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New to the site, and like many others here, I've been bouncing around a few ideas for a hard sci-fi short story with some friends, centered around an interstellar voyage to colonize a habitable world. We figured that laser propulsion (a lightsail-equipped spacecraft being boosted and then decelerated by a powerful orbiting laser in the Sol system) is one of the better ways to propel an interstellar spacecraft; it's safer than antimatter, can get to relativistic velocities, and it's not limited by the rocket equation. It seems like the natural choice. But its dependence on an outside power source, back home in the departure system, is just something that gives me plenty of anxiety to think about. What if something went wrong with it? So that's the idea I was thinking of exploring.

The Setting

About the mid 23rd century, the Sol system has been heavily colonized, and the various great powers within it are now racing against each other to colonize habitable worlds found around other stars (at sublight speed, no FTL here). The successful terraforming of Mars and Venus last century has left humanity with an enormous amount of energy infrastructure, namely several extremely powerful solar-pumped laser arrays. And I mean powerful; they were previously used to melt almost all of Mars's surface to liberate the oxygen and water in its soil, and to cut Saturn's moon Enceladus into pieces to be deorbited onto Venus to make its new oceans. With the two planets now habitable, the new use found for the laser arrays is to accelerate interstellar colony ships leaving the Sol system, and then to decelerate them once they're near their destination. By the point of the story this has already been done successfully several times, and a number of nearby star systems have been colonized through it.

The Spacecraft

The ships themselves are sleeper ships, at about a kilometer long are rather small by interstellar ark standards, but they carry an accelerator sail with a diameter of 1000 kilometers and a decelerator sail with a diameter of 300 kilometers. Reminiscent of Robert L. Forward's design, the laser array first boosts the ship away from the Sol system for six months at 1G by the accelerator sail (with the decelerator sail folded up inside it), until it reaches 50 percent of lightspeed and the laser beam shuts off (in the interim period, it's redeployed to accelerate/decelerate other ships elsewhere). Once it's near its destination, the laser array back in the Sol system again is shined towards it (or was a few years/decades earlier, with the beam taking that long to cross the lightyears to the ship). When the beam is detected, the accelerator sail detaches from the ship and is pushed ahead and away from it, and the decelerator sail is unfolded to receive the laser light reflected off the accelerator sail to slow the ship down (the accelerator sail has built-in servomotors to slowly change its shape as it races away from the ship, to keep the light reflected on the decelerator sail). After six months of deceleration, the ship has reached its destination system and the decelerator sail is detached, leaving the ship with only its built-in fusion engines intended for interplanetary flight (they have nowhere near the fuel capacity for interstellar velocities).

A standard colony ship carries a few dozen asteroid mining probes, surface construction equipment, 3D nanoprinters (many are built into hull-mounted robots that roll around the ship and repair any damage from the interstellar medium during the voyage), four VTOL SSTO spaceplanes for landing on and launching from habitable worlds (their built-in reactors can make their liquid hydrogen fuel from water or even just atmospheric humidity), and 10,000 humans in cryostasis (judged to be the minimum starting population needed to maintain a healthy gene pool in the long term). Food brought along can sustain all onboard for a year, and an onboard hydroponic farm can keep a few hundred of the crew fed during the six-month deceleration period (they're woken up early to monitor the deceleration and survey their destination) and in emergency situations (all others onboard are expected to be kept asleep until any emergency is resolved). Onboard gravity can come from either rotation or linear acceleration; the gimballed passenger compartments can either face forward or backward during the 1G acceleration or deceleration, or outwards for rotation during cruise and when stationary. An onboard fusion reactor powers the ship, and for contingency it carries enough fuel for twice the expected journey time.

The Ill-Fated Voyage

At the time of departure, it's been about fifty years since these interstellar colonization voyages have begun, and the Sol system has already received radio transmissions from the first missions in their destination systems up to 15-odd lightyears or so away, confirming they arrived safely and are settling into their new habitable worlds.

This particular mission is to an Earth-sized habitable exomoon found orbiting the gas giant Upsilon Andromedae d, named Majriti after 10th-century scientist Maslama al-Majriti (look it up, the IAU has already named it that). The moon is named Fatima after his supposed daughter. At 44 lightyears away, the journey will take nearly 90 years, and it's by far the longest interstellar journey attempted so far. But with interstellar colony missions having a perfect safety record so far, and any habitable world being too good a target to pass up on, they feel confident enough. The acceleration up to cruising speed goes off without a hitch, and the passengers and crew climb into their cryopods for the long journey.

About 87 years later, with the ship (I'll have to think of a name for it at some point) hurtling through interstellar space at 0.50 c, a skeleton crew wake up as planned in anticipation of the laser beam arriving, ready to monitor the deceleration, do some maintenance on the ship, and see what they can survey of Fatima before arrival.

Only, the laser beam never arrives. Something happened back in the Sol system, and the laser array never shined a deceleration beam at them. They don't know what happened; they didn't receive any radio transmissions since shortly after departure, and whatever it was, it was 44 years ago (maybe I'll explore what happened there in an eventual sequel story). Either way, the ship is still plugging along at cruising speed, rapidly approaching its destination and seemingly about to overshoot it, with nothing to slow it down.

So, with no laser beam coming from Sol, with the equipment they have, and with the few hundred crew that the hydroponics can sustain awake, what can they do, if anything, to slow down? Will they inevitably overshoot their habitable world? If so, would they be able to stop in another star system (not ideal for them given that it likely wouldn't have a habitable world, but better than endlessly coasting through space)? That's what I'm hoping to explore, and I'm hoping it's a good starting point for an interesting story.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Jan 27, 2023 at 20:38
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    $\begingroup$ Since you've gotten some feedback that the plan might be implausible (it is a lot energy after all), perhaps a minor tweak -- the plan is to instead wake up part of the way through the acceleration, just to check in on things. It is discovered that Earth mysteriously stopped transmitting when the ship was only at .1c. Now you have quite a bit less energy to bleed, and a lot more space to do it in. But you'll have a much, much longer journey -- some of the colonists might get freezer burn, etc etc. You also have to go back to sleep on a ship that has been lightly MacGyvered -- stressful! $\endgroup$
    – Zwuwdz
    Jan 27, 2023 at 20:57
  • $\begingroup$ Could the deceleration laser have been automated, and still present despite whatever happened on Earth, but unmaintained? So that it fires and starts the deceleration on schedule, but then fails midway through? This would give you a lot more to work with. $\endgroup$ Jan 28, 2023 at 12:37
  • $\begingroup$ Great username, btw. 👍 $\endgroup$ Jun 1, 2023 at 17:21
  • $\begingroup$ 50% of the speed of light is brutal for slowing down. Gravity wells? Only neutron stars and black holes are deep enough; tidal forces would tear up the ship unless the hole was very massive. Aerobraking? That would be lethal...for anything on the surface of the planet as well. Solar sails? It's still way to small a "net" to catch you. 40 hours (75AU at 0.25c) of light to slow down (very generous, even atom-thick nanomesh for an extremely bright star may not be enough) means over 100gs. $\endgroup$ Jun 2, 2023 at 8:14

12 Answers 12

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Taking a page from The Mote in God's Eye, they'd keep the big sail, turn ship 180 degrees (ought to be possible, though it'll take a while), and dive straight into the oncoming star.

It's a gamble, as to whether they can decelerate enough for a useful capture without getting so close they get cooked, but the closer they get to the star, the more thrust their sail gives them. The big sail, with 9+ times the area, will give that much more thrust as well. The strategy is to start angling the sail to slide past the new sun at the last possible moment when they can avoid excessive heat stress, while still holding the sail at an angle that eats away their incoming velocity.

Whether they can slow enough to capture in the correct star system, how they can lighten the ship enough to do so (what can they afford to jettison?), whether they can use the second, smaller, deceleration sail as a heat shield (and how effective it will be) sound like a lot of the plot of your survival story.

There are several objections in comments that, effectively, this is impossible with real world physics -- the acceleration of a light sail is too low even close to the star. Solution to this is to travel slower. Instead of 0.5c, if they travel at 0.05c they'll have only 1% of the energy to lose, and have ten times as long to decelerate even without any of the heroic measures like dumping half the colonists or burning all the lander engines and then jettisoning the landers. Plus, there will be ten times as long for civilization to fall, records of their voyage to be lost, etc. to account for the braking laser not lighting up on time.

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    – L.Dutch
    Jan 27, 2023 at 20:38
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If there were anything they could do to make an unplanned safe deceleration from 0.5c, they probably wouldn't be reliant on a giant external laser beam to handle the acceleration to 0.5c. If they didn't have a backup braking system for just this scenario, their only real option is to wait for the beam to arrive, late. Hopefully the trajectory was designed with enough margins for them to compensate for the delay.

If they do have such a backup system (perhaps one only capable of braking a fraction of their colony payload), their destination remains the same. There most likely is not a star aligned with Upsilon Andromedae closely enough to be reachable, and close enough to be reached in a reasonable time. If it existed, they wouldn't have an easier time stopping at such an alternate destination.

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    $\begingroup$ I haven't decided exactly what happens back in Sol yet, but whatever it was it was bad. One can safely assume that the laser array won't be sending any more light, late or otherwise. Good to know that going to another star isn't a realistic option though. It's Upsilon Andromedae or death. That ups the stakes a bit. $\endgroup$ Jan 25, 2023 at 20:37
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    $\begingroup$ Spaceflight is a domain of tight margins, and only rarely can you completely change your plans mid-flight. And when the portion of flight that was successfully performed involved accelerating to 0.5c, you're pretty completely committed to at most a minor variation on the original plan. This actually becomes easier if the beam fails during acceleration...with sufficiently advanced technology, an improvised braking system might stand a chance of getting them down from 0.05c or something. However, surviving long enough to reach the destination then becomes a major problem. $\endgroup$ Jan 26, 2023 at 0:43
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Spacecraft typically don't carry around "backup" mission delta-v (especially 0.5 c's worth), so I think these folks are at the mercy of the winds.

The only plausible alternative I can see (if Zeiss's answer isn't good enough) is a magsail brake, and I could maybe see how you'd pack one of those "just in case". The system consists of thousands of kilometers of superconducting wire, perhaps massing as much as the main sail, and a powerful reactor to run it continuously for years. Combined, it would be a sizeable fraction of the vessel's mass.

What threatens the plausibility of hauling one of these along is the reactor. Why would a laser-propelled starship needs such powerful on-board energy production? Unless there's some other process going on from which energy can be diverted away.

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    $\begingroup$ Not that the designers back in Sol would have ever intended them to be used in this way, but if the hull-robots could be retooled to print superconducting wire all over the accelerator sail, then cut holes into it until it's a mesh grid of sorts, then connect it to the onboard fusion reactor, I mean, it wouldn't be anywhere near as good as a purpose-built magsail, but along with the other deceleration methods I imagine it could be something? $\endgroup$ Jan 25, 2023 at 21:00
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    $\begingroup$ the cryo chambers use lots of power, so they have a fusion reactor running and are all connected with superconducting wire - but we would need to power them down to repurpose the wiring and don't have enough food for everybody. What a pickle. $\endgroup$
    – Christian
    Jan 26, 2023 at 10:45
  • $\begingroup$ @Christian Those cryopods also represent a lot of mass you need to slow down...assuming they are still on your ship when the time comes. $\endgroup$
    – Nosajimiki
    Jan 26, 2023 at 19:45
  • $\begingroup$ Spacecraft typically don't use sails either... "the ascent engine emerged as the least complicated of the three main engines in the Apollo space vehicle, including the LM descent and CSM service" "Designed for 2 restarts" ... and they lit it once (yours, with a safety factor of one, isn't). If it really...really, really... just has to work, then it has to be the no-fail type, which doesn't involve deployable tinfoil sails. $\endgroup$
    – Mazura
    Jan 27, 2023 at 1:34
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    $\begingroup$ @Mazura I think this is one of these no-return missions. It's uncommon now, but I think it was quite common in previous times. Sail to America or die trying. Walk over the Alps or die trying. Paddle to Australia with your canoe or die trying. And now it's fly to the next star or die trying. And this voyage looks like it's one of the "die trying" ones, but we gonna TRY HARD. $\endgroup$
    – Christian
    Jan 27, 2023 at 8:23
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Expand the tethered acceleration sail, release and unfold deceleration sail, rig out a mass driver, and slow down with interstellar matter and/or the heliosphere of a destination star.

In fact, since your ship is travelling at 0.5c through interstellar space, AND accelerating from Sol's laser, it's already sturdy enough to withstand collisions with neutral HI (H-one, hydrogen) atoms and not corrode, this deceleration could well be used as interstellar brake. So you grab that accelerator sail, expand it to its largest configuration, possibly steer with it so that you won't miss the star (I assume they won't need the steering as the reference design is depicted as being unable to steer while decelerating), and let neutral matter slow your ship down.

After all, delivering 1G of acceleration via light gives your ship flux no less than 1/3e7 worth of the ship's total mass, to radiate heat produced by reflection of that much and not melt in the process would require truly interstellar sail radius, I can't properly predict how big it should actually be. Thus the space "air resistance" would also be pretty significant. I think an example calculation using existing materials would lead to an empty solution, so I'll skip materials and use only dimensions and weight. Let's say your sleeper ship has its capsule mass of 100 Mt = 1*10^11 kg, its acceleration sail is 1e7 m wide and also has a mass of 1e11 kg, its deceleration sail can expand to about 1e6 m, and the acceleration sail can expand if needed to 2e7 m in radius. This configuration would require 2e11*10/3e8/2 = 3.3(3)e3 kg of light per second, this amasses to 3.3(3)e3*3e8*3e8 = 3e20 W of incoming flux, with the total square of PI*1e14 m^2 it'll be somewhat manageable 955 kW/m^2, if the material would be 99.9% reflective and not ablative under this hard flux (after all we're saying laser, thus we can produce a nano-material that'll be almost 100% reflective in the selected band) it could even start up. So the initial parameters look decent.

Now to deceleration. We can calculate the resistance of interstellar space vs our ship based on data on average interstellar space density of 1 atom per cm^3. I have found that there is a range of interstellar gas density from 0.003 to 100k cm^-3, yet 1 is well within range, and as not even Voyager 1 is yet in there, we here don't have direct observation data on actual gas density near the Sun. The fully accelerated ship with fully expanded acceleration sail will cross a cylinder 2e7 m in radius and the length of 1.5e8 meters per second. That volume will contain 1e3*PI*2e7^2*1.5e8 = 18.85e25 atoms of HI, or 3.13e3 g or 3.13 kg of hydrogen per second. If all that hydrogen is absorbed, it would net already a thousandth of incoming acceleration, this also means that such a ship will decelerate against interstellar space with its sail expanded to 1e7 m at 1/4000 G or 0.025 m/s^2 at top speed, and since this procedure has been performed several times, the Earth's scientists know that. We still need more, as we only have about 40% distance left until we hit the destination star, yet now we could actually try to steer our ship with rigging of the acceleration sail to have a sidewards force component, so that we miss the star and whatever planets we were initially aiming at, but by both using local solar wind, heliopause and other space-volume effects of the star's heliosphere, the crew might hope to divert the ship's course to the star in close vicinity of their initial target, in the mean time the ship would slow down enough to allow the sail to work at normal speed range and as a manner to actually explore the second star's system. This will however take quite a lot of time, even if the second star is as close as 1 LY to the first and is aligned for the ship to not miss it, the ship would have to spend some several hundred years in space before fully decelerating.

As an additional means of deceleration, assuming the ship has fusion reactors with a means of adding more fuel, the crew can use its DEceleration sail as a means to focus incominng flux of matter into a rigged collector that could then process HI into either fuel for ion engine or plain for the reactor itself. This process would gradually slow down as the ship would cross less and less space per second, yet it could provide both some exercise for the crew to not lose their minds, their computer to not go awry, and the next generation(s) to have a go at alternate approaches to reach at least the second target with them intact.

As a last resort, they could strap a relatively small compartment of about 1.5 Mt to the sails, and slow down at twice the calculated rate, retaining a chance to actually contact the first target, leaving the rest of the ship adrift into endless space (hi ethics, bye ethics) as an unguided asteroid (hi Oumuamua) that will eventually cross the galaxy most likely unbroken, and could attract some other galaxians' attention to the Milky Way and its interesting inhabitants. After all, the emergency crew of several hundred is pretty enough to colonize a planet.

(As a story element, nothing could happen at Earth, it's just the ship had passed through a small undetected nebula with increased gas density, making the ship fly slower and miss the rendezvous point by several light-days, thus the Earth just missed the ship with its ray.)

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  • $\begingroup$ Thanks, that's a very detailed answer. It's pretty wild to imagine outer space having "air resistance", but that does make sense with such a large structure. Several hundred years to travel to a different star rather than their chosen system should be survivable for those in hibernation; the cryopods can sustain them indefinitely as long as they have power and maintenance. Only they would need that from humans eventually; I imagine a small number of crew members would have to make the sacrifice and stay awake to keep the ship maintained and on course to the second star. $\endgroup$ Jan 26, 2023 at 21:03
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    $\begingroup$ PBS spacetime did a video on space "air resistance" in this video: youtube.com/watch?v=wdP_UDSsuro $\endgroup$
    – DarkNeuron
    Jan 30, 2023 at 9:10
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  1. Probes and spaceplanes are placed to run engines and decelerate ship. Hydroponic water is used as fuel for spaceplanes.

  2. Fusion reactor with double needed fuel is McGyvered into 3d printed catapult device to hurl any mass that can be torn loose from the ship in their direction of travel as hard as it can be hurled, to slow ship. Hopefully cryostasis passengers are not in the "torn loose" category but desperate times... Maybe they can be retrieved later?

  3. Gravity assist deceleration.
    https://en.wikipedia.org/wiki/Gravity_assist

Voyager

Depicted- Voyager's path, using various planets to increase velocity using gravity assist.

A gravity assist around a planet changes a spacecraft's velocity (relative to the Sun) by entering and leaving the gravitational sphere of influence of a planet... To decrease speed, the spacecraft approaches the planet from a direction away from the planet's orbital velocity – in both types of maneuver the energy transfer compared to the planet's total orbital energy is negligible

The incoming ship would use outer bodies in the system to shed energy.

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    $\begingroup$ Hmm, that's a good idea indeed. A good thing about the Upsilon Andromedae system is that it has four very large gas giants, so some good bodies for a gravity-assisted deceleration. As for jettisoning the passengers, I would imagine anything thrown out at 0.50c is gonna get shredded by the interstellar medium, so they're probably just throwing out equipment. $\endgroup$ Jan 25, 2023 at 20:48
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    $\begingroup$ Wake them up and chuck the cryo chamber? They have enough food not to starve. They will probably be glad not to have been chucked with their chambers. It would be cool if some of them had additional new ideas - like aerobraking in the gas giant as Zeiss proposed they do with the star. Also some additional bad ideas. $\endgroup$
    – Willk
    Jan 25, 2023 at 21:13
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    $\begingroup$ They've got 0.5c to shed. The whole reason for the laser beam is that this is impractical to get by throwing part of the ship away as reaction mass. If you can achieve an effective exhaust velocity of 100 km/s (pretty impressive for a jury-rigged mass driver slinging random objects), you need a mass ratio of 2.8e651. There's only about 1e80 atoms in the visible universe. And a gravity assist can only change your velocity by some fraction of the planet's orbital velocity...you need around a hundred thousand times that much. $\endgroup$ Jan 26, 2023 at 0:59
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    $\begingroup$ And the faster you're going, the smaller the benefit you can eke out from a gravity assist, because gravity assists are about bending trajectories and velocity vector addition. It's pretty much just stellar remnants that have the gravity and density to bend something coming in at 0.5c relative to any noticeable degree, and then you've got to deal with wicked tides and general relativity, and still have the aforementioned orbital velocity limitation. $\endgroup$
    – notovny
    Jan 26, 2023 at 11:06
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    $\begingroup$ Gravity assists can, at most, change your velocity by twice the planet's orbital velocity. If you're coming in at 0.5c, even a "hot Jupiter" won't slow you down very much. $\endgroup$
    – Mark
    Jan 26, 2023 at 22:19
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If there were any viable plan B it would have been plan A in the first place. There are a few cases where a partial propulsion failure doesn't doom the mission (a recovered Falcon 9 can lose a first stage engine and still claw it's way into orbit--but at the cost of expending the booster) but there's never a backup for a total propulsion failure.

The only energy source even in the ballpark of stopping your runaway ship is antimatter.

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If you're set on hard sci-fi, and only want to use technology currently available, with no technobabble references, then your two best bets would be:

  • Print your own Electric Sail. This basic idea is similar to how Xysticus spiders fly. Out to the strength of the cable, the ship could just continuously print electrically conductive wire and spin the ship to get them to radiate out in every direction.

  • Print your own Magnetic Sail. Your craft apparently has fusion generators, so we're already somewhat outside of "currently available" so the commonly held superconductor requirement necessary for the large currents might not be a huge issue. If the warning was early enough ("we were supposed to have regular updates for 85 years of transit, and they stopped") then you might be able to take action earlier. Another option might be to print multiple expanding radius loops trailing behind the ship, or to print a helical pattern with an ever expanding radius (kind of a horn shape to enhance sideways deflection of the bow shock).

  • If either of the above were deployed early enough they could interact with the interstellar medium long before you reach the star. The interstellar medium's not moving very fast, yet the ship is. So you've got a bunch of particles that from the ship's perspective are moving at 0.5c.

  • As a note for acting early enough, most missions with humans would probably not have them go to sleep and have nobody check on the status during the entire 85 years of transit. Even if it was just, we wake 5 people up for a week every year, just to make sure nothing horrible has gone wrong. Very few manned missions with 10,000 human lives at risk, and the enormous financial risk, would ever have such enormous time-frames without at least a status check. If anything, would probably keep a socially-stable number of crew members active continuously. You also posit that there is a hydroponics system capable of feeding 100 crew members on-board. You don't leave a hydroponics system alone for 85 years. Plants can be really finicky over 100 year time-frames.

  • In some ways, having the status updates might be better for story. Then there's the possibility for gradually, but perceptibly increasing tension.

    • "Status from Earth's OK"
    • "Status from Earth sounds like there's some social issue"
    • "Status from Earth sounds really bad"
    • "Status from Earth sounds like a holy-**** situation"
    • "No status from Earth, maybe they'll call back?"
    • "Uh oh"
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They can't stop after a total failure, but a partial failure is doable.

The ship is already packed with the most advanced breaking system mankind has ever made to bring themselves down from 0.5C to re-entry speeds... there is no chance that they have anything on board that a few hundred crewmen can MacGyver to replace what took the best of Sol's industrial and scientific capabilities to make. If another tech existed to stop them that were simple enough to improvise on the spot like that, they would have planned on using that instead. That said, if they can mostly stop before or after something goes wrong, that opens up all sorts of options.

There are many possible reasons the beam might not fully slow your ship down. Maybe it drifted off course and could not get back into position before the beam arrives; so they miss part of thier deceleration window. Maybe one of thier sails does not deploy properly and they don't get enough reflection. Maybe everything is going smoothly, then something damages one of the sails. Maybe Sol had to stop transition early. Whatever the case may be, they did not get a full 6 months of 1G deceleration.

So instead of having to make up 150,000,000 m/s of deceleration, they might need to only make up 10 million or 1 million m/s or 100,000 m/s just depending on how compromising the problem is... the good thing here is that you are no longer bound for plot reasons to stick to relativistic problems, and can instead just define how much you are off by based on how you want them to fix the problem.

This way instead of trying to pick the most feasible solution for a 0.5C slowdown, you can mix rule-of-cool with hard science and figure out the most dramatic way to stop the ship, and then work backwards from there to figure out how much speed you need to compensate for to make it happen. So if you want to go for maximum drama, you could add up everyone's ideas into one super Hail Mary: jettison the cryosection to reduce weight, use your bots to turn your solar sail into a mag sail, bounce off the sun while depleting your fusion fuel powering the mag sail, then do a gravity assisted back flip, snap the bad guy's neck and save the day! ... or something like that ... but my point is you can add all of this up and still be in at way less than 0.5C, but that is okay because once you figure out how much all of your stunts should buy you together, then you can just fill in the blank for how much the initial slow down failed by.

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    $\begingroup$ Another plausible partial failure mode is one of the accelerator mirror - the amount of precision it needs to accurately reflect the beam to the space craft (especially in the latter parts of the deacceleration phase where they might be most than a light year apart) is simply insane - it's quite plausible that it loses alignment $\endgroup$
    – poncho
    Jan 26, 2023 at 21:39
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If they have a magnetic scoop such as you have in a Bussard ramjet to supply matter for the expected beam, then that would make a good brake against the incoming solar wind. But it may only have been designed as a scoop for interstellar hydrogen, and not be capable of taking the stresses for real deceleration. They might try stiffening the craft - maybe adding a rotation to stiffen the scoop, and taking the magnetic field beyond the design limits. The might overshoot but still end in a very elliptical orbit, so the braking may take years and many overshoots.

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Here is an "as well" strategy to possibly be employed alongside the measures in other answers.

One strategy that might be interesting is to launch one of the probes or space planes ahead of the main ship (accelerate it slightly so it pulls ahead). Angle it for collision with an asteroid. Then raise the mainsail and triple check all of the heat shields, its going to be a big bump. The probe collides with the asteroid at 0.5c, que an explosion that dwarfs any nuclear weapon ever built. The main ship is some distance away (the exact distance would be easy for them to control by deciding how much the probe pulls ahead) and catches the blast wave on the sail from the closest possible safe (survivable) distance.

Depending on details (that for the story can be handwaved) the puff of vapour gas from the asteroid+probe might be even better for slowing down than the radiation blast. I could imagine a scenario where you do 3 such slowdowns. The first with the probe leading the ship by a long way (so the gas/dust cloud is very diffuse when you hit it and you don't burn up), but on subsequent hits you put them closer because you are going slower and can survive a thicker tempary atmosphere to slow down in.

This strategy is nice for a story because it gives several moments of tension/drama (one for each slow-blast), and larger fragments of the asteroids colliding with the ship provide a convenient plot explanation for any ship machinery to break (as needed by the story). It also just "looks desperate" in a way I think matches the situation nicely.

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    $\begingroup$ There's a serious problem in this answer, namely the ship's crew needs to detect the asteroid first. When they'll be reaching the destination star system, there should be a plenty of them in the local Oort cloud, comets or what, but at BEST this ship would be reaching it at 0.4c, at worst at 0.5c minus a little, this raises the detection problem seriously. You can't see a something until the reflected light comes back at you, and that light you'll have to emit yourself. Yet there's a point, if you can't meet a gas cloud, you maybe can make one. $\endgroup$
    – Vesper
    Jan 26, 2023 at 13:44
  • $\begingroup$ @Vesper Detection is a good point. That might be tricky. I think the best handwave for that is that these future people must have some kind of device to see Oort cloud comets etc., because plan A already (presumably) included avoiding them. $\endgroup$
    – Dast
    Jan 31, 2023 at 12:36
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If they have an energy source up to the task, they could split their spaceship in two halves (or any ratio) and install a laser in the front half.

The front half of the ship would then cast the laser backwards, speeding itself up further, but slowing the back half down.

While it means losing many resources for the settlement, the improvised laser only has to be half as strong as the one on earth (or less, depending on how much mass you sacrifice into the front end). Depending on that ratio, a later start of the braking phase (allowing refitting the ship and setting up the laser) may also become viable as there's less mass to decelerate.


Slightly off-topic, a reading tip: check out "Tau Zero" in which a colonization vessel employs a "buzzard ramjet" engine (scooping up interstellar hydrogen and igniting it magnetically, i.e. the faster you go the more power you get). Their decelerating drive becomes damaged mid-journey.

Now they can't turn off the acceleration drive because the moment its magnetic field shuts down, any speck of interstellar dust to annihilate the ship, so they keep accelerating, planning a slingshot around another galaxy (time dilation allowing them to experience the journey) planning to repair the drive in the high vacuum between galaxies...

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  • $\begingroup$ Re Tau Zero - as I understand the concept of Bussard ramjet, you can leave the magnetic field scoop on while deactivating the charge-up-and-accelerate part of it, that would make the ramjet a brake, at effectiveness of about 5-10% of the solid shield like OP has on his ship, the size of the ramjet's field. Also the heroes in there could turn the ramjet the other side forward mid-flight, either in intergalactic void or somewhere within the galaxy, if they'd makeshift a rather simple heat shield to withstand interstellar gas pressure while the ship turns. $\endgroup$
    – Vesper
    Feb 2, 2023 at 14:42
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    $\begingroup$ In the book's universe, the ship had dual drives (one on each end) and somehow the magnetic field was generated outside of the ship by the buzzard process itself (or so it was explained; with the forward one damaged, they could neither stop accelerating nor rotate the ship too far from its heading), conveniently explaining why they wanted to fly a huge extra-galactic loop. The story also needed the self-fed magnetic fields as a plot device because that removes any limit on how strong they could become as the ship gets ever closer to light speed (referred to by the ship's crew as "Tau Zero") $\endgroup$
    – Cygon
    Feb 4, 2023 at 22:37
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I might be way off on the physics here, but... How about the skeleton crew realise they've got very few options, so use the robots to almost completely repurpose the ship to become a heat based decelerator. That is, using the fusion engines to produce as much heat as possible and to push that heat out of the front of the ship as efficiently as possible. If you can spare any hot gases or particles, then all the better. You're essentially making a rocket engine out of what you have available.

The trouble is that to do this and stand any chance of decelerating by even half of what's required, the cryopods all need to be deactivated, waking the entire human compliment. There's no way they can run all the leisure facilities or lights, not least because all that space is being used by the repurposed equipment. Those humans now need to live in cramped, utilitarian surroundings, possibly under a pseudo-military leadership, eating nothing but rations and yet working every day towards efforts to decelerate the ship. Life for the next six months is very hard.

As others have mentioned, you can repurpose the things you have to do part of the work - a large sail to catch solar wind, perhaps a "fly by" to catch a bit of gravity too, all of which add to the unpleasantness (and length) of the trip, and the sense of hopelessness as you sail right past the proposed system, back into the cold empty space beyond as you decelerate further to come to a stop, before turning around and cruising to the final destination. Of the 10,000 original compliment, maybe only 1000 survive (or whatever). Family members are cremated in the ships engines to aid the deceleration efforts, all the memories of Earth that they brought with them are consumed leading to future traditions and cultures on the destination planet etc. On arrival they don't have all the pleasantries they hoped for, so have difficulty even getting to the planet surface, let alone "setting up shop". etc.

In summary, from the moment the skeleton crew wake up, no decision is easy, no task is as-designed and all bonds with Earth are gradually further and further severed, even to the point of not being able to send the "we've arrived" message back. Some want to re-establish relations with Earth, some don't, tensions simmer, etc.

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