Prelude TLDR; A fleet of ships is passing through our galaxy, stopping off at particular stars and causing small dips in the light from stars.

This is related to a couple of other questions (and a little suspension of belief to make it fit the story I want). The other questions are refueling and detection.

So the idea is that explorers from another galaxy have sent a fleet of their ships into ours - the ships need to refuel regularly by collecting fusion fuels from red giants (I've decided the ships also use the light and heat from the stars so I can 'explain' using stars rather than gas giants for fuel).

The fleet$^{\dagger}$ obscures some of the light from a star as it collects the fuel, making a small dip in the intensity of the light we receive from that star. Then it moves on, using up fuel to power cryo, travel, AI etc and then stopping to survey and refuel at the next red giant.

$^{\dagger}$ One big ship can have a large volume without presenting such a large area to block out light. As explained in this answer we need a large area...so I'm making it a fleet with a main mother ship rather than one big ship.

The situation to reality check

As the fleet pass through our galaxy they obscure very small amounts of light to the point where they're almost lost in the noise of solar flares, sun spots and general brightness changes (I'm going to say they orbit their chosen star fairly quickly so we can fold the data and get a better signal).

A scientist in my story has come across this unusual trend of a small but very regular dip which appears for a specific amount of time, then disappears and turns up in another system...this trend seems unnatural to our scientist but has been overlooked by others. For the sake of argument I'm going to say that, so far, the ship has passed three star systems in the hundred years of data our scientist has but if you think he needs more I can change this.

I want our scientist to decide this is a sign of intelligent life and track the path of their ship (joining up the dots of red giants) and send out a message to them by predicting where they will be when a message can get to them.

(I want this message to include information about the star system he thinks they'll be in and information about ours so they can find us...but that is just background and for another question).

So how realistic is it that he would be able to track their path? Does he have enough information about their movements to predict where they will be?

(I'm aware he will be long dead before the aliens reach us, I want to give humanity generations of expecting the Aliens before they actually arrive).

Edit: I was imagining slower than light travel between stars and the initial group of red giants being close to each other.

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    $\begingroup$ A fleet dimming the light from a red giant even in the smallest amount that we can detect is enough to blow out all my capacity of suspension of disbelief. $\endgroup$
    – Rekesoft
    May 2, 2017 at 11:17
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    $\begingroup$ @Rekesoft a fleet flying here from another galaxy likely comes from a civilization that already explored all, or at least a large part, of their own galaxy. I don't find it hard to believe that a fleet sent out across the incredible vast absolutely nothing between galaxies would be seriously huge. At that tech level, I would even find a fusion candle driving a gas giant somewhat believable ;) $\endgroup$
    – Syndic
    May 2, 2017 at 11:22
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    $\begingroup$ @Syndic My capacity of suspension of disbelief has always been exceedingly frail. :D I'm a sceptic by trade. ;) $\endgroup$
    – Rekesoft
    May 2, 2017 at 11:25
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    $\begingroup$ @Rekesoft I know its a bit of a push, I just wanted it for my story...I'm thinking they could be refugees from a whole galaxy torn apart by a massive war. The fleet would be huge there is no arguing that...but it is also needed for my story :P $\endgroup$
    – FreeElk
    May 2, 2017 at 11:31
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    $\begingroup$ Your visiting fleet is star-hopping to refuel. Yet they managed intergalactic distances without the need to refuel (if only because there was no star they could refuel at). I have trouble believing that. $\endgroup$
    – Burki
    May 2, 2017 at 11:57

7 Answers 7


My first issue is: A hundred years of data - in a hundred years, detection and filtering technology should advance a lot (if we start from a point during which a large fleet around a distant star is observable), so it might be a bit hard to justify that fleet being "almost lost in the noise" both at the beginning and the end of that timespan. But maybe funding for space sciences was cut and no new developments were made.

Second: three sightings is definitely not enough. If it's almost lost in the noise, other possibilities would be considered more likely and that scientist would be seen as a nutjob - those three stars were probably close to each other, so anything from a thin nebula crossing the line between us and them (dimming the light slightly) to those stars having various separate reasons to dim (planetary collission throwing up debris and dust? stellar activity variations? who knows) would seem a more likely explanation than "a fleet large enough to dim a star". I don't know where I'd draw the line, but we'd definitely need more than three sightings. And the "nebula with varying density between us and those stars" theory would likely hold power well into the double digit of data points.

Third: tracking their path... tracking would be simple, but not prediction. For this they would need a clear goal they're working towards, rather than going to the nearest interesting-seeming star. Or very specific criteria for "interesting". If they're coming right for us (or planet XYZ) and only deviating slightly from a straight line to refuel, it would be doable. If they're always going to the nearest yellow star that wouldn't require backtracking, then too. Exploring with a wider area of possible interests would seem too erratic for a clear prediction.

And lastly: how far away do you want that fleet to be when it is detected? Our closest neighbouring star is 4 light-years away, but the edge of the galaxy (the close edge, mind you) is some... 24 thousand or so light years away, I think. Any signal he could send without FTL-technology would take a year to travel a lightyear, so if he discovers them while they're still far away, he'd better be REALLY good at his predictions, have a VERY strong signal source, and be very lucky to have those aliens listen for exactly the kind of signal he intends to send, too. If he has to hit them with the signal while they're between star systems it gets even harder - less crew awake, ships likely in power conservation mode, and less time spent in any given location than while they orbit a star. And he might have to do that, since he can't slow down or speed up his signal so if the aliens are X lightyears away, the signal will travel X years. If the aliens will be right between two stars in X years and it will take them years to decades to get to the next star, would he be willing to wait those years or decades before sending his signal?

  • $\begingroup$ For the first issue I was under the impression that new tech wouldn't give us that much more of a boost from this comment. Second issue, three sightings is very few but it was the lowest boundary I could think of (else the ship might have to be moving FTL). I've no idea how many we would need though. Thanks for the answer though, it brought up a lot of points to think about! $\endgroup$
    – FreeElk
    May 2, 2017 at 11:54
  • $\begingroup$ @FreeElk that's why I aded "and filtering" to the technologies. More computing power and better algorythms (at least partially from better understanding and models of stellar phenomena) allow us to reduce the noise from measurements a lot better today than a few years ago. People are even going through old measurements from NASA again to get new data with the new technologies we have now. $\endgroup$
    – Syndic
    May 2, 2017 at 11:57
  • $\begingroup$ So could it be that they have only just got to the point of being able to pick up the trend in the data now? (I'm okay with him being seen as a nut job for a while just as long as he can send out his message) $\endgroup$
    – FreeElk
    May 2, 2017 at 12:05
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    $\begingroup$ @FreeElk that would definitely be a good way around point 1. Your scientist noticed in a recent reading a dip in the brightness of the star, remembers having seen something similar (though widely doubted back then due to lower filtering abilities) in a paper from a 20 years ago, decides to filter data from that area of space with modern technology and finds that even a hundred years ago there was already such a phenomenon, although it went unnoticed back then. $\endgroup$
    – Syndic
    May 2, 2017 at 12:08
  • $\begingroup$ @FreeElk Syndic's good point about requiring many more than three data points for anyone to think it's ET life (even a crazy nut) only helps the other point: if you need 15 or 30 instances before anyone is suspicious, then by that time you have enough data to start trying to predict where they will be next (maybe not good enough yet, but start trying). If you have your main character look into it after seeing 2-5 instances him/herself, finds that over the last few decades there were several others, re-analyzes old data to find a pattern back 100+ years, then he has 30, 50, or more instances... $\endgroup$
    – Loduwijk
    May 2, 2017 at 18:19

Time is against you

The nearest red giant to Earth is 37 light years away, so the signal he has is at least 37 years late but more likely about 10,000 years late. His signal to them has much the same problem.

Your scientist is looking for the dips in light from specific stars, the light from those stars takes 1 year to reach him for each light year it is distant from him. Hence the use of light years as a measure of distance. Considering him to be on Earth, the nearest red giant is 37LY away, so 37 years before the dip in light from that star reaches us. Given that the Milky Way is around 100,000LY across, even if the passing fleet is relatively close to Earth, only 10% of the galaxy away, the fleet would have passed 10,000 years before the scientist would be in a position to notice that it had done so.

Assuming that any species capable of intergalactic travel won't be moving STL between stops:

  • If they're moving towards him, they're already past him by the time he notices they're coming.
  • If they're moving away from him, his signal can never catch them.

My instinct says that he can never get a message to them, due to the double lag of starlight and speed of light messaging.

  • If they don't go straight overhead then the light from the stars has to travel further and slower.
  • If they do pass straight overhead then they're travelling faster than the light anyway, so he still doesn't detect them until they're gone.
  • $\begingroup$ Can't they have visited a different set of red giants? Not the 88 light year away one yet? The point about 88 years vs 10,000 came across as a little cryptic, maybe I'm missing something vital though, could you explain that a little more? $\endgroup$
    – FreeElk
    May 2, 2017 at 12:01
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    $\begingroup$ Arcturus is actually closer, at ~37 light-years. $\endgroup$
    – HDE 226868
    May 2, 2017 at 12:54
  • $\begingroup$ @HDE226868, that's what I get for trusting google on "nearest red giant" $\endgroup$
    – Separatrix
    May 2, 2017 at 12:56
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    $\begingroup$ Thanks for the update. The stars don't have to dip within the scientist's lifetime, he can look at data from a long time ago. I still don't quite see why they can't be moving towards us and we see the light way before they have had chance to get to us or pass out the other side of the galaxy. I know they will have moved from the specific star by the time we get their light but if we do a dot to dot of red giants can't we say where we think they will go next? (Even if that will be in a few hundred years?) $\endgroup$
    – FreeElk
    May 2, 2017 at 13:06
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    $\begingroup$ And if they are at SLT speed the time between fuel stops might be way too high to draw the conclusion of observing a fleet movement. If your astrological observations have even been running for a time long enough to observe 2 different stars that is. $\endgroup$
    – Morfium
    May 2, 2017 at 13:33

I'm no astrophysicist / astronomer, but wouldn't your scientist chalk this abnormality up to some combination of:

  1. Observation error / Statistical anomalies
  2. Solar system debris (meteors, comets, kuiper belt objects, asteroids, and etc.) transiting between the target stars and Earth

The odds of them believing they've just observed evidence of a massive fleet of space ships seems infinitesimal. The odds of them ever even mentioning the data anomalies -- much less claiming they're alien ships -- seems even smaller.

If they tried to make the scientific claim that they were intelligently powered devices, that could well be the end of their professional career.

Also, given the three dimensional nature of space, how hard would it be to even get this effect to be observable from Earth, across stars in a pattern we could see? Remember, stars are not equidistant, so the ships might transit one star from our point of view, but then go behind the next three before transiting another (for example).

  • $\begingroup$ Oh, those durned clumps of interstellar dust! $\endgroup$
    – user535733
    May 2, 2017 at 22:59

As mentioned; we already know there are rogue planets and possibly even black holes out there. Is the fleet in question cover an area the size of Jupiter?

Why wouldn't the better explanation be that these red giants have large planets orbiting them, with total orbit duration > 100 years? We already know most stars have planets.

Plus, a straight line to Earth eyes may be a triangle in 3-d space; and a closest-neighbor walk for the aliens would seem like a drunken walk among stars to us; until we map them in 3-D to see a clear path.

That said, I think a solution to your problem may exist: Make the changes to the Red Giants, as made by the aliens, produce a more permanent feature which your scientist can observe, and might plausibly be the first to measure. for example, a particular signature combination of reduced or increased elements in the spectra of the red giant; or something else permanently observable about the star. As if the aliens extract something useful from the star necessary for their propulsion, or manipulate the star in some way.

Then your observations can be millions of years old; covering hundreds of stars, and your scientist's measurement clearly partitions red giants into Mined vs. UnMined, and furthermore he can see the nearest-next-red-giant path in the 3-D map. There is no natural explanation for such a well defined path; it must be due to intentional selection.

Then further than that: He can see, by comparison of current spectra to spectral observations taken less than 100 years ago, that the last two red giants on only one tail of this path were the most recently changed, just a few decades apart, the last point just a few years ago, so the next star on that path is one we could signal in time to communicate with these aliens before they arrive.

Perhaps further plot developments allow him to measure the trip-time between red giants, say he discovers a subtle decay rate in the spectral signatures, and using that he discovers the aliens sometimes travel at light speed, but other times take decades longer than expected before arriving at the next red giant: He concludes they must stop and visit places for some reason, perhaps they discover life, or just something else worth mining besides red giants.

Feel free to use that if it is useful; I am just spit balling here...

  • $\begingroup$ /Make the changes to the Red Giants, as made by the aliens, produce a more permanent feature/ yes. $\endgroup$
    – Willk
    May 2, 2017 at 18:30
  • $\begingroup$ The better explanation might be other things, like orbiting planets, if there weren't the very specific trend that OP claimed in the question. This actually is a technique that we use to detect exo-planets, so yes, such a dimming would be consistent with planets, but having it happen with the same signature to different stars along a rough line, the phenomenon moving along the line with time, that would not be consistent with a planet or other object. It would be super difficult to notice, but if noticed would be very suspicious. $\endgroup$
    – Loduwijk
    May 2, 2017 at 18:34
  • $\begingroup$ @Aaron: Agreed, but that would demand noticing it on 3 RedG within 100 LY of each other, and not necessarily even in a straight line. say from earth I see three RG in a flat kind of triangle, from left to right A,B,C have distances of close, far, medium distance. The aliens visit A, then C, then B, to us apparently reversing course; but to them, following a direct course of nearest-star. for them it is a pretty straight line, but won't look like one to us, and no reason to think it is not just something similar that happens to red giants that may have a neutron star companion, or big planets. $\endgroup$
    – Amadeus
    May 2, 2017 at 18:54
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    $\begingroup$ "no reason to think it is not just something similar that happens to red giants that may have..." Except the specific trend. Without the trend of time, location, and most importantly specific signal signature, all bets are off and it could be anything as you say. With the trend (and as noted elsewhere it would require more than just 3 instances; that aids your argument), it is specifically something moving and acting on these separate stars, probably something alive; it would be difficult to think otherwise. If your argument relies on the fact that actually noticing the trend is an ... $\endgroup$
    – Loduwijk
    May 2, 2017 at 19:55
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    $\begingroup$ ... is an insurmountably difficult task, so it would not get noticed and would be passed off as the natural phenomenon that you describe, if not as simple noise or error, then that I cannot argue against. Yes, detecting the data and actually noticing the trend may be one of the most unbelievable parts. But if you do (big if, yes) notice the trend, especially for many more than 3 stars, then it would be almost unmistakably something alive. $\endgroup$
    – Loduwijk
    May 2, 2017 at 19:58

You would need to observe the giant all the time to notice it.

And MOST IMPORTANT. They would not see it. Because there is a thing called lensing. The armada would need to be very close together to obstruct the light from star. The closest to star they are the smaller portion they cover.

Than, it's also another problem. Your scientists would need to be specialist in the field of sunspots to distinguish the ships from the real sunspot.


I don't see this working out

To cause a measurable dip in the light curve of a star, you need to blind a non-neglible area of its surface. The choice of red giants makes this especially hard, since they literally are giants with a giant radiating surface. We can currently barely detect super-earth size planets orbiting small stars closely.

Also, short time orbits around a red giant are completely impossible. The sun, for example, is expected to expand to somewhere near earth's orbit when it reaches its red giant phase. That means the shortest possible orbit will be around one year. And thats assuming the ships basically scrape on the surface.

Then there is the issue of stellar winds, red giants have strong stellar winds, causing condensation of tiny dust particles in the winds which can dim the starts light easily enough to be detectable (observed decades ago). This makes observing any transiting bodies harder, as there are more sources of noise. Same goes for the star itself, red giants have a very thick outer layer where energy is transported through convection, adding another source of noise.

Then you are assuming an unlikely distribution of stars: "I was imagining slower than light travel between stars and the initial group of red giants being close to each other."

Red giants are comparatively rare. There are multiple reasons for this, one is that small stars, namely red dwarfs (which form the majority of the galaxies population) do not go through a red giant phase, and those that have enough mass will not do so before the universe is many more billion years old. Next is that the red giant phase is short, compared to a stars lifetime spend on the main sequence. This means three red giants close to each other is not common. Another good indicator is that there are no red giants in the suns immediate neighborhood.

The general problem is that its hard to believe the fleets combined coverage area would be, lets say as large as the moon - unless you plan the technology level to be in the realm far beyond current physics, ships will not be moon sizes, not even large asteroid sized. The absolute upper limit would be where the gravity from their own mass forces them into spherical shape (for natural bodies this is somewhere around 400km diameter).

You propose a fleet of smaller ships instead, this relaxes the problem a bit, since the physical structural limits do not apply to small ships, and you get more surface area for less mass. But it brings up new questions: Each ship would have its own systems, making it largely or entirely self reliant. So why would the fleet consist of a huge number of ships? The logical way to build the individual ship would be to make it just as large as needed to perform its mission. Splitting functionality between multiple ships (e.g. tankers, maintenance and passenger ships) would only worsen their mass efficiency and make the system more complicated, as it adds the requirement to interact with each other frequently).

Then there is the aspect of signal travel time. By the time your scientist would observe whatever effects of the alien fleet, the observed position would be decades to millenia old. Then sending a message to the fleet takes the same amount of time it took the light from the fleet to travel to the scientist. By the time he could possibly get an answer he would have already died of old age. The galaxy is huge, the light from stars in the center takes approximately 27000 years to reach earth. So if your alien fleet isn't in the immediate neighborhood of earth (or wherever your scientist lives) this will not work, the timespans involved are just too long.


The best way to travel to another galaxy is to go very fast. The actual time is long but the subjective time is not.

Once you get to the new galaxy you have to slow down. Red giants may be good for that; they are not dense so you could be going literally through their outer layers. This could be observable depending on the size and weight of the ship. I think a moon sized ship plowing through a red giant star at nearly the speed of light would be noticeable. Perhaps smaller for a less noticeable affect.

Lets also assume they are by chance heading nearly straight toward the observer and by the time they get close their speed is perhaps 1/2 the speed of light or less.

Given these conditions, signalling them would definitely be possible. The line would be pretty straight due to the momentum so you could predict the path, especially if you were pretty sure they were aiming for the next red star in the line.

The initial encounters would arrive at nearly the same time since both are so close to the speed of light, the latter events would have more and more time lag as they slow down.

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