It's the height of humanity's space-colonization golden age when casual interplanetary travel has become possible and humans have either colonized or established large space stations around nearly every planet in the Solar System. Then, humanity is beset by the sudden onslaught of an unknown alien aggressor that quickly lays siege to Earth and Mars, leaving the spacers who occupy the Asteroid Belt and the various stations past the orbit of Jupiter as the only surviving members of humanity.

Acting quickly while the aliens are busy finishing off their planet-bound brethren, they fashion asteroids and existing space stations into massive starships and promptly leave the Solar System, forming a "Great Fleet" comprised of hundreds of kilometers-long and/or wide seed ships spaced many AU apart relative to each other that occupy almost a light year's worth of volume. It is in this Fleet that they hope to leave unnoticed and spread out into other solar systems to rebuild, traveling for thousands of years, ships breaking off from the Fleet only when they've arrived at a potentially habitable solar system.

Could this work? Could millions or even billions of people actually flee the solar system unnoticed by any alien aggressors, or would they be caught and either shot down by relativistic projectiles or tailed for multiple AU until the faster alien ships get into firing range? Knowing that there's little to no stealth in space, how could they leave unnoticed and stay unnoticed as they commence their centuries-long journey through interstellar space?

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    $\begingroup$ Obligatory reference: There Ain't No Stealth in Space $\endgroup$
    – kingledion
    Commented Oct 13, 2016 at 19:52
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    $\begingroup$ A light year isn't a measure of volume. Do you mean a cubic light year? Then if each ship is 10 au from another ship you still have about 10^11 of them. Also, how fast can the people and the aliens travel? $\endgroup$
    – Lacklub
    Commented Oct 13, 2016 at 19:57
  • $\begingroup$ Even assuming the aliens where to lazy to chase after the fleet, they would all die before they found another earth candidate. The next closest star is 4.3 light years. (Pluto is 8 light minutes from our sun, so 65,880 times that distance) $\endgroup$
    – Tezra
    Commented Oct 13, 2016 at 20:00
  • $\begingroup$ It's way more than ten AU per ship by the time they're well into their journey. And that's sort of the quandary I'm facing. The aliens can achieve relativistic speed somewhat with ease, but unless my human fleet is towing a neutron star made of antimatter that they can use as fuel, it's unlikely they could accelerate many hundreds of kilometer-long ships up to relativistic speed even over the course of years, which is why I'm paying so much attention to how their escape would work. $\endgroup$ Commented Oct 13, 2016 at 20:17
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    $\begingroup$ A lot of this will come down to warfare approaches. What is the opponent trying to do. If their goal is Xenocide, they will likely care greatly about corralling our ships. If their goal is to acquire a planet full of water, you can get away with a lot. $\endgroup$
    – Cort Ammon
    Commented Oct 13, 2016 at 23:02

5 Answers 5


No, they can't go unnoticed.

A lot of asteroids suddenly changing their orbits would be noticed all right. We monitor them already, and we do it without any immediate need. Aliens who travel between solar systems would want to make maps, will want to make sure they routes are safe. And if they think they are at war, they will have rear guard, too.

It's not believable that their observation tech is too low to do this cheaply, when today's human one makes it affordable. And if they are successful space conquerors, they can't be that stupid...

Of course if they only want to have Sol, or harvest X millions of people, they may ignore this fleet. But they will know there was one,and flee or prepare for our comeback.


I find it difficult to see how this could become plausible due to the distances involved and the massive technology, industry and power base to be able to make the phenomenal distance.

Best case scenario

The aliens are form the nearest star to our own, Alpha Centauri, which is a mere 4.367 light years away. The furthest man made object from earth is Voyager 1, launched in 1977, and its now 119.8 AU away, or 0.00189 light years. This is about 0.00043% of the distance to Alpha Centauri. Nothing human could make the distance to Alpha Centauri, even if we become something like this Type II civilisation you describe, the massive distances involved make this a mammoth task for anything to get here, so presumably their technology would be orders of magnitude above ours, and could detect kilometer long starships which will have a high energy output across a lot of visible and non visible spectrums.

Worst case scenario

These aliens are from HD40307g this is the furthest discovered star that could feasibly have life, and is 42 light years away, just under a factor of ten further away compared to Alpha Centauri, so assuming they are conquering Earth for expansion of territory, they will be coming a long way, and on the assumption they can make this distance, which could well take thousands and thousands of years, which even for what could well be about a type 2.5 civilisation (only 0.5 difference, but the Kardashev civilisation system works on a log system, so its a HUGE difference), and given that the estimate for us becoming a type II civilisation is a few thousand years, and this alien armada coming towards us could feasibly receive messages, and improve their existing technology, they will always be orders of magnitude stronger than us.

Conclusion / Tl;Dr

Massive distance from potential nearest planet containing life, massive improvements in technology and industry and manufacturing to be able to make this distance, therefore they will be able to wipe out our planetary bases and will wipe out our space faring brethren with similar ease, even assuming they don't, it could take years to build these kilometer long starships you described, which would take years and years, could be stopped by then, but even assuming they aren't, they would easily be spotted.


The aliens are clearly superior from a technological standpoint and anyone who does escape does so because they were most likely considered irrelevant to the aliens plans. Think of it like a human killing an ant hill. Do you chase every little ant down? No you just wipe out as many as you can and assume the rest die off.

So with the assumption they don't chase you down because if they did they would most likely succeed. I would then argue that the only course of action is to create and deploy as many generation ships as possible. Any travel through the galaxy at speeds that are well below the speed of light will take many generations. Your survivors are going to be space nomads who with a lot of luck may someday reach a place they can settle.


You can't go entirely unnoticed while using thrust, what you can do is form large clusters of thrust capable "vehicles" which accelerate together on the same vector for some time before the inner groups, which are the arks carrying people, cut their engines and commence their long drift out into the galaxy while the outer shell of "ships" (actually empty) scatter and continue to run their engines until caught or out of fuel. The arks aren't exactly invisible but they have a much lower target profile with their engines shut down than the rocks that are powering out of the system at maximum acceleration. The Arks retain a maneuvering reserve some of which can be used for further acceleration once they feel they're safely away.

To increase the chances of being ignored as a target you also want to avoid vectors that intercept nearby stars, and any star with a known planetary system, in fact vectoring for any visible star on departure is a bad idea.

  • $\begingroup$ Just change your vector so that your engine output points where the agressors cannot see it. In the end you want to reach one of the stars you didn't vector for in the beginning ;-) $\endgroup$
    – Henning M.
    Commented May 22, 2018 at 21:48

Any thrust large enough to propel a generation ship at a decent speed is going to be easily visible (you can read about Torch Ships to get an idea of the problems of doing this stealthily. They could burn while their ships where behind Jupiter, but wouldn't get up to much in the way of speed.

Solar sails will be reflective and noticeable as well.

Four ways that I can think this might work. Only the first and fourth ones really strike me as super practical.

Method One

For some reason the aliens know the humans are leaving, but can't pursue in a timely fashion. Perhaps their ships arrived to Earth after a long journey in a condition no longer fit to fly. Their weapons still work, but their ships are no longer fully "space worthy," or they need to refuel, or anything else that keeps them from immediately pursuing. Any pursuit would take at least a generation to prepare, and by then the humans have a large Delta V lead.

** Method Two **

Rail guns on Ceres originally used to launch asteroids towards Earth for mining instead launch asteroids out of the solar system. These asteroids have humans living inside, but are not generation ships. Once out of the solar systems they use small thrust to maneuver closer and link up.

Smaller asteroids near the edge of the solar system are simply hard to detect. There is a decent chance they don't reflect much in the way of light, and if the humans are careful won't emit much in the way of heat. So if the aliens are preoccupied with fighting humans on Earth, they may not be focusing too much observational time to look for unpowered objects moving away from the solar system.

They can afford to go slow at first because they are already planning on effectively spending forever to get anywhere. However, a full generation could pass before they were at a safe range to link up and begin thrusting for real.

Actual numbers are impossible without having more details about the aliens and just how technologically superior they are to humans.

Method Three

Deception. They burn hard, but do so to intercept comets, asteroids and other objects already leaving the solar system. Land people on them, burrow in, and then hope for the best. At some future time build and activate thrusters on your new home.

Now, the problem here comes in not being super obvious. If your people are clever they might be able to burn to match velocities with these objects while sight lines to Earth are blocked.

While I see this method being interesting

Method Four

Over whelming numbers. You scatter in every direction. Space is big, and Newton is their friend. An alien ship might hunt down and kill a human ship, but for it to then reach the next nearest human ship might require massive amounts of fuel. Launching from the asteroid belt already gives the humans a huge delta V advantage over the Earth's gravity well. Maybe the majority of the ships get hunted down and destroyed. If even 10% survive, that could still be many humans.

Method Five

I lied, here is another method that would work. Just run. The aliens might notice them, but it doesn't matter if they can't catch them.

One thing to note about relativistic projectiles. It takes light about 8 minutes to travel an AU. If the humans did a synchronized launch from Jupiter, the aliens would not even know for between 52 and 36 minutes depending on where Jupiter and Earth are because of the light delay. At that point they could launch projectiles at near light speed

If the humans are pulling an uncomfortable 3 gees during an initial "Holy cow, lets get out of here" speed burst, they will be going at 93km/s faster (3% speed of light) than they had been going by the time the aliens know they have started accelerating (During which time they can have changed their direction). During that time they will have been able to travel about a tenth of an AU!

How are they achieving such awesome accelerations? Fusion power perhaps. Or maybe they are harvesting naturally occurring anti-matter from Jupiter's magnetic field. Of course they will run out of this quickly, and then away from Jupiter can no longer get more and become stuck on the slow path. But by then they already have a decent lead and can just "random walk" their ships to avoid non-guided ordinance.


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