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Say we have a group of space travelers who have been frozen inside a sleeper ship for an enormous amount of time that left when Earth was recognizable. When they thaw out and reach their destination, they land on a planet which has only one super-continent and possesses a desert ecosystem which evolution has rendered almost completely alien, compared to present day flora and fauna.

Since the solar system has orbited the galactic core multiple times, the stars are in completely different positions and the amount of time that has passed has noticeably increased the physical size of the Sun. Also, during the time that passed, an extra-solar gas giant passed close by Earth in which the Moon was knocked from orbit to become a planet in an independent orbit, as well as sending Mercury crashing into its parent star.

Civilization it turns out, collapsed shortly after the astronauts left for the stars due to severe and abrupt climate change and any artificial traces of humanity has naturally eroded away into nothingness while the surviving species reverted back to something reminiscent of a house-cat sized purgatorius.

Disregarding exactly how the ship and crew managed to survive time itself, how would our unfortunate star sailors realize that the planet they are on is not alien at all, but instead they have traveled in one unfathomably huge circle?

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Serban Tanasa Jun 20 '17 at 13:25

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Even with Moon and Mercury out, the rest of Solar system should still be recognizable. Venus, Mars, Jupiter, Saturn and the rest should still be on their orbits, though those orbits may shift a little. Also, Andromeda galaxy would not go away, and while its presence has no relation to Solar system, the distance to this galaxy may serve as a somewhat accurate galactic clock.

P.S. Also, according to modern science, with Sun getting more luminous, in one billion years Earth would be unsuitable for human habitation.

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    $\begingroup$ +1 for Andromeda galaxy, it would be far better visible after 600 million years and its shape is quite distinctive, so the astronaut would know that they are at least in the Milky Way. $\endgroup$ – Thorsten S. Jun 17 '17 at 12:55
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    $\begingroup$ Would the mass and size of the Earth and other planets change? I guess not. It would be very strange for a foreign planet to be precisely one Earth mass and diameter. And don’t forget all the planets with their moons. Finding a planet with rings and 62 moons (if they are able to detect them) just shouts Saturn. $\endgroup$ – Michael Jun 18 '17 at 20:11
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    $\begingroup$ @Michael We have very little idea about exactly how common Earth-like or Saturn-like planets are, and neither Saturn's rings nor moons are considered to be stable over astronomical times. Even without extra-solar Jupiters going smack through the middle of the Solar system disrupting everything. Finding something that looks like the Solar system with a missing planet and something like the Moon in its own (stable) orbit may very well seem more likely than recognising it as our own system 600 My later - what's the chance of that, huh? :D $\endgroup$ – Luaan Jun 19 '17 at 14:14
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    $\begingroup$ @cst1992 «Who says Saturn wouldn't have 100+ moons in the future» the Cassini mission. All bodies down to some small size are certainly charted now. $\endgroup$ – JDługosz Jun 20 '17 at 0:07
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    $\begingroup$ @marcellothearcane The pole star actually changes - Vega will be the pole star around 14000 AD. $\endgroup$ – Matthew Daly Apr 13 '18 at 20:46
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Even though evolution surely would have replaced all lifeforms on earth with new ones in 600 million years (assuming there are any left), the lifeforms would still have something in common with their distant ancestors -- if not visibly then at least on the cellular level.

Among other things, I find it likely that the cells would still be divisible into prokaryotic and eukaryotic cells, with roughly the same structure; that all cells would store their genetic information as DNA or RNA and have cell walls made of phospholipids, while the eukaryotic cells would get their energy from a form of mitochondria.

These are traits which have survived for hundreds of millions of years, and which are very dominant in today's world, making me assume that they would be very hard to replace.

I do acknowledge that this question has one significant weakness, and that is that the astronauts simply could assume, that they just have come to an alien planet, where evolution has led the alien cells to use similar survival techniques as earth-cells. The problem is obviously that we -- since we don't know of other lifeforms than those on earth -- do not know if other planets could evolve life with significantly different traits, but I do at least find it hard to believe that the cellular structure we see on earth is the only possible way of constructing life, and therefore I do think that the astronauts, eventually, if they continued to discover such strange cellular similarities between the lifeforms of the planet they had arrived at and their own cells, at least would start suspecting that they might not be on an alien planet.

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    $\begingroup$ In support of this answer and knowing a lot about genetic patterns and structures, it is inconceivable to me that the precise arrangement of the genes for various proteins on the chromosomes would be the same for an independently evolved species even if the genetic code we have is the only possible code in the universe. The ordering of the genes serves no known purpose and seems utterly random: If the ordering is even nearly identical, it had to evolve on Earth. $\endgroup$ – Amadeus Jun 17 '17 at 18:49
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    $\begingroup$ Possibly more relevant than just the use of DNA/RNA and other cell structures: the assignment of codons to amino acids is believed to have survived with little or no modification since the time of LUCA, i.e. around 3.5 billion years ago. There are good reasons to suspect that it will not change in the future, either, yet is (assumed to be) effectively random, i.e. it would be extremely unlikely for any non-Earth-originated life to have the same coding, even if they did use DNA, RNA, and build proteins from the same set of amino acids as us. $\endgroup$ – Jules Jun 19 '17 at 0:14
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    $\begingroup$ The biochemistry as a whole would be a dead giveaway. Any single piece (DNA/RNA, bilipid membranes, chirality, choice of amino acids, etc.) could reasonably be expected to be found somewhere, but the full combination is almost certainly unique to Earth. $\endgroup$ – Mark Jun 19 '17 at 4:25
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    $\begingroup$ We may be able to eliminate independent evolution, but what about colonized worlds? This method wouldn't be able to tell us whether the planet was colonized by Earth life in the last 600 million years. $\endgroup$ – J Doe Jun 19 '17 at 19:13
  • $\begingroup$ It could still be massively useful to narrow down the search by excluding any planets with truly alien biology, far enough such that fuzzy matching of planet orbits becomes feasible again. $\endgroup$ – MauganRa Jun 20 '17 at 9:04
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Really a comment, but too long: You have some big problems here.

1) The astronauts don't wake up. After six hundred million years of suspended animation. You're looking at .17 mSv/year from the potassium in your body. x 600,000,000 years = about 100,000 Sv of radiation. LD50 is 5 Sv. That sort of dose is a drop-dead-on-the-spot type dose.

2) Pangaea has probably broken up again by then. Wikipedia

3) CO2 concentrations will be very low. Your ecosystem is descended from grasses and succulents as all the rest are extinct or close to it.

4) It's unlikely there is anything wolf sized alive then. There won't be a lot to eat and smaller stuff gets in more life cycles and thus evolves faster--the big stuff will die out because it can't adapt.

5) Unless something has happened the moon is still there, just farther out.

6) It's too hot. I'm not finding a temperature for 600 million years but extrapolating from a value I did find we get an average temperature of 93F (33C). That means most of the planet will be too hot for human survival outside protected environments.

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    $\begingroup$ The radiation dose will be a bit lower than you say, because 600 million years is half of the half-life of potassium-40, so the dose per year will be noticeably lower at the end of the period than at the beginning. $\endgroup$ – Mike Scott Jun 17 '17 at 10:20
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    $\begingroup$ "It's too hot (33C)", I'm imaging the Levantines can say otherwise. The average temperature in Iraq and Saudi Arabia can reach 45C in the summer and they don't need space suits. $\endgroup$ – user38070 Jun 17 '17 at 13:37
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    $\begingroup$ On point #4, all the big stuff on Earth did die out once already, but more big stuff came along. $\endgroup$ – martin Jun 17 '17 at 14:24
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    $\begingroup$ @DavidRicherby Yes, but the accumulated dose will be maybe 15% lower than is claimed in this answer. It doesn't make any practical difference, of course -- 85,000 Sv will kill you just as dead as 100,000 Sv. Not to mention having 40ppm of the potassium in your bones turned into calcium or argon. $\endgroup$ – Mike Scott Jun 17 '17 at 15:46
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    $\begingroup$ I think the potassium issue can be hand-waved away... instead of literal sleeper ships with suspended animation, have everyone in stasis. Radioactive decay in a stasis field could be minimal even over half a billion years for the same reason that radioactive decay of relativistic objects is slower. $\endgroup$ – Michael Jun 17 '17 at 20:10
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The night sky will still tell you, your planet is orbiting our Sun.

Just ignore the stars of the Milky Way. The Milky Way is just one galaxy of many. It is moving, but in 600MY it will have moved about 11 times its width.

But it is only 100,000LY across; other galaxies are a million (M100 is), even 1.5 million LY across (Hercules A). They are moving, but Hercules A (or M100) will be so close to their original positions, from Earth's POV, that they can't be anything but those galaxies; no matter how their stars may have moved about in that time. Nothing else that big could have appeared that quickly.

Given just a few dozen large distant "lighthouse" galaxies in the universe, we can compute our position by their relative positions and angles with each other; the lines between them will form a kind of 3D mesh that is unique to each position in space. Their rate of movements is already known, we can compute how this map will look at any future point in time: In one million year steps would suffice. Using that, they can discover they are 600M years in their future, and using that, determine their location is our Sun, in our orbit within the Milky Way, and that the planet they are (due to its size, mass [measured by strength of surface gravity] and orbit) has to be Earth.

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  • $\begingroup$ FInding and identifying nearby galaxies might be able to let you dermine you're still in the milkyway; and approximately how far from the center of the galaxy you are but not anything more precise than that. $\endgroup$ – Dan Neely Jun 18 '17 at 4:15
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    $\begingroup$ @DanNeely I think if we have the tech to make humans survive 600M years in suspended animation, we also have the precision necessary to compute the angles and rates of movement of such galaxies. Just 24 of them would give us 2024 unique triplets to use for triangulations; and a billion steps of positional evolution for hundreds of such galaxies would be a trivial exercise, even with a single common laptop of today (I routinely run programs to solve equations using many billions of steps; it is truly not a problem at all). $\endgroup$ – Amadeus Jun 18 '17 at 16:10
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    $\begingroup$ Measuring the movement of the galaxies only gets that we're in this galaxy. It can't get that we're in the solar system because baring some sort of theoretical breakthrough that allows directly solving the n-body problem predicting where the sun will be in the galaxy is impossible. The methods we have to approximate it go off the rails very quickly with minuscule variations in input. For the solar system moving the position of 1 planet by the diameter of a hydrogen atom is enough to randomize the positions of the planets after a few hundred million years of simulation. $\endgroup$ – Dan Neely Jun 18 '17 at 20:49
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    $\begingroup$ @DanNeely I think you just want to be argumentative. We don't have to solve an n-body problem; we need to know how much time has passed (and can figure that out from enough known galaxies), how many rotations our galaxy has made, and the approximate position of the Sun. Barring some rogue star or black hole (which hasn't happened in five billion years), there is nothing else within several light years of us that looks remotely like Sol! It has to be Sol! I am a mathematician, I am aware of the complications, and simply claiming I am wrong does not convince me. Sorry. $\endgroup$ – Amadeus Jun 18 '17 at 21:34
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    $\begingroup$ @Typhon I am not 'threatening' you I am predicting what will now most likely happen. To answer your question, I have repeatedly said we don't have to be exact, we just need an approximation; because both Sol and its planetary systems are unique enough to recognize. If I tell you in July that there is a man (and only one man) dressed as Santa Claus walking around the Mall all day, you will be able to find him without an exact location, even though the Mall is a very large place to look for a single person, because you will have exactly one person that fits even that inexact description. $\endgroup$ – Amadeus Jun 19 '17 at 18:21
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The astronauts could determine they had landed on Earth because of sampling the isotopic ratios of minerals. This will match the isotopic ratios of the time they left Earth. yes the proportional amount of radioactive decay will have shifted the content of radioactive elements but that should be enough to confirm they were 600 million years in the future (sort of).

Also, they will know they are in the solar system because of the presence of the Moon. Even if it is now an independent planet (high unlikely, if considered realistically) many of its surface will be similar to those of the earlier. Plus the isotopic ratios of lunar minerals will clearly indicate they are part of the Moon.

Saturn's rings will be long gone by this time. There may be other changes due to human activity on the moons, planet and asteroids. Yet many aspects of the solar system will still be recognizable. For example, Pluto and its moons may be relatively unchanged.

The positions of the visible stars will have greatly changed in 600 million years. the nearest star to go supernova is most likely to have blown up by then. This can be a marker for how far in the future they are now.

Despite the changes to the local astronomical environment, there will be enough indicators to clearly show the astronauts are on Earth 600 million years later.

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    $\begingroup$ The Stars will be the least dynamic fingerprint element. We can also predict them surprisingly well. Having a simulator would be very helpful. $\endgroup$ – user39453 Jun 17 '17 at 8:27
  • $\begingroup$ @user39453. Good point. The astronauts in the sleeper ship might not have the data to predict the stars around Earth after 600 megayears, but if they did it would be easy. It would reinforce their return to Earth. $\endgroup$ – a4android Jun 17 '17 at 12:02
  • $\begingroup$ @a4android If they had tech to even survive 600 megayears in suspendend animation, I would at least expect the ship engineers to tackle in some navigation systems and some stellar maps and simulations. $\endgroup$ – T. Sar Jun 19 '17 at 19:54
  • $\begingroup$ It is very unlikely that the local stars will be recognizable, nor in the same positions, nor even the same stars any more. 600 million years is about how long it takes the Earth to orbit the galaxy once. We really know very little about the stability of loose local star groups like ours, but it seems unlikely that they all would stat together for that long (we don't even really know how eccentric or how stable the earth's galactic orbit is). $\endgroup$ – RBarryYoung Jun 19 '17 at 20:10
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    $\begingroup$ @RBarryYoung 600 million years is three times to orbit the galaxy. The supernova remnant is likely to be a pulsar. It is problematic whether it will still be in the vicinity of the Sun, or relatively close enough for identification. Not a definite suggestion, more of a possibility to be given consideration about its utility. $\endgroup$ – a4android Jun 20 '17 at 4:31
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Fossils.

The oldest known fossils of living organisms are at least 3,000,000,000 years old (more than five times older than this 600,000,000 year period in this mental exercise), and have survived all the geologic and physical processes that wear away rocks in that time. About 540,000,000 years ago (slightly less than the 600,000,000 year period in this mental exercise), life suddenly evolved into many complex forms and a great number of species, and these fossils are still around today, such as trilobites.

So, it's no great leap to think that in 600,000,000 years, humans awakening and landing back on Earth would find fossils of the creatures they already know that are alive here on Earth today, even though the living things they would see around them in the near barren environment of that future time would appear quite different from organisms today. (But maybe not all that different... crinoids first appeared 530,000,000 years ago and are still around today and have hardly changed in appearance.)

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    $\begingroup$ All of today’s accumulated fossil knowledge is based on a century-long research (excavation, analysis, classification) in which participated countless teams of experts from various fields of knowledge (archeology, geology, biology..) spread over countless locations and geographic regions. While astronauts from the OP won’t need to classify every life form that ever existed, we have to remember that they do not know what they are looking for! $\endgroup$ – Fingolfin Jun 19 '17 at 12:39
  • $\begingroup$ They will still need to commence a colossal project in order to have even a basic mapping of the organisms which used to live on this planet, since finding random bones from the upper earth layers (not an easy task either) won’t help them too much. A single team of non-experts won’t even have a motive to begin such a project without an already existing strong suspicion that they are on Earth, cause even if they did - they would have never been able to reach the correct conclusion in a plausible time frame. $\endgroup$ – Fingolfin Jun 19 '17 at 12:40
  • $\begingroup$ @Fingolfin it is quite plausible that futuristic technology might be able to scan for and locate bones. We have imaging software for locating underground caverns and various things using satellites. I know I saw it on tv once, granted they couldn't say the exact nature as they were essentially borrowing a government system. I don't find it far-fetched to say that the technology or math behind it might progress to the point that we can obtain rough shapes of fossils beneath the surface. I suppose if we have relativistic engines we can scan for fossil evidence... and make basic conclusions. $\endgroup$ – The Great Duck Jun 19 '17 at 19:41
  • $\begingroup$ @Typhon That solution only (attempts to) solve the excavation issue, and this is not nearly enough. Even if that was possible without satellites (nonexistent in this setting), their findings will consist mainly of fossils from the last few hundreds of millions of years (the future in the OP setting) instead of fossils from our current era. Also, they probably won’t find entire specimens, but only partial samples, and without the appropriate knowledge they will NEVER be able to reach conclusions. It's like searching for a needle in a stack of hay, without even knowing what you’re searching for. $\endgroup$ – Fingolfin Jun 20 '17 at 8:23
  • $\begingroup$ @Typhon If challenged with 3 skeletons - a brown bear, a (prehistoric, now extinct) cave bear, and a fictional somewhat bear-resembling creature, would you be able to know which is which? Would you even be able to tell that they all belong to a bear-like creature in the first place? And now try that with only a rough simulation of these skeletons… And now try that with a rough simulation of partial parts of these skeletons. $\endgroup$ – Fingolfin Jun 20 '17 at 8:23
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I think that it is very poor navigation if the ship lets the crew sleep much longer than intended and winds up on their home planet instead of the destination they were supposed to reach.

At one percent of the speed of light the ship could have traveled 3 million light years to a nearby galaxy and back in 600,000,000 years. If the mission was to travel for 10,000 years to a star 100 light years from Earth, wake the crew and have them explore, put them back in cold sleep, and return to Earth after another 10,000 years, and the ship instead traveled for 600,000,000 years (30,000 times the planned duration) before returning to Earth, shouldn't the ship have woken the crew and asked their advice or orders about the "minor" change in the mission.

The travelers should ask the navigation computer where they are and be told it is Earth in the distant future. The ship would have calculated and kept track of Earth's ever changing position and landed the ship back on Earth after 600,000,000 years.

Couldn't the navigation computer have been aiming merely to reach any star and planet in our galaxy, and only stumbled upon Sol and Earth by accident and thus not know where it was? That is theoretically possible, but with hundreds of billions of stars in our galaxy, the odds against it would be "astronomical".

It seems to me that they will soon deduce that the universe is 600,000,000 years older. The expansion of the universe will mean that it will be slightly thinner. The galaxies will be spread out more. Deep field images of the farthest visible galaxies will show significantly fewer per square arc second. The nearby galaxies will all be father away, except for the very nearest that are gravity bound to our galaxy and will be measurably closer.

Our galaxy is like a gigantic solar system in that every astronomical body orbits around the center of the galaxy. Some orbit it with periods of a few thousand years, some orbit it with periods a million times that long. Most present open star clusters will have been dispersed by the gravity of passing stars and only a few massive present ones will remain among the newer ones in the future.

But globular star clusters can hold together for many billions of years. They often orbit the galactic center at distances several times that of the The Sun. The Sun is estimated to take about 225,000,000 to 250,000,000 years to orbit around the galaxy once. In exactly 600,000,000 Earth years the sun should have orbited about 2.4 to 2.66 times.

Globular clusters with 400 million year orbits will have orbited 1.5 times and will be in the opposite points of their orbits. Globular clusters with billion year orbits will have orbited 0.6 times. And so on.

If external galaxies can be identified well enough to show that the spaceship are in our galaxy, the positions of the globular star clusters in their orbits in the external galaxies should tell how far into the future they are.

The angles between where they are, the center of the galaxy, the center of the Andromeda Galaxy, the centers of the Magellanic Clouds, etc, should show them where they are in our galaxy. And they may note that in 600,000,000 years our solar system should have orbited to a similar position in our galaxy.

If they take samples of rocks and minerals from the Earth, Moon, Mars, etc. they should deduce that their chemical composition and isotope ratios are consistent with those planets 600,000,000 years in the future when the radioactive isotopes have decayed for 600,000,000 more years.

If they dig up fossils on the Earth, they will discover that the geologic layers 600,000,000 years old have fossils identical with recent Earth organisms. If they analyze the DNA of the life forms they will discover it has the same base pairs as on Earth. And if they compare the genetic codes of the future Earth live forms to those of present Earth life forms they will find it is consistent with the number of genetic mutations in 600,000,000 years.

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It wouldn't be that hard.

You wouldn't even have to get off the ship.

Note- this answer predicates on the continuing functionality of the ship's central computer, however, given the successful navigation to and landing upon the surface of Earth, the computer seems intact and functioning.

Timing

The timing would be the easiest part- radioactivity! There are a surprising number of sources for radioactivity, most of which could be traced for timing.

A radioactive engine would be too easy (trace the material, etc, etc), so let's get creative.

A common staple for smoke detectors is Americium-243, as it ionizes particles well and without harm to humans (smoke doesn't conduct electricity, so the flow is disrupted, causing the detector to go off). It has a half-life of 432.2 years, so that would be out, but the decay has a product (as every decay does). Neptunium-237 as a half-life of 2140000 years, which would last us a significant amount of time. From there, the Neptunium Series is followed, and at any point the amount of any element along the way could be analyzed to conclude the timing.

Current day, many weapons systems are based off of Uranium-238 or Plutonium-239 and their reactions. It would take an entirely separate answer to explain how critical mass works, so let's just focus on the weapons systems. Much of the Uranium-238 would have long decayed into Thorium-234, which would then follow the Actinide Decay Series. Plutonium-239 would also follow the Actinide Decay Series, just starting at a different part.

Placement

Okay, the timing has been identified. What now? The stars have moved about considerably, but there is still hope. The ship is close to Earth, as it lands there, which means it traveled near to it, or has at least returned to it. If the civilization is in any way gifted with foresight, they would have plotted all coordinates off of galactic center. The ship has been flying this whole time, plotting points and taking note of it's mission. Being 600000000 years in the future, one can assume the reasonably intelligent astronauts would know the galaxy rotates, but also know that, and I cannot stress this enough, they rotate with it! Even if the mission was (most likely) extrasolar, the rotation of the galaxy is predictable enough that one could identify the earth's likely position, and then compare it with one's own likely position. All in all, no part is that hard.

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  • $\begingroup$ I like this answer and it goes very well with basic planetary science. The only flaw with this is if the humans on earth do something that causes the earth to move or something disrupts its orbit (such as a meteor). Great answer though! $\endgroup$ – The Great Duck Jun 18 '17 at 5:17
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    $\begingroup$ @Typhon That's true, except the question stated civilization collapsed, but there was that gas giant. That would disrupt orbit, but if one plotted to the star in the center of our system (colloquially known as 'The Sun'), then it wouldn't be a problem. $\endgroup$ – Imperator Jun 18 '17 at 11:53
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    $\begingroup$ I was referring to something disrupting the orbit of the solar system within the galaxy. Hence, one might not know the motion of the solar system. $\endgroup$ – The Great Duck Jun 18 '17 at 18:29
  • $\begingroup$ @Typhon A meteor big enough to actually have an appreciable impact on even Earth's orbit would be gigantic, or moving at relativistic velocities relative to Earth, or both. Remember Theia and the giant-impact hypothesis? That's what it took to impart significant momentum (in that case angular momentum) on the Earth. $\endgroup$ – a CVn Jun 19 '17 at 11:22
  • $\begingroup$ @MichaelKjörling No, I don't "remember" those hypothesis. Why do you keep saying things as if I already know them? I'm just referring to something causing the solar system's orbit to be disrupted. It could be a solar flare for that matter causing the sun to swing around. $\endgroup$ – The Great Duck Jun 19 '17 at 16:24
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Space Junk

Nope, not the run of the mill debris around the planet, after 600 Million years, the minimal atmosphere will have degraded the orbit of LEO satilites enough to cause it to return and burn up in the atmosphere. Even out past geostationary orbit, the tidal drag will have deorbited any satilites after this amount of time.

Instead, you look for the Junk in Solar orbit. There are quite a few peices out there, assuming your future space ship has the required technology, spotting an Apollo booster stage in solar orbit (or possibly a satillite left in solar orbit, we've done a few of those too) would be a good signpost that you're in our solar system.

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  • $\begingroup$ Correct me if I'm wrong, but I would think that atmospheric drag is exactly zero in geostationary orbit. After all, junk there rotates with the same angular speed as the earths atmosphere below. If there is recognizable space debris around, I would bet on it to be either in geostationary orbit, or in the associated graveyard orbit (which is a tad higher, and should thus have minimal negative atmospheric drag, actually slowly accelerating the junk into higher orbits). $\endgroup$ – cmaster Jun 19 '17 at 8:53
  • $\begingroup$ @cmaster you are correct. I have amended the answer - there is no atmostpheric drag, but there is tidal (gravitational) drag which is lower, but also in effect. $\endgroup$ – Baldrickk Jun 19 '17 at 9:02
  • $\begingroup$ @cmaster Something disrupted the Earth-Moon system enough to kick Moon on a clear orbit around the Sun, stable over hundreds of millions of years. It's a safe bet it would also have disrupted any possibly surviving geostationary satellites, and far more drastically. $\endgroup$ – Luaan Jun 19 '17 at 14:34
  • $\begingroup$ @Luaan that too. It's a fair bet that it's 'cleaned' a whole load of junk from the Solar orbit. It has however not managed to prevent the Earth from being habitable, so it can't have been too big. So I figure that some of the debris will survive. $\endgroup$ – Baldrickk Jun 19 '17 at 14:38
  • $\begingroup$ @Luaan The moon is at a height of ca. 380 Mm, geostationary orbit is just at a mere 36 Mm, less than a tenth of that distance. Lower orbits are really a lot more robust to being disturbed from the outside. If a heavy object passes behind the full moon within, say a week, it can easily transmit enough differential momentum to kick it out of orbit. The geostationary satellites will only experience a tenth of the differential acceleration tugging on their orbit, and they will fully circle the earth seven times, making the accelerations received on both sides of the earth cancel each other out. $\endgroup$ – cmaster Jun 19 '17 at 15:39
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Depending on exactly when Earth's civilization burns out, there may be recognizable remnants of Human technology. Nothing large, I'd guess, but very small items may survive, such as nanobots, and crystalline storage devices. Also twinkies, undoubtedly.

Erosion and oxidation would make it difficult to find any obvious traces on the planet's surface, but potentially space debris, a moon base, or underwater cities could have surviving remnants. If there is any biological life, it night still have genetic traces of bio-engineering, such as a copyright notice written into GM corn DNA.

Our particular configuration of planets, their sizes and compositions, and ringed / unringed natures may persist and be observable by the travelers. Even if the Earth looked unrecognizable, the neighborhood may be familiar. Jupiter for instance protects the Earth against a large volume of incoming space debris. In many other solar systems, the Jupiter-sized planets seem to be on the inner rings, and very hot. The moon may also have key features that are still identifiable, and is in synchronous rotation with the Earth, so we essentially only ever see one side over reasonably observable timespans. The astronauts might notice this and it may be anomalous.

There may also be some geological aspects of Earth that are fairly unique.

For instance, Earth is able to maintain its atmosphere and water against solar winds because the magnetic field is especially strong. That field strength is thought to be generated by a combination of the molten core, the speed at which the Earth spins, and possibly our core's iron-nickel composition. The right combination of factors to generate the right field level, particularly for liquid water (if it still exists) appears to be fairly unique. On top of which, the molten core thought to be kept molten in part due to the moon's gravitational pull.

Earth appears to be the only known planet which has plate tectonics, which is also what released Earth's water to the surface, previously trapped within. Interestingly, the oceans also enable the plate tectonics. These types of dynamics and the balancing act between the geological effects may be rare.

Also have a look at some cryptohistory, in particular-

  • The Coso Artifact, a human-made object discovered inside of a fully formed geode. Geodes are estimated to take about 500,000 years to form.
  • Fossils such as the human footprint found in a shale deposit in Delta, Utah. Estimated age 300 mil - 600 mil years old.

You can find quite a few more interesting examples here.

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Following on from @some-dude's answer, crocodiles haven't changed a lot in the 65 million years from when dinosaurs roamed the Earth. Someone coming to Earth in the far future may be able to recognise descendent creatures from what exist today.

Of all the reptiles alive today, crocodiles and alligators may be the least changed from their prehistoric forebears of the late Cretaceous period, over 65 million years ago--although the even earlier crocodiles of the Triassic and Jurassic periods sported some distinctly un-crocodile-like features, such as bipedal postures and vegetarian diets. https://www.thoughtco.com/crocodiles-the-ancient-cousins-of-dinosaurs-1093747

Cockroaches may also have had a similar form to those that exist now and could have been around for an even longer period of time in our pre-history than crocodiles.

The Carboniferous, around 300 million years ago, is sometimes called the Age of Cockroaches because insects broadly resembling modern roaches flourished during that era. However, the first really modern roachy remains appear later, in the early to mid-Mesozoic, perhaps 200-180 million years ago. Modern cockroaches have changed little from those ancestors. http://guides.library.harvard.edu/c.php?g=310507&p=2072128

So perhaps our sleeping astronauts may be able to use their knowledge of modern creatures and extrapolate from there.

But, the one thing I'm pondering - what would make the astronauts want to think they're on Earth and not some other destination? Why would they suddenly decide they want to investigate the creatures they are seeing and want to determine if they're still on Earth? If they were expecting to be arriving at some other destination they may believe life has a similar, common form throughout the universe (especially if they had never encountered alien life elsewhere before). They may not even think to ask the question, "Are we back on Earth?"

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    $\begingroup$ Perhaps in trying to compute how far from Earth they are, using various astronomical methods mentioned in other answers, they come up with an average estimate of two light years: Not even halfway to the nearest star, today. Or 10 LY, still too close to be coincidence when they were supposed to be hundreds of LY away. It occurs to them they haven't gone anywhere... $\endgroup$ – Amadeus Jun 19 '17 at 9:07
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    $\begingroup$ @Amadeus ...that and the 600 million years old Windows dialog they found when they woke up saying: "Are you sure you want to travel to X? [Accept] [Cancel] [Quit]" $\endgroup$ – xDaizu Jun 19 '17 at 11:31
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Very, very carefully.

If you're a space traveler, you'd had to've gotten there somewhere. Right? They are space travelers, unless they're just roaming randomly looking for earth they have to have some measurement or calculation of their historic travel. So then you can just run over your path, calculate the orbit of everything, and boom. Hopefully at least. They have to have at least some general idea of how far they've traveled, for how long, and in which direction from their origin

The most efficient way to conclude, would be to check your previous measurements and estimations. Assuming they're proven and correct, you can ascertain the truth that you must in fact, be on earth.

Otherwise, one would never know.

The claims of using the stars are wildly inaccurate. Most stars may have burnt out, and/or completely changed orbit; in comparison to the fact of probability that there must be other planets also that share similar constellations. This would inevitably become a wild guess within the realms of what's possible.

There would be almost no similarity outside of the certainty that, you must be very good at record keeping.

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  • $\begingroup$ Stars don't randomly change their orbit around the galactical center. $\endgroup$ – a CVn Jun 19 '17 at 11:32
  • $\begingroup$ @MichaelKjörling You mean, they haven't in the past couple of centuries? We're talking millions of years in this case. We can only assume there's slight movement until we can conclude in the next 4 million years, that at 2 million years there is in fact no orbit around the galactic center. $\endgroup$ – Jacob Gaiski Jun 19 '17 at 15:41
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    $\begingroup$ Is it possible that there exists some hitherto unknown force in the universe, or some (possibly hitherto unknown) massive object, which could perturb stars in the Milky Way over such a timeframe as the OP stated? Yes, that's possible; it would be ludicrous to categorically rule out much of anything over a time period of over half a billion years. However, is it likely that our understanding of celestial and orbital mechanics is fundamentally flawed, which would likely be required for "most stars [to have] completely changed orbit" by that time? IMHO, not so much. $\endgroup$ – a CVn Jun 20 '17 at 3:50
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When they 'Thaw Out and reach their destination' the Captain will know exactly where he is. At the destination he programmed the ship to go to. (And will send his compliments to the refrigerator company).

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    $\begingroup$ That depends on whether Earth actually was their destination (the OP didn't specify). If it was, this is an excellent outside-the-box answer. $\endgroup$ – F1Krazy Jun 19 '17 at 16:32
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Have the astronauts catch a .Coelacanth. They have existed in basically their current form for 400 million years or so. If they survive the death of humanity they could potentially last another 600 million years.

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    $\begingroup$ Welcome to WorldBuilding! If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! $\endgroup$ – Secespitus Jun 20 '17 at 9:01
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Given a (reasonable) bit of luck, The solar system will still be recognisable. There will be five naked-eye visible planets whose orbital periods will be remarkably similar to ours. Oner heck of a co-incidence, if it's somewhere else! Use of a telescope will reveal Uranus and Neptune, and the sizes of the five planets, and their major moons, once again pretty similar to our Solar system. Mars will almost certainly still be red; Olympus Mons and other major features largely unchanged. Earth's day will be longer, but its year will be only slightly so. The Moon will be considerably further away and "smaller", but like Mars it will look much the same. (There's no reasonable natural way for it to vanish, that doesn't wipe out life on Earth at the same time it does).

Why "with a reasonable bit of luck"? Because all orbital mechanics with more than two bodies is mathematically chaotic. There is no guarantee that orbits within the Solar system won't change dramatically in the far future, possibly with the result of one or more planets smaller than Jupiter being expelled into interstellar space. Best observational data and computer modelling says the current arrangement is stable for about 200M years, so no cause for panic. Beyond that, we can say nothing, because the model is "sensitive to initial conditions" inside the error bounds of the observations.

Fossils may be another give-away, but after a billion years, plate tectonics will have subducted most of the sedimentary rock formed in the present day. More likely indicators will be seen from biology and biochemistry. The genetic code of life will still use the same codons for the same bases. The major enzymes shared by all life will still be the same, modulo accumulated mutations which do not damage the functions (because any damage to the fundamental mechanisms of life, will result in death at a very early stage). Biology will supply similar clues at a higher level. For example, mammalian (and bird, reptile) eyes have a "design defect". The blind spot, where the optic nerve connects to the retina. The same compound eye design evolved independantly in molluscs (octopuses and squids), minus this defect. There will probably still be recognisable mammals, birds, reptiles and squid. There are many similar "design defects" which once evolution has taken a particular track, are "designed in" and cannot evolve out except by extiction of a whole branch of the tree of life and re-evolution of some similar organism from a different branch. That has not happened often over the last 600 Myears.

I've ignored the problems others have pointed out. A billion years it too far in the future: the sun will have become so hot that unless chaotic orbital mechanics provides a miracle, the Earth will by then be dead. Half a billion years, and the Earth will probably be hotter, dryer, but still habitable.

Suspended animation for that time is surely impossible. It might be more plausible to employ handwavium. Maybe a prototype warp drive malfunctions, and instead of crossing light-years in an instant, it merely provides a one-way jump to the future. (ISTR Larry Niven(?) wrote something similar as a short story ... it was a passenger spaceship with a faulty drive).

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The night sky will be pretty similar, with sufficient vision and record it should be rendered unique, if they know the order of magnitude of how long they were asleep and how fast they were going.

It would be unique and relatively predictable, though not the same.

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    $\begingroup$ The night sky will be completely unrecognisable, and nothing like it is now, except for the visible planets. $\endgroup$ – Mike Scott Jun 17 '17 at 10:22
  • $\begingroup$ This does not make much sense. $\endgroup$ – Radovan Garabík Jun 17 '17 at 10:23
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    $\begingroup$ You are wrong, The night sky will have changed completely because Earth and the Solar System are revolving around the galaxy every 220 million years. And no, it is not predictable with celestial mechanics because our outlook into the galaxy structure is limited (Hint: suns in the way) and our data is not remotely precise enough to allow predictions in such a far future. -1 $\endgroup$ – Thorsten S. Jun 17 '17 at 12:51
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    $\begingroup$ @ThorstenS. Right -- the Lyapunov time for the solar system is in the tens of millions of years (depending on who you believe), so we can't predict where the planets will be in 600 million years. You might be able to get a decent idea using the Milky Way and the outer planets if you have very very precise measurements beforehand and can replicate them afterward but this depends on how far in the future the astronauts go to sleep. $\endgroup$ – Charles Jun 17 '17 at 19:25
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    $\begingroup$ @Amadeus Suns. The whole white band of the Milky Way consists of innumerable stars and each one is a sun. While suns in the vast distance are almost points, they still have dimension and like trees in a wood they hide and outshine suns behind them. Even worse is the intergalactic dust, there is a small optical area called "Baade's window" which has luckily much less dust and allows to examine the galactic center. $\endgroup$ – Thorsten S. Jun 17 '17 at 22:22
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Depending on how many explorers there are to start with, and what technology level they have brought with them that they can use to bootstrap their society, it might take a very long time. In the worst case, there is not enough population to maintain the same education level through generations, and unless the ship can maintain itself indefinitely, there is a risk that all knowledge of the old earth will be lost.

Assuming away those issues, some clues will be in the earth and some in the heavens, and it all assumes the explorers even bother to care about this question.

  • While the moon no longer orbits the earth, but is a "dwarf planet" now, once it can be observed through telescopes, it may be possible to prove that it is the moon. At the very least, its size and surface composition will be pretty much the same. Though it will be more subject to surface impacts on both its faces now.
  • Once some good space telescopes can be set up, extrapolating the proper motion of observable stars backwards should produce a past snapshot that agrees with today's map of the skies.
  • If the ship included several radioactive samples on board with various half lives, the approximate length of the voyage can be calculated if one of those is a Uranium 238 sample.
  • At some point it is plausible that new ancient stone fossils will be discovered. Either they would agree with the historical archives of the ship (dinosaurs etc) or they would match known "present day" life forms (humans, dogs, horses, pine cones, etc) and make a convincing case that the planet is earth.
  • It is not unthinkable to locate a stable stone monument of human construction. A partial pyramid is probably most likely to survive that amount of time, through earthquakes etc, particularly if completely covered by earth. Anything left exposed to wind and rain has much less chance. If buried and not near water, deep stone engravings could survive.
  • Crocodiles, cockroaches, lancelets and other creatures are found in forms similar to present day forms in the fossil record. So there is a good chance at least one of them will be found, or something much like them, this far in the future.
  • The gross chemical composition of the atmosphere, soil, deep rocks, and ocean will match any records on the ship of the earth.
  • The approximate mass of the oceans, overall mass of the earth, average radius of the earth, distance from the sun, approximate length of the year, and other parameters should all match ship records of earth. (I interpret the question to stipulate that earth's year is not changed.) However, without the moon, parameters of earth's rotation (including eccentricity of earth's shape and of its orbit) may not. Whatever trauma was sufficient to remove the moon from earth's orbit could have significantly altered the axis of rotation and other parameters of rotation that are stable and predictable today.

All of these factors taken together will make a good case that the planet they have found is in fact the earth. Assuming they even care.

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The stars.

As soon as he gets a telescope and starts looking at the galaxies further away, she/he should realize that she's/he's either on earth or on a planet much closer to earth than he/she intended. At this point the astronaut in question should start looking for a way to prove what planet she's/he's on. The make up of the ground shouldn't change that much and the earth should look something like what it's expected to look like 600 million years in the future. There should also be human artifacts here.

The real question is shouldn't the sun have swallowed the earth by that time?

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  • $\begingroup$ The sun will be a few percent brighter by then (it will be about 10% brighter in 1.1billion years). This is the right answer though. $\endgroup$ – crobar Jun 20 '17 at 9:33
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...how would our unfortunate star sailors realize that the planet they are on is not alien at all, but instead they have traveled in one unfathomably huge circle?

They can never be sure about that. After all what are the chances that there is another solar system with some planets and a brighter sun and no artificial traces of a past civilization, totally different star patterns and cat-like species (as you describe it). It could be that in one of the hundreds of millions other stars in one of the million other galaxies this will be the case in 600 million years.

Getting the time right might not be a problem, we measure the age of the universe already now (with some error) as 13.8 billion years, so maybe it could then be measured again as 14.4 billion years.

But being certain about the location, that would require some statistical estimation of how unique our solar system is currently, how quickly that changes and then how significant the reasoning is that if you see certain species and fossils that it can only have been earth. It all depends on how unique we are and we don't know that.

The best idea would probably to colonize the new planet and then spread out to find something that resembles the original earth more. If you don't find anything, you can still think you are on the original earth, but you may not be sure about it.

In principle if we map enough stars and galaxies and have precise enough informations about their evolutions for the next 600 billion years we might have enough evidence to conclude we are still on earth, but I doubt that.

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protected by Serban Tanasa Jun 20 '17 at 13:25

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