# How would you have a space ship set out on a journey to a distant planet, but find themselves arriving back home without realising it?

I've been thinking of doing a kinda riff on Planet of the Apes, where a space faring civilization sets out to colonize the stars only for one of the ships to wind up back on the home planet.

I like the idea of the round trip happening in geological time (like hundreds of millions of years) so that by the time they return the remnants of their civilization are not obvious, and in fact don't get discovered until the final act in an archeological dig or something like that.

I'm not sure how to contrive the return in such a way that it is not obvious to the returning craft. My first instinct is that maybe the ship gets cast a-drift somehow. Perhaps a miscalculation sends the ship ever so slightly off course leading it to drift in deep space in a low-zero power mode. I could imagine it drifting long enough that any internal clock may overflow an indefinite number of times before the ship approaches near enough to any star that it might recharge it's solar cells. In that time perhaps the navigation systems get corrupted, perhaps it uses time to measure how far it has traveled? But that seams a little contrived.

I figure that if we suppose enough time has passed that a whole new set of species are thriving on the planet, then any recognizable astronomical objects may not be in positions that the crew recognize them once they are awakened from stasis. Are hundreds of millions of years too short a span to suppose the solar system would look sufficiently different to fool trained space farers?

Edit:
So many really cool answers! I'm partial to the orbital mechanics solution, that feels subjectively nicer than any of the universe is actually curved solutions. Not because those are bad, just because you need such a small universe for that to feel like the right solution. I'm really torn about who to award this to, because the orbiting the galaxy idea is great in my opinion, fits the time scale I need perfectly!

A couple of issues seam to crop up about the details of the ship and how the navigation computer could last so long. Honestly I hadn't settled on anything there yet, and was toying with ideas along the lines of the ship is a (relatively) tiny object, just a collection of high density information stores and a couple of nano-machines. Small enough to not need astronomical amounts of energy to get reasonable fractions (like $$0.01 \space c$$) of light speed. Basically the ship would rebuild the crew and survival units from available resources on the planet it lands on, without having to transport them physically (of course they were scanned pre-travel with memories etc).

Either that or it is a standard issue large hulled ship full of frozen people, over engineered to the nth degree. Such that the original designers had a round trip of 100 000 years in mind but used a safety factor of 1000 for everything. Because if a thing needs to be frozen for 100 000 years, what are a few 100 million?

### Totally plausible if you lose your navigation computer - space journeys aren't straight lines!

As much as star-trek and the like may make us believe: Space isn't a big open 3d region where you just pick a direction and head that way and get there. If space was that simple, it would be basically impossible to come back to your point of origin unexpectedly like you want in your story. You'd need wormholes or a curving universe or some bizarre malfunction.

But long distance space flight isn't simple 3D cartesian space, 2 points, straight line, done. It's a network of curves and ellipses.

Much of the universe is essentially best modeled as network of things in orbit around each other. A ship orbits a moon, which orbits a planet, which orbits a sun, which orbits a galactic central point, and there's probably more above that with the galaxy orbiting local clusters or some higher level barycentre. All moving in circular paths.

All of these objects are in circular or elliptical paths relative to their parent or child objects. Baring FTL drives, the most fuel efficient path through this system is almost always also one of curves and ellipses.

The path that 20th century space probes take to travel around the solar system will resemble the paths your ships will take between the stars. Your ship will be travelling through a curved journey its entire time, it will almost never be "facing the destination".

In this example journey; what does the purple ship see out it's front window?

Your destination is behind you half the journey!

In this exact scenario, the purple ship enters the green objects gravity well, but if they miss it (or arrive a few minutes early or late), the purple ship will continue on the ellipse it's currently on, which if you trace it out, will return to the blue ring.

Now, someone is supposed to be navigating, and some computer is supposed to be intersecting these ellipses to plot the course. Not everyone knows orbital mechanics, and they're very counter-intuitive, many people plausibly think "face destination, thrust, arrive." If some major disaster occurs on the ship, and all the adults and their experience and knowledge is lost, the ships navigation information is lost, and the teenage children have to step up and save the day (you know - the setting for some teen bestseller book), it'd be plausible for them to repair much of the ship, to locate their destination's star correctly using telescopes, (or even using charts and sextants), orient the ship towards it ("Back on course!"), apply thrust, and assume they were back on their way.

So if the ship's teens try to correct their course here by accelerating toward the green object along the blue arrow:

The ship will pass behind the green object. If the green and blue objects are distant stars or galaxies even they might not ever realise they've undershot, and even if they do, they are unlikely to be able to correct it that late due to the extreme delta-v requirements.

The incorrect orbit will eventually intersect with the blue objects orbit, eventually returning them to their point of departure.

TL;Dr: Rule of thumb: If you turn towards your destination planet/star/galaxy and apply thrust, you're probably going to miss it.

• Very good answer! The gifs really helped me visualize it! – fartgeek Feb 24 at 14:46
• If I read OP question right the ship was not design to have a travel duration lasting 'geological ages'. If a ship is capable of interstellar travel (even if under FTL) its acceleration capabilities would likely make acceleration through slingshot manouver (using gravity wells to accelerate like we use today to accelerate our probes) most likely irrelevant. I do agree on the trajectory being curves and not a straight line though. – Duncan Drake Feb 24 at 17:32
• I almost wanted to down vote because of the young adult fiction premise, but then realising that they actually ruin everything by interfering fills my cynical heart with joy. – Andrew Micallef Feb 24 at 20:10
• You can determine your position in space from star charts and a sextant, much like seafarers have done for hundreds of years. Even without a navigation computer, it strains credibility to suggest highly trained and disciplined astronauts couldn't determine where they were. – stix Feb 24 at 21:44
• @stix you mean the star charts on the nav computer you just lost? But sure. Lets say they were printed out or something, the ability you mention is kinda key to the screw up. I expect them to be able to find where they are and their destination and use that to set a path towards it. That's the problem! "Towards it" in space is almost always the wrong way. – Ash Feb 24 at 21:56

Note: I have edited this answer to remove the need for explicit maths.

The solar system is orbiting the centre of our galaxy. Suppose the travellers set off towards the opposite side of the galaxy with the intention of exploring the local area when they get there. While they are travelling across the galaxy, the entire solar system is at the same time travelling around the galaxy. By a terrible stroke of bad luck the two orbits coincide.

By adjusting the trajectory of the spacecraft, you can adjust the time before it arrives at Sol again all the way from zero years to 230 million years*.

Why didn't the astronauts realise this would happen?

Because it is very unlikely to happen. However it could happen through sheer bad luck.

A suggested plot:

Theorising that anywhere is as good as anywhere else, they spin a bottle and tell the computer, "Head thataways for 100 million years and wake us out of cryo-sleep when we arrive at a habitable planet. We don't care where we end up as long as it is habitable for humans."

The computer starts off in "that direction" accelerates up to speed and then switches off the main engines. It keeps a look out so as to avoid obstacles and makes suitable corrections when necessary. However space is pretty empty and a collision with a star is highly unlikely.

Meanwhile the solar system continues on its way around the galactic core. By an incredibly unlikely coincidence, they are heading for the same spot at the same time - they are on intersecting orbits! These orbits will coincide in 100 million years!

After the required time, the main computer wakes up and starts looking for a nearby habitable planet. Surprise! There is a solar system nearby with a perfect planet for the job! It automatically puts the mother ship into orbit and sends the cryo-sleepers to the surface in the transit vehicle where they have been stored all this time.

The explorers wake to a beautiful sunrise - it looks a lot like the Earth that they remember! Of course, at night the constellations will be completely different because the component stars are orbiting the galaxy at different rates.

At some point, they realise what has happened - D'oh!!!

*The solar system takes 230 million years to orbit the galaxy centre. Even half of this is plenty of time for big changes to happen on Earth. Under the scheme I suggest, you can choose any time from 0-230 million years!

• @Matthew - I understand. However the crew don't. They have come to rely on computers so much that they no longer have to think for themselves. They could just say, "Go thataway, and wake us in 100 million years." Their intention is to come out of hibernation in a brand new place where they can explore locally. The computer doesn't correct them - it calculates their destination as Earth but assumes that that they want to get back to Earth in the future. This is a way of glossing over any complicated maths and using it as a plot device.... – chasly - supports Monica Feb 24 at 14:42
• ... They don't have to travel in straight lines and we don't need to know the actual path they follow. – chasly - supports Monica Feb 24 at 14:46
• @Darrel Hoffman (1) My version allows any length of time from Zero to 230 million years depending on the initial speed and direction. (2) Someone would notice if Ash's craft was circling the solar system for millions of years (3) The OP wants geological time. Even if the navigation system fails in Ash's craft, how do you explain the perfect working over millions of years of the ship, its air conditioning, waste disposal, and of course the people for that length of time? – chasly - supports Monica Feb 24 at 18:45
• As I commented on the OP, leave it too long and they simply won't recognise earth. The irony is, they might recognise the moon and other planets, though. – Stilez Feb 24 at 22:43
• @Stilez Would they though? It's been theorized that the Moon is an essential component of Earth's habitability so "it exists" probably isn't enough to tip them off, and over the course of geological time, enough asteroids would hit even the near side to change the crater pattern. Add in that the Moon is very slowly leaving Earth's orbit, and the eclipse coincidence will no longer occur by the time they return, muddying the waters further. – No Name Feb 25 at 6:29

Are hundreds of millions of years too short a span to suppose the solar system would look sufficiently different to fool trained space farers?

Nopes.

First, the planet. The plate tectonics theory and the continental drift theory clearly state that the continents are in motion, and that there has been a cycle of formation of supercontinents and then return. Even otherwise, 100s of millions of years are enough for multiple ice ages, which could cause their own patterns of glacial erosion and life formation, different from the current Earth.

Second the solar system. Since the interlapsing period is 100s of millions of years, its quite possible many more planet systems with 8 planets have since been discoverd. Just like geography of earth changes, it is hard to predict what kind of impact a series of faraway asteroids, or even another alien species does to the planets within earth's solar system. Thus unique identification is hard.

Third the galactical system - one may utilise the 3D star patterns to identify if the star of this plant matches that of Sun. However, this can be difficult for 1 reason - the 3D map of stars itself need not be unique across the universe. Say 10 different stars were used to map the relative position of Sun, one would need to pick the same 10 stars when mapping the Sun again, and even then, there could be a replicas in some other part of universe. Thus again, unique identification is hard

The bottom line is the ship and humans may not realise that their destination changed if their initial destination was a star system similar to solar system. If instead, they were headed to a 3 planet system or a 2 star system, it should be fairly obvious.

I'm not sure how to contrive the return in such a way that it is not obvious to the returning craft

For things to be obvious, either the ship's AI, or the humans themselves have to be able to see some markers from their onward journey during their return journey. This is quite common with people getting lost in jungles. Thus any such markers can be avoided if different directions are used during approach and takeoff.

How would you have a space ship set out on a journey to a distant planet, but find themselves arriving back home without realizing it?

There was a dark forest strike on the ship's AI (not the planet) during its millions of years of voyage, which wasn't completely successful, but managed to change the parameters of the voyage enough for the ship to return to home planet, without the AI or the humans realising it.

When Apollo flew to the Moon, NASA made use of a phenomenon of orbital mechanics called a free return trajectory.

Basically, the Apollo spacecraft was given a course and speed, during the trans-lunar insertion phase, which meant that if anything went wrong, for example if the spacecraft missed the Moon, it would follow a large elipse which, without a further fuel-burn, would eventually bring it back to the Earth.

Because gravitational field strength is curved, an amount of thrust used to move away from any massive body, such as a planet or star, will cause the spacecraft to follow a large curving path through space, if the mission begins with a single burn and the spacecraft then essentially coasts. The Apollo spacecraft were in effect placed in a highly elliptical orbit about the Earth, in which the apogee of the orbit was at the orbital altitude of the Moon, which the perigee of the orbit was in low Earth orbit, and the spacecraft would basically coast up to the Moon, then back to the Earth, in a repeating cycle, taking about 7 days to complete each orbit.

Termed a free return trajectory because the energy for the return to Earth is provided by nature for free: simple gravity would draw Apollo back to the Earth, in the second leg of its orbit, if it had for any reason failed to go into orbit around the Moon.

Any space mission which does not employ continuous thrust can make use of the force of gravity to slow its outward path: in fact that is what must happen, until eventually the craft passes the apogee point and gravity then begins to draw it back towards its point of origin.

This has nothing to do with the universe being curved. It is a simple effect of gravity, and must occur if a mission does not have unlimited fuel.

Alternatively, if you do have unlimited fuel, any spacecraft which approaches the speed of light will encounter time dilation, whereby the on-board time passes much more slowly than on its planet of origin. A mission might last 5 years, ship-time, yet the crew could return to find that thousands of years have passed at home.

Try Gene Roddenberry's tv series 'Andromeda', where the starship Andromeda passes too close to a black hole, and when it fights its way out after the close encounter, Captain Dylan Hunt finds that three centuries have passed on his homeworld, and the civilisation he comes from has been destroyed by the enemies who he had entered the black hole to escape, because of the time dilation caused by the ship's passage through the gravity well of the collapsed star.

Space ship journey was a hoax, they were just orbiting Earth or Moon, and hyperwarp space they seen in portholes was just 3d render. Government issued this "Generation ship" launch to make sort of human resources backups. But, things changed on Earth, and expedition took longer they expected...

• See "Ascension" on netflix... – Ash Feb 24 at 15:36

They went through a wormhole

While in sci-fi wormholes are shown with bright, flashing ligths for the convenience of the audience ("Ah! it's a wormhole!") so little is actually known about them that may well be completely invisible.
You can make the passage through it as fast and as smooth as you may like but the result would be that the ship & crew would still be in good shape after the passage, because of plot armor.
They would see that the position of the stars has changed, maybe they have lost some of their sensors in the rough passage. Anyway they detect they are not on the original route anymore but can't figure out they are on a vector home.

Also the wormhole has messed up their position in time but they have no way to detect it. Now star positions have deeply changed. That contributes to them not realizing they are coming home.
They may see that the star they are headed to has a similar spectrum than the Sun (if they started from Earth) but not recognizing any of the nearby stars would induce them into thinking they are going towards a different star system.
A character may say: "Hey, that really looks like our Sun!". And another: "Oh yeah? And where are Alpha Centauri, Tau Ceti, Epsilon Eridani?"

Still once in the solar system they should be able to see that is exactly like the one they left. Planets don't disappear even after hundreds of millions of years. Yes, Saturn will have lost its rings and some satellites will be a bit further away / closer but a system like ours should be clearly recognizable.
Only solution to this would be that their home system is so boringly common and / or deprived of special landmarks like Jupiter or Saturn would be. Maybe just a single planet orbiting its star?

1. On the scale of 100s of millions of years, Earth's position in the Galaxy is chaotic.

2. On the scale of 100s of millions of years, Solar system orbits are chaotic.

Start off with a civilization deciding to leave the galactic disk. They manufacture a white dwarf star for power and use exotic materials to build a Matrioshka Brain around it, then launch it (powered by a few supernovas) beyond galactic escape velocity.

Some people from this civilization decide to head back to the Galactic disk. This is expensive; but maybe that is part of the plan (create a branch civilization, send it out of the Galaxy where it is safe, then have it send seed pods back into the Galaxy).

The beings in the Matrioshka Brain would be uploads, naturally, and the seeds sent back would be star wisps. When they'd arrive back, they'd start assembling from raw materials an industrial base.

The goal would be that even if Galactic civilization collapses, a branch of your civilization reseeds it.

They'd arrive a geological time later in a solar system rendered chaotic by time, newly printed out flesh copies of trans-galactic explorers. Possibly their brain state was frozen for most of their trip outside of the galaxy (and only woken up after the star wisp had managed to print bodies for them, a million years after arrival).

When I was in high school I attended a seminar from a theoretical physicist, who gave a lecture on the structure of the universe and how it is a closed geometry, roughly like the surface of a sphere.

When I asked him why we don't then see the light from the stars coming "from the other way around", in the same way I can fly from New York to Los Angeles across the States or across Europe and Asia plus a couple of oceans, he answered with a disappointing "when you will be at the university I will explain it to you".

In your story the universe is much smaller than ours, with light and matter being able to cross it in both directions, the shortest one and the "other way around" one. This is what happened to them: they travelled so long that they completed a whole round of the universe, in a bit more than 80 days, I assume.

• Did you ever find out why we don't see the light from the other way around? – chasly - supports Monica Feb 24 at 10:57
• @chasly-supportsMonica, I think it has something to do with the inflationary period after the big bang and the expansion rate of the universe, but I am no cosmologist, so somebody else might give a better explanation. – L.Dutch - Reinstate Monica Feb 24 at 11:58
• This. Also Dutch is correct in the comment above. This is all supposing the universe is curved and finite, but if it is, and if you could move faster than light, then if you moved in a "straight" line about the diameter of the universe, you would end up in the same place. – The Square-Cube Law Feb 24 at 14:18
• @TheSquare-CubeLaw, more than "straight line" I would say "great circle" – L.Dutch - Reinstate Monica Feb 24 at 14:19
• @L.Dutch-ReinstateMonica yeah I put "straight" into quotes because it is a circle, but from the traveller's view point it might look like a straight line. – The Square-Cube Law Feb 24 at 14:32

For reasons I've explained, this won't easily work.

1. Human artifacts don't last long - order of a hundred thousand years or thereabouts. See the many many questions about how long artifacts last, for this.
2. Despite minor changes the solar system will almost certainly be immediately recognisable for hundreds of millions if not a billion or so years, minimum.

So its hard to have a belated discovery moment as you planned.

One way it might work is if the original planet wasn't earth, or at least not earth as we know it. If they lived in sealed cities , domes, or underground, and for some reason knowledge of the outside world was forbidden, then your reveal becomes more practical.

For example, suppose it was a buried lunar dome, as described in many works of fiction, and that it was dictatorial, and the ship was an escape.

Or suppose the dictatorial nature came from a fanatical religion, where space was God's and it was blasphemy to seek to be in God's place, and those leaving stole one of the few mothballed ships prior to the religious revolution (they'd last ages forgotten on the taboo'ed surface and we can handwave away fuel with some kind of ion drive or whatever). Those leaving wanted to find God and ask Her some questions/see Her face/get to Paradise the fast way. So they don't have a clue, and the knowledge of space is a forbidden thing, no way to learn except interrogate the computer.... which is unreliable due to age and lack of maintenance, doesn't have stellar maps which they don't find out till after they hastily leave...

To avoid observation they dare not have emissions other than the ion drive or can't yet figure the observation system until interstellar drive is enabled (who knows if the leadership would take a 2nd ship and some gladly hellbound sacrificial volunteers to prevent blasphemy on this scale), which means no looking out of observation plates until the ship says they have entered interstellar space. A lunar dome/construct, unlike an earthly one, might last a very long time indeed. Or at least is easier to handwave its age. Especially if in a location protected from impacts.

That provides all your elements - a juicy plot and character hook, a basis for a richly written interesting universe and society, a way to have them not recognise the solar system at all, multiple ways (inexperience, computer fault) to have a navigational route or error that takes them home again and when they get to this unknown system, what more natural than to seek a similar environment to the one they know - the 4th planet is dangerous now for some reason (radiation?) But fortunately the 3rd one is a binary planet and one of its 2 planets is similar to their home environment.......

Given the time involved you have to involve something that can fling you through time. Nobody would set out on a voyage meant to last that long, therefore something must have gone badly, badly wrong for them to be unaware of the situation.

Thus you're going to have to go into the realm of speculative physics to get an answer. Two ideas come to mind:

1. Normal travel between the stars is via wormhole. Wormholes are quite hard to detect, survey ships map them and regular ships simply go to the right spot and go through. What the crew didn't realize is that there was another wormhole passing through at high speed and they inadvertently entered it. It took them through time and dumped them back close enough to Sol that it was the star they found. The time travel is because the wormhole has been moving at it's insane speed for a long, long time and the ends are subject to special relativity. (See Timemaster by Robert L. Forward for more on this idea.)

2. A failure of a hyperdrive of some kind--the jump went much farther than they intended and the interaction with Sagittarius A* took it a way forward in time--they jumped across the galaxy at a time when Sol was on other side. This is close enough to the One Face story I won't explore it further.

# The universe is flat and infinite

We still don't know the shape of the universe. From what we can see it seems it is flat and infinite (we cannot rule out that the universe is finite or curved yet though).

If it really is infinite, then consider that any volume within it with a finite amount of atoms has a finite number of ways those atoms can be arranged. If you move faster than light1 in a straight line or an open curve long enough, there is a greater than zero chance that you will find a volume within that universe that looks enough like the volume of the universe that we can see right now, even having evolved much in the same ay. Maybe with a few little changes.

1 Consider a wormhole, or handwave it as some sort of space anomaly.

For a system considering a finite universe with a closed curvature, see Dutch's answer.

• "Any volume within [the universe] with a finite amount of atoms has a finite number of ways those atoms can be arranged" - that's only true if spacetime is discrete, which it is not. There are an infinite number of ways to arrange a finite number of atoms in a finite but continuous volume. – Nuclear Hoagie Feb 24 at 14:48
• How about the universe has periodic boundary conditions. – alessandro Feb 25 at 5:23
• This is a bit too much like infinite typewriters and exceedingly low odds, for my liking. If the question was about travelling to a space like ours but not the same, without inter dimensional handwaving, its a nice idea, but the emphasis in this question doesn't seem to be asking for such long odds as this solution would require. – Stilez Feb 25 at 12:38

The light from the far off solar system could have actually been bent by an undetected black hole half way between the two systems. The light happened to be bent back into an orbit which pointed right back at their origin solar system. The ship constantly travelling towards the light would essentially be retracing that same orbit. Only after having gone full circle would they realize their folly.