 2, right before Main Book 3, which is the latest one in the chronology. Thus, once it gets to the landing in Main Book 3, you already know from Prequel 2 why the landers have little to no fuel left, and/or why some of them might be missing / broken. After that, Prequel 3 connects everything back to the beginning of Main Book 1, thereby completing the cycle.

The “no fuel left” explanation however won’t work for the eventual landing, as by that point, the ship is in the target solar system — and could relatively easily scoop up hydrogen by circulating around the host star. It is only during the interstellar travel that picking up hydrogen from interstellar gas clouds etc. would not be possible, given the ship’s speed.
Once in the system they would probably be forced to go for the star itself to renew their hydrogen reserves, as red dwarves seem to be unlikely to have enough matter left in their vicinity to create gas giants. (I had postulated two gas giants in the Teegarden system originally, so that I would have a Jupiter-like planet to protect the inner rocky planets from meteors, and a Saturn-like planet to keep the Jupiter from wandering inwards and kicking the rocky planets out. But without any gas giants, fewer meteors might be attracted into the star system to begin with.)

As far as I know, Proxima Centauri (another red dwarf) may have a gas giant, but for Teegarden’s star, only Teegarden b and c (both rocky) seem to be known thus far.

The ship leaves. Back on Earth, the AI becomes sentient and kills everyone, including itself. (Or some other suitable doomsday scenario.) The empty Earth is now far more habitable than anything the colonists could expect to find on the new world. So they turn around because Earth is now the best & closest destination.

That sounds more like a combination of The 100 and NieR: Automata…

 

[Algr]

The 100 and NieR: Automata

I don't know anything about either of those shows, but I guess there are only so many ideas out there.

Taking a page from issues on this forum, you could have pseudoscience as part of the plot. The generation one rebels have some theory, but you need real math to understand why it won't work. And they distrust all the academics who try to explain the problem.

The landers don't have to transport the ship's fuel, engines, frame, water, or radiation shielding. So the mass that actually lands would be a small fraction of the ship in flight. Also, the landers would heavily rely on atmospheric breaking rather than fuel. Compare the space shuttle on its launch pad vrs when it is starting re-entry. Given the time-scales involved, it would make sense that anything returning to the ship would make its own fuel on the planet first. So using lander fuel to move the ship sounds unlikely, unless you write a reason for it to work.

As for other planets in the solar system, the destination would have been chosen as the best of thousands of possibilities. So there is no reason to make it a "typical" system. Jovians? Six planets in the habitable zone? That's why they picked this system.

 

[DaveC426913]

you could have pseudoscience as part of the plot. The generation one rebels

I just finished reading 'The Stars My Destination' and it had a really cool idea: a future Cargo Cult.

These colonists, cut off from the rest of humanity still believed in the Scientific Method but had lost all understanding of what it was.

They would wave shards of glass around and mix pretty colored liquids and invoke the deity Occam, all while crying their mantra "Insuff Data! INSUFF DATA!"

 

[Strato Incendus]

Taking a page from issues on this forum, you could have pseudoscience as part of the plot. The generation one rebels have some theory, but you need real math to understand why it won't work. And they distrust all the academics who try to explain the problem.

I’m glad for every point I can give to the opposition, so that I can flesh out their stance and make them more of an obstacle, due to having good arguments on their side. Also, given the young age of the Gen-One rebels, their idealism, and their romanticised view of a planet they’ve never seen with their own eyes, it’s more than appropriate for them to be naive about the technical / logistical possibility of returning there, too. Thanks a lot!

As for other planets in the solar system, the destination would have been chosen as the best of thousands of possibilities. So there is no reason to make it a "typical" system. Jovians? Six planets in the habitable zone? That's why they picked this system.

The number of alternatives is severely limited by a) the travel speed of 0.125 c, b) the type of propulsion (nuclear-fusion drive), c) the maximum duration of the trip considered manageable for human psychology (it’s a lot easier to lose track of the mission over a journey of 1,000 years than over one of 125 years), and d) the projected time Earth has left until it gets hit by the gamma-ray burst. Ideally, at that point, the first settlers should already have arrived at the new planet, so that others can follow.

Combined with Teegarden b having the highest Earth-Similarity Index in the interstellar neighbourhood — the only one with an even higher ESI is several hundred light years away — this makes it the top choice by a long shot. In the prequel, the scientists discuss a few other popular candidates — Proxima Centauri, Tau Ceti, 82 G. Eridani, Ross 128, Trappist-1 — but ultimately find reasons to reject them all. At least for the first mission.

They would wave shards of glass around and mix pretty colored liquids and invoke the deity Occam, all while crying their mantra "Insuff Data! INSUFF DATA!"

This idea isn’t as fictional as we might like to believe; it’s only that in the real world, people tend to say mantras like “follow the science”, often combined with “the science has settled” — rather than understanding the scientific method as a never-ending debate, fuelled by the constant willingness to be proven wrong.

That’s kind of the premise of the first prequel, by the way (about Generation Zero boarding the ship): Since the ship itself embodies the story’s misbelief, the message one could derive from prequel one is actually the polar opposite of what I believe. That’s because the actual intended message of prequel one is “steelman your opposition and always be willing to be proven wrong”.