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Apologize in advance if this sounds too much like a story-related question, but I'll try and limit it to logistics-only as much as possible.

I have a spaceship that's travelling to a colony planet with its passengers in cryosleep. The aim is for them to miss their destination and crash-land on another planet instead. To that end, I'm wondering exactly what could go wrong, mechanics-wise, within the ship to throw them off-course.

For reference, the planet they land on is about 5 light-years away from their intended destination. Does a glitch in the navigation system make sense? There are passengers in cryosleep but I'd assume that a colony ship like this would have a skeleton crew awake at all times on a rota basis to keep an eye on basic tasks and repairs etc.? Perhaps an issue with deceleration where they couldn't stop at their intended planet and had to go on to the next habitable planet?

What kind of mechanical error could make them miss their destination and land on this other planet?

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    $\begingroup$ A mechanical error would most likely leave them drifting through interstellar space until they reached intergalactic space and then they'd drift through that. I'd suggest not an error but a deliberate act of sabotage to send them where someone else wanted. $\endgroup$ Commented Apr 12, 2020 at 18:40
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    $\begingroup$ @PeteBecker Also, there's Abbott and Costello go to Mars where the two thought they were going to Mars but actually crashed on Earth. Not that they realized it, of course. $\endgroup$
    – Spencer
    Commented Apr 13, 2020 at 18:18
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    $\begingroup$ Despite what movies have told us, Space+Crash=Death, the energy involved is just too great, nothing can survive crashing on an unintended planet. A more realistic scenario would be being forced to land because of mechanical difficulties. $\endgroup$ Commented Apr 14, 2020 at 1:33
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    $\begingroup$ @RBarryYoung great point, and I'm actually fine with that as well. Being forced to land works even better for my story $\endgroup$ Commented Apr 14, 2020 at 13:52
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    $\begingroup$ I'm surprised that the film en.wikipedia.org/wiki/Pitch_Black_(film) hasn't been mentioned. $\endgroup$
    – Drise
    Commented Apr 14, 2020 at 19:58

19 Answers 19

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Failed slingshot maneuver

Space vehicles even now frequently use a gravity assist technique to gain velocity toward their target. The Galileo probe used 5 such maneuvers (2 around Earth) to get to Jupiter.

But this requires a burn of precisely the right intensity and length at precisely the right time in the maneuver.

So, if some mechanical issue causes the burn to not go off perfectly, the ship might burn up in the planet's atmosphere, or be flung off into space. People on board may be able to survive by landing (or crashing) on the planet itself.

This happened in Larry Niven's story "At the Bottom of a Hole" where a Belter smuggler tried to slingshot around Mars to get into a desired trajectory, but ended up crashing on the planet instead. The rest of the story involves him trying to reach a failed Earth colony on Mars to get some survival resources.

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    $\begingroup$ Good answer. To elaborate a little, a slingshot is more effective the closer you get to the body you're slinging around, so there is a logistical reason to get as close as possible. But if you get too close to the planet, you run the risk of deceleration due to atmospheric drag, or if there is no atmosphere, you might get so low that you accidentally lithobrake on a tall mountain. I've done that in KSP when trying to slingshot around the Mun, without taking into account local topography... the mean planetary radius + 1 is not a safe orbital altitude! $\endgroup$ Commented Apr 13, 2020 at 15:15
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    $\begingroup$ Though, if you are in a slingshot around a planet you have quite high velocity already, which means a crash on that planet is not likely to be survivable. $\endgroup$ Commented Apr 13, 2020 at 23:58
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    $\begingroup$ It's more likely that the ship is conducting its gravity assist around a different planet than the one that they actually land on. The computer reaches its "outside parameters" state and metaphorically throws up its hands, waking the crew and giving them enough time to figure out how to land on the planet in a semi-safe manner. $\endgroup$
    – ltmauve
    Commented Apr 14, 2020 at 4:14
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    $\begingroup$ @PaŭloEbermann Pretty much any crash landing will happen at a minimum of escape velocity, slingshot or not. So long as you came from somewhere other than that planet, you'll be going at least escape velocity by the time you hit the ground. For a planet with a thick atmosphere, the crew could have time to accidentally aerobrake, killing their slingshot opportunity, and then be forced to make an unplanned landing as they fall into the planet's gravity well. I don't think the fact that it's a slingshot makes much difference, compared to crash landing any other way. $\endgroup$ Commented Apr 14, 2020 at 13:20
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    $\begingroup$ It doesn't make any sense to do a slingshot for interstellar distances. 'Star hopping' is just not feasible at these colossal speeds, which also means 'colossal time'. The question stated 5 light years as the error distance - I would imagine this alone would ring alarm bells when thinking of a simple slingshot giving us slow speeds and long time frames (I'm talking millions of years plus). $\endgroup$
    – flox
    Commented Apr 14, 2020 at 16:10
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This is less about the mechanical problems that cause the crash, and more about why there was a convenient planet there to crash into, but...

They navigate by "star-hopping"

To avoid exactly the problem of being stuck in interstellar space, the ship's journey is plotted as a series of "hops" from one star to another. This failsafe design ensures that, if something goes wrong, the ship will (eventually) come into a solar system, where it can get the energy and possibly materials it needs to correct the situation. Of course, this assumes the ship is functional enough to enter orbit, or else gets nicely gravity-captured by the target star...

All sorts of things could go wrong to force a ship down on a planet orbiting the star previous to their ultimate destination. Debris strike. Manoeuvring engine failure. A bizarre concatenation of an off-by-one error and a planet that happened to match the parameters of the target.

PS Why don't they just do one "hop" towards the destination star? They'll still get there eventually, right? Nope. These hops have to be relatively short, because of uncertainty in measurements over long distances. It's simply not safe, given the accuracy of the current technology, to fling yourself straight from your point of origin towards the destination star if it's more than, say, half a dozen light-years away. (Also, time is an issue. The difference between "nearest star" and "destination star" is probably an order of magnitude; depending on your propulsion technology, that could be "a year" versus "a decade".)

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What Not to Do

Randomly finding another planet is REALLY unlikely:

From what we know so far, the odds of 2 habitable planets even being within 5 light years of each other seem pretty slim; so, even if your ship can pick any target within that range of the target, you're already looking at something kinda like a 1:1000 chance of there even being a second option that close to your destination.

To make matters WAY WAY worse, if this is a totally random other planet because you lost flight controls, 5 light years means you are off by a margin of is ~4.73e13km. The radius of the gravity well of an Earth like planet is ~1.26e4km. Square those numbers and compound them with the odds of even finding a potentially habitable planet you are looking at about 15 quintillion to 1 odds of randoming running into another sort of habitable world by chance. You have better odds winning 2 games of powerball back to back than this happening.

How to Increase your Odds

Make it a survivable but not inhabitable world

There are a lot more worlds in the universe that are survivable with the right equipment than there are step off the ship and take a breath of fresh air type worlds. If your colonists were already going to a Mars or Venus like world instead of an Earth like one, then they would have all sorts of stuff like O2 generators, inflatable habs, arable soil samples, etc. Everything they would need to survive on a hostile world. Once you do this, then the odds of them finding an alternate world they can make work go way up and 5 light years becomes a reasonable expectation.

Make it a stopping problem

Your ship needs to accelerate and decelerate over VERY long distances. This means you need to make for a situation where it can not stop in time to go to the target. One option is to damage one or more of it's stopping thrusters. This would mean that it can not stop in time to go to the intended world and has to decelerate slower, overshooting its target bringing it to a farther away star system. A second option is that something happened to its fuel supply, and it needs to pick a planet that will be moving away from it instead of at it to conserve fuel and make it not have to slow down as much to match the planet's speed.

Damaged thrusters especially would contribute to why it is not able to land where it is going though, forcing the crash landing. A fuel shortage could cause the same, but would have to be short by a very specific margin to not kill everyone.

enter image description here

A: Original Destination, B: New Destination.

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  • $\begingroup$ Sorry, i should have added this to my post, but the second habitable planet has already been discovered. For the purposes of the story, it's a secret, off-limits world, but through a series of events, our colonists find out about it and decide to venture there after failing the landing on their actual destination planet. $\endgroup$ Commented Apr 13, 2020 at 15:34
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    $\begingroup$ "why it is not able to land safely where it is going" or even get to its original destination. E.g. a fuel container ruptures halfway through a burn (which might last a very very long time with, e.g. an ion thruster. You're now headed at a speed to high to turn around, but too low to get to the original destination in a reasonable time. However, you have just enough fuel to alter course to some alternate destination (but not enough to land "safely"). $\endgroup$
    – Him
    Commented Apr 13, 2020 at 17:40
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    $\begingroup$ @cowlinator, I agree, 1:1000 is believable despite being very steep odds. I bring it up because it is important when rolling loaded dice on odds like that, that the characters should at least acknowledge the magnitude of thier good fortune unlike in Star Trek for example where there always seems to be an M class world just around the corner to crash land on. This also means that if you are writing a series, campaign, or whatnot that doing it more than once instantly defies credibility. $\endgroup$
    – Nosajimiki
    Commented Apr 13, 2020 at 19:17
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    $\begingroup$ @space_cadet the odds are 1:1000 assuming you are in a well charted area of space. If you were to just pick a random near by star and hope for the best, the odds get even worse. $\endgroup$
    – Nosajimiki
    Commented Apr 13, 2020 at 20:29
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    $\begingroup$ @Nosajimiki-ReinstateMonica The funny thing about Star Trek is that its ostensibly a show about space yet nobody is ever wearing a space suit. $\endgroup$
    – papirtiger
    Commented Apr 15, 2020 at 12:12
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We're going to be in the Hudson.

were going to be in the Hudson

https://www.youtube.com/watch?v=JSlbHK07fkY

There is a skeleton crew awake. Something bad happens to the ship. I am thinking of the best scene in Passengers where the colony ship (with everyone asleep) traverses a cloud of asteroids but better is when Captain Sully realizes their plane is going to crash, and so he lands in the river as the best of bad options.

You skeleton crew of 3 realize that the ship is going to fail catastrophically, and soon. Maybe part of the ship has already broken away. They take it down in the nearest place they can find where they and their passengers are not going to die outright from the atmosphere.

When I send out a colony ship I am going to include missiles to break up big asteroids like that in advance of ramming them.

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Currently, when we launch a object towards a distant destination (such as the moon or mars) all of the major trajectory decisions are made before the object starts moving. Additionally, when we launch such an object, it's initial motion is comparatively slow. So to miss a target by five light years given our current scientific understanding, you would not only have to make the error very early in the journey, but also have that error go undetected until the ship reached a great enough speed that pursuit and capture by other vehicles was impractical.

That is not unbelievable in a space-punk genre where super corporations care so little for individual lives that they would write off hundreds or thousands of lives because the cost of the pursuit needed to save them exceeded some financial threshold. However, most space castaway stories are far from the dark halls of the space-punk genre, so writing the launch sequence with such dark undertones might clash with the rest of the novel's writing style.

As an alternative, having the ship's course unavertably altered by some external force, might provide the same effect without the sinister overtones. Perhaps a rogue black hole or other large mass object passed by the ship just close enough to pull the ship off course. Perhaps the ship was struck by a smaller object with just enough force to alter its course but not enough to penetrate its hull. Maybe something on the alternate planet reached out with gravitational or magnetic tentacles and deliberately dragged it off course.

...but those are answers for another question.

To answer the OP's current question, nothing mechanical aboard a ship built based on our current understanding of physics and our current plans for deep space travel, could alter its post-launch trajectory to such an enormous degree.

Following my posting of this answer, I had a short conversation with a much more scientifically minded friend and she convinced me that I was totally wrong in my closing statement. I'm leaving the rest of my answer which I believe is still valid, but am now adding an actual answer to the OP's original question.

Although the major trajectory decisions are made prior to the initial launch, the actual execution of actions born from those decisions can go on throughout the journey. In order to accelerate in space, thrust must push the vehicles to every greater speeds. Journeys may start slow, but they get faster only because some force, either from the engines or upon the sails, push them up to those greater speeds. Mechanical problems in any of those mechanisms, over a long period of time could alter a course drastically. The earlier it happens and the longer it goes unnoticed, the greater the alteration might be; but if the journey is long enough, 5 light years might not be that much an error, relatively speaking.

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The most straightforward answer is "anything that causes you to miss the initial target", which can be just about anything.

  • Thrusters don't fire.
  • Thrusters fire but a micrometeorite punched a small hole in the outlet nozzle that alters the exhaust direction slightly.
  • Thrusters fire but some fuel leaked out or was in the wrong tank, and the thrust ends early.
  • Thrusters fire but the cooling systems partly break down, altering the thrust delivered and/or causing the engine to shut off early.
  • Sensor malfunction, and the ship thinks it runs out of fuel prematurely.
  • Previously undetected mass (especially early on) changes the trajectory slightly.
  • Mass of ship is incorrect. (Maybe somebody smuggled something aboard and hacked the systems to not include its mass?)
  • Any other sort of navigational error.

Anyway, the point is that once you discover that things didn't go as planned, and are too late to course-correct (which is very likely if any deviation isn't corrected quickly; you probably don't have the fuel or time to turn around or even make large swerves), then your only option is to find a new place to land. And you'll have to do it quickly, while you still have time and fuel to course-correct. (Time and fuel trade off against each other -- the earlier you adjust, the less fuel it'll require, but that also means making the call when you're farther away from the target and therefore your telescope may not be able to resolve things with enough precision.)

If you want a more "out there" answer, you could plant a large mass between your origin and intended destination, such that gravitational lensing shifts things around enough that your preplotted course is just dead wrong. Then it's too late to correct, etc; see above.

Or for a more "in the box" answer: the builders screwed up imperial vs metric again, and the calculations are all wrong until it's discovered and corrected for.

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  • $\begingroup$ last paragraph is really funny yet sadly the most likely $\endgroup$
    – Topcode
    Commented Apr 14, 2020 at 14:54
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A computer virus: It sets an alternate destination, but show the original destination as selected to the crew. If there are no or few windows then the view on screen can also be altered. And then if they realize the error maybe it's to late and they have no choose other than go to the new destination. Maybe they try to fix the problem or change the landing location, but that causes a crash landing.

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    $\begingroup$ To add to this idea, the error could be deliberate - sabotage by someone who wants them on that world instead. $\endgroup$
    – SeanC
    Commented Apr 14, 2020 at 20:04
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For a real-world example, consider the Pioneer 11 space probe. For reasons that puzzled scientists for decades, the probe has been drifting slightly off-course. The current explanation for this drift is that one side of the probe is generating more heat than the other. A probe in deep space can only shed excess heat through thermal radiation, and the process of emitting that radiation provides an ever-so-slight push to the probe. When heating is uneven these "push" forces are uneven, which causes the probe to accelerate in an unintended direction and it veers off-course.

For Pioneer 11, the overall course disturbance is rather slight. The probe is small and doesn't generate a lot of heat to begin with. What you're talking about is a much larger craft, one that must maintain an internal temperature warm enough for living creatures to survive in. A craft like that would generate significantly more heat and could receive a much larger "push" off-course. The push would still be imperceptibly small from the point of view of an awake pilot, but over a long trip it could add up to a significant deviation. A ship without attitude control thrusters (or without the fuel to burn them) would be unable to correct this deviation. It could overshoot the destination, or get too close to a planet during a flyby/gravity assist and fail to escape its gravity well.

Other mechanical problems could result in the same sort of slow push over time, such as a pinpoint gas leak. Small errors like these that build up over time are harder to detect, so they're more likely to go unnoticed until it's too late.

Other more-conventional mechanical problems could include:

  • Attitude control thrusters/gyros that are asymmetrical or otherwise unable to adequately control the craft's orientation
  • A navigation system error. Your navigation aids in deep space are guide stars, but an unexpected bright light (like a distant supernova) could be mistaken for your guide star. Alternatively, an unexpected object could temporarily eclipse your guide star, and your navigation system veers off course due to lack of a reference point.
  • A tiny propellant leak could leave the ship without sufficient fuel to slow down to the point where it could land without crashing. Your only option is to skip through your target planet's atmosphere (where drag will slow you down some), drift onward at a now-slower speed towards a remote planet, and attempt to land there.
  • Many long-distance spacecraft are spin-stabilized to avoid drifting. If your spin rate never reaches a high-enough rate, you might not have enough rotational inertia to resist drifting.
  • Some spin-stabilized spacecraft use yo-yos to stop spinning. If one of your yo-yos fails to fully extend (or doesn't deploy at the exact same time as the others), your center of mass will be off center and your own rotational inertia can cause the craft to tumble off-course.
  • There is a discrepancy between the amount of speed that a spacecraft gains during a gravity assist and the amount that our mathematical models predict. This phenomenon currently has no explanation. If your craft's voyage includes several gravity assists, the cumulative effects of this phenomenon could alter your speed enough to change your course unexpectedly.

Some of the most fun mechanical problems are the ones that cause your issue without directly impacting the spacecraft.

  • The clock in the cockpit is an analog dial clock and it slowly loses/gains time. The clock's drift is not really noticeable, but it accumulates over the course of a month's long trip. Your pilot notices the discrepancy compared to his cheap wristwatch, but assumes the cockpit clock is the correct one since it's more sophisticated. The crew makes their entry burn several minutes too early/late, misses the target, and goes drifting through space towards another planet.
  • A mid-flight mechanical problem requires a crew member to replace a hose that feeds your engine or control thrusters. It's a rather routine repair, but the crew member uses the incorrect replacement line which is slightly longer than the original. When you fire up the engine/thrusters in preparation for final approach, the longer line causes that nozzle to fire slightly later than the others. Your thrust forces are now asymmetrical and your course drifts towards the side with the longer hose.
  • Your flight plan involves approaching your destination with that planet's sun directly ahead. Your cockpit's solar screens jam and fail to fully extend, and your pilots have to land with a giant fireball blinding them and half their sensors. Understandably, their aim is slightly off and you miss your destination.
  • As your craft approaches the point where it needs to make its final course adjustment burn, the mounting bracket on an overloaded cabinet fails and it crashes to the floor, blocking the doorway to the empty cockpit. By the time the crew clears the heavy equipment and regains access to the ship's controls, they've missed their turn and don't have enough fuel to correct their course.
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  • $\begingroup$ add on to the 6th bullet, the spacecraft attempts to correct it to stay on course but fails and gets further off course but thinks t is on course causing all sorts of bad things to happen and navigation shuts down and the skeleton crew wakes everyone and crashes into a planet near the one the gravity assist failed on, or if its a gas giant a moon of the planet. $\endgroup$
    – Topcode
    Commented Apr 14, 2020 at 15:00
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Danger, Will Robinson! Danger!

The extra weight of a stowaway (Dr. Smith) causes the spaceship (Jupiter 2) to go off course. The damage caused by the reprogrammed robot will require the stowaway to awaken the crew and land the ship on different planet.

That was the basis for the (original) TV Series Lost in Space (1965).

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Emergency Hyperjump

The ship has experienced a sudden loss of fuel and cannot maintain the cryopods if it heads to where it was programmed to go. Everybody will thaw out and die.

The ship uses a large share of its remaining fuel to calculate a hyperjump to the best possible planet it can reach on the charts and activates the emergency transponder.

Only once it has landed (which it does badly with the minimal amount of fuel available) the crew awake to find themselves on the wrong planet.

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Emergency course change

Here is another very unlikely szenario: Even when mass in space is sparse, it is theoretically possible to encounter asteroids, meteroids or even a roque planet, that are undetected/unknown and require a course change of your spacecraft. Changing your course in space is hard [Citation Needed], so your spacecraft may need a lot of energy/fuel to change it's direction (and back).

This idea is based on the Film Passengers(2016)

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So, I'm going to expand on Spencer's answer of "poor gravity assist."

The ship is traveling through this system where it crashes because it is intending to perform a gravity assist off the system's primary

This explains why your ship is close enough to the star in the first place. Space is mind-boggling empty. You may think it's a long way to walk all the way around the world on foot, but that's just peanuts to space. Unless you aim for a star, you will not hit it. Look up at the night sky, and even in the darkest nights there's so much space there isn't stars.

(This brings up huge collision and radiation risks, but just go with it. If the ship isn't a torchship or other sort of vessel that we'd dub "realistic" then it might have its own operating principles that would allow it to travel to this star.)

Something goes wrong well before perihelion

The problem must happen well before the closest approach to the sun. The earlier you make a burn, the less delta-V you have to spend.

I recommend that there is a problem with a burn intended to put the ship into a course correction around the system's equivalent of Neptune - a large body on the edge of the system. This gravity assist is intended to allow the ship to adjust its course to make the solar gravity assist.

The system is a binary system, or it has a hot jupiter

A ship intending to go through the system and reach another one should be moving fast. In order to slow down without horrible G-forces, it will want to use something almost as fast. Gravity assists, to quote Randal Munroe, are "like bouncing a ball off a truck." If you come up behind a hot jupiter and "bounce" off it, you will lose a lot of speed (while imperceptibly speeding up the planet.)

51 Pegasi b, the first hot jupiter (and first exolplanet) found orbits its primary at 136km/s. (though if the ship is moving at that sort of speed, it will take thousands of years to get to the target star after this. Cryosleep, indeed, and there wouldn't be a skeleton crew awake at all times. You'd wear out the passengers before you got anywhere.) Some binary stars may have higher orbital speeds, but I would stick to something like AR Scorpii, where the B star is a white dwarf instead of having to fly between two giant fusion reactors.

Hot jupiters and fast-orbiting companion stars also have the advantage of being only slightly off from the primary, which means recovering to them is more plausible than other locations in the system.

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The ship's spectroscope has been pointing at the wrong planet all the time

The colony ship has many redundant systems on board to deal with unexpected circumstances. One of them is a spectroscope that keeps track of the target planet by its water signature.

Unfortunately, during configuration and testing, an engineer had trouble calibrating the system and used the other planet (which is very close to the target planet when seen from the starting point) to more easily fix it. This testing configuration was never switched over to the real target planet.

For a long time, nothing seemed amiss as the deviations between the navigation data sources were within expected parameters, but when they did conflict, the computer gave priority to the data from the spectroscope and adjusted course.

When the crew finally discovers the error, the ship is already too low on fuel to re-route or even land under power.

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In many current space maneuvers (Apollo) most thrusters are switched on and off by computer control. In addition, there is usually a pilot standing by to activate the thrusters manually if the computer control fails to activate or shut down correctly.

Manual control is, of course not as precise. Many maneuvers have a mid flight course correction planned. Any inaccuracy could explain an altered course, as could a black hole. A hyperdrive could shut down when encountering an unexpected black hole, or a black hole’s gravity could alter the course.

You have to assume some reason for not detecting the altered course, or not being able to correct the course.

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Two versions out of the mind:

  1. Ship's frame was slightly bend due assembly error, so instead of straight line the ship traveled via huge-radius curve during the hyper-jump
  2. Input controls issue: let say we use limb-like rotatory disks to set the course so even small imprecision could change the destination on the long run (and gravity traps can make it even worse then)
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You cannot 'crash-land' on another planet

Space is not like the ocean where you may encounter land if you are adrift or off-course. It is incomprehensibly bigger, three-dimensional, and almost completely empty, with you travelling at deathly speed.

A reminder: a 5 light year distance = 47300000000000 kms.

To reach your destination in 1000 years your speed = 5399543 km/hr

And that's for a 1000 year journey

Trajectories, in particular over very large distances, must be incredibly precise. The distance between stars (and also planets) is unfathomably huge.

Many answers here concentrate on methods for spacecraft to be adrift or for there to be an error in calculations. However, even if this is the case, the spacecraft will not 'crash' on another planet - the odds are incalculably small due to the following factors:

  • the colossal distances between star systems
  • the massive distance between objects in a star system
  • the enormous speeds the craft would be travelling to reach one (even a journey that lasts a 1000 years your craft will be travelling at extreme velocities)

Whatever scale your are thinking of here you need to rethink it - we are talking really really really enormous numbers here.

These factors alone mean that journey's must be precise to the utmost degree, let alone the slowing down of velocity to safely 'land' on a planet (a mammoth task even at only interplanetary speeds, let alone interstellar ones).

The only likely scenario where this would occur for your story is the AI in charge of the mission making an intentional decision to land, very precisely, on another planet in a star system 5 light years from the intended destination (but it must have enough fuel to alter trajectory precisely given the enormous speed the craft would be travelling already, thus meaning the mission planners would include this ability prior to launch - meaning it is not, and never could be, an accident).

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Ship ran out of a critical resource

While in flight, a tank of oxygen, cryosleep coolant, reactor fuel, engine fuel or some other important consumable resource ruptured and spilled its contents into open space. Problems like that tend to happen during spacetravel.

There is not enough left to last until the destination. So the skeleton crew needs to find an alternative destination immediately. Their options are to die in space or try their luck on a barely habitable, unexplored planet.

Why would they crash-land instead of land properly?

The damage which forced them to cut their voyage short might also prevent a proper landing. For example, they might have barely enough fuel left to deorbit the ship, but run out a couple seconds too early. Or the exploding tank might also have damaged the landing gear.

Or the ship wasn't designed to land on a planet at all. The plan might have been to deliver the colonists to a space station in orbit and return to Earth. The colonists would then get shuttled down to the surface with a different vessel which only travels between the station and the surface. Two specialized ships might be far more economical than one ship which can do both interstellar travel and planetary landings.

Or perhaps the ship is capable of landing, but not on that kind of planet. It might have a higher gravity, lots of micrometeroids in its orbit, a denser atmosphere or some other hazard the ship wasn't designed to handle.

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What was not what happened

Since you are specifying that it is a mechanical issue, I understand that it is not:

  • a software error/bug.
  • a sabotage.
  • an accident caused from someone screwing up the things.
  • someone who wrote the wrong number on some file and nobody noticed.

At least, it is not any of those in isolation. Those might happen only if there was indeed a mechanical issue.

Further, I think that a small calculation error that accumulates over time is not a reason for that. They surely would have computers evaluating the route continuously and they would alarm when they notice the slightest mismatch, and it would be soon enough for the correcting maneuvering to be very slim and done with no problem. And no doubt, the ship would have enough extra fuel to be used in the case of something going bad. By the way, New Horizons did a lot of correcting maneuvers in order to be able to visit Pluto and Arrokoth, so no doubt that a much more advanced civilization with a manned ship would also do.

The odds of crashlanding and why

Let's say that the ship was coming from A, targeting C and nearby the star B which hosts the planet D.

As other answers notes, since space is very empty and distances very long, the odds of randomly crashlanding somewhere are absurdly slim, so it must have a reason to pass near that planet, so it was not very random at all. A gravity assist from B is (as many other answer notes) a good candidate for being a place where the ship deviates. Otherwise, it would have no reason to go to "the wrong star" and much less "to the wrong planet". So something goes wrong when the ship passes near B and it crashlands on planet D.

Also, even if/when the ship happens to crashland on D, it is surely not a random event. Even if B happens to be a small red-dwarf star with several planets in close orbits (like TRAPPIST-1), space is still huge and mainly void.

What happened

There are many things that might go wrong in a gravity assist without destroying the ship nor sending it to be lost in deep outer space. My best candidate is this:

The ship is struck by a micrometeorite near B, and it damaged some important part of it, making it impossible to finish the journey to C.

So, they are in one of those situations:

  • (a) the ship is in the eminence of being destroyed and must be repaired ASAP. They did some provisional duct-tape and glue repair, but it won't last long enough.
  • (b) the ship is in a wildly off-course uncorrectable trajectory, so they must either land somewhere nearby or get lost in deep space forever without anywhere to land.
  • (c) the cryosleep system or some other crew-survival system is damaged and nobody would be alive when the ship reaches B.

So the crew must do some emergency maneuver. Concluding that it is impossible for them to reach B without fixing the ship and that the ship can't be fixed without landing, they decide to land into the nearby planet D.

But, a soft landing is impossible because the micrometeorite damaged the part needed for proper landing. However, they can still maneuver the ship into a crashlanding (as long as it doesn't destroy the ship), but that would surely add further damage to the ship.

After landing, hopefully with all or at least part of the crew still alive, they can fix the ship, wait for a rescue or make that planet their new home.

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Micro meteorite damage - say the fuel tanks were punctured and the ship's computer determined it won't be able to get to the destination. Or it would get to the destination but it will not have enough fuel to slow down and land. Then the computer finds the closes habitable planet and redirects the ship there.

Same thing but instead of meteorite, it turns out the interstellar medium is not as expected. For example, the ramjet engine was unable to collect enough fuel. Or it was too dense and thus more fuel is need than expected.

Human error. The coordinates were entered wrong, for example a digit was transposed.

Computer problem. This is a new model ship and unfortunately there is a software bug. This could be anything - an unit conversion gone wrong, memory leak crashed the computer and caused the computer to lose the target coordinates, the AI went rogue and decided to go exploring.

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