Assume we developed a perfect rocket, say a ramjet using a very efficient matter to energy conversion drive, within a few centuries. How far could settlers get with such a technology? The local group? Vrigo Supercluster? Laniakea Supercluster?

Given that the universe is expanding and that one would have to slow down again to settle a region, how far away is the farthest reachable place today?

I'm aware that the velocity and vector of the distant objects will be quite relevant in answering whather or not a specific target can be reached. So I'm looking for a rough boundary.

Can humanity reach any of these?

Can humanity reach any of these?

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    $\begingroup$ SF author Charles Stross did the math a few years ago: antipope.org/charlie/blog-static/2007/06/… Even with perfect matter to energy conversion, the answer is pretty dismal. $\endgroup$ Apr 24, 2021 at 19:43
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    $\begingroup$ VTC:NMF. (a) You're asking more than one question. Frankly, you're technically asking more than a dozen questions. Please pick one. (b) This question falls into this category: "How many angels can dance on the head of a pin? Answer: as many as wanting." Humans can keep moving away from Earth for as long as your world's rules allow them. For example, generation ships or cryogenics would permit infinite expansion. So... what's your question? $\endgroup$
    – JBH
    Apr 25, 2021 at 4:26
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    $\begingroup$ @JBH infinite expansion eh? Aren't he asking it because of universe expands, there may be physical limitations which prevent inhabit even part of observable part of universe. Not saying it is a great question, but it is not your pin-q $\endgroup$
    – MolbOrg
    Apr 25, 2021 at 15:25
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    $\begingroup$ I agree with MolbOrg; there are clear limitations, both on a ramjet traveling through the intergalactic medium and on how far a spaceship can travel before the expansion of the universe breaks apart superclusters (which aren't gravitationally bound). There's a very specific race against time if you want to got beyond the Local Group. $\endgroup$
    – HDE 226868
    Apr 25, 2021 at 15:56
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    $\begingroup$ @JBH I'm not sure what other questions you mean; it's just asking how far humanity could spread. All of those structures (the Local Group, the Virgo supercluster, etc.) are at different distance scales, so it's like asking "How far can a car drive? Down the block? To the edge of town? To the next state?" Those are technically multiple questions from a purely syntactic point of view, but they're all just facets of the same question. $\endgroup$
    – HDE 226868
    Apr 25, 2021 at 18:14

4 Answers 4


Per this source (https://phys.org/news/2019-09-fast-universe-mystery.html) the universe is expanding at between 70 and 90 km/s/megaparsec. With the middle value being 82.

A rocket, even a perfect one, is limited by the rocket equation: dv = ve (mo/mf)

Substitute c for ve, and dv of 0.3c has an mf / mo of 74%. Just on the acceleration. Squared to include braking 30%c requires about 96% of the craft to be fuel.

So, let’s call 30%c a practical upper-limit to rocket-based space flight.

So, anything far enough away that it’s expanding faster then 0.3c is beyond us forever, with rocket technology. Roughly 300000 x 0.3 = 82 x distance. That gives a distance of roughly 1,097 megaparsecs. 1 megaparsec = 3.2 million light years. Our reachable universe with rockets, then, is 3.2 billion light years.

  • $\begingroup$ Interesting answer. However I propose using an ramjet, a rocket that doesn't carry any fuel but feeds of the interstellar medium instead. $\endgroup$ Apr 25, 2021 at 7:20
  • $\begingroup$ That value for the Hubble constant is just one study, and it's very far from the two clusters of values, which (as the article mentions) are really about 67 km/s/Mpc and 73 km/s/Mpc. I think 70 km/s/Mpc would be better for a back-of-the-envelope calculation, although I guess it wouldn't be that significant (just a couple hundred million light-years off). $\endgroup$
    – HDE 226868
    Apr 25, 2021 at 15:58
  • $\begingroup$ I checked out the specific impulse equation for the ramjet. I can see how you might be able to get to infinite specific impulse. That being said, conservation of energy is the next limiter at around 80-90%c. This would triple the range above. $\endgroup$ Apr 25, 2021 at 16:17
  • $\begingroup$ Possibly a little more than that if the Hubble constant is smaller. $\endgroup$ Apr 25, 2021 at 16:18
  • $\begingroup$ Disagree. Your rocket velocity does not limit what you can catch. The thing is you aren't limited to a single burn. Stop along the way, refuel and set off again--each hop has a velocity difference below your limit. $\endgroup$ May 6, 2021 at 3:57

Part One of Four:

Assuming relatively realistic and probable space travel.

Assuming that humans will eventually be able to build spacecraft that can accelerate to speeds of 0.001 to 0.1 of the speed of light, and decelerate at the end of their voyages, it should take those spacecraft about 10 to 1,000 years to travel one light year.

Assuming that humans can colonize star systems without habitable worlds by building space habitats, and that the average star has several star systems with 10 light years, and a voyage of 10 light years taking 100 to 10,000 years is the maximum length, a world will colonize all the star systems with 10 light years within 100 to 10,000 years of sending the first colony ships.

Assuming that it takes 100 to 1,000 years for each colony system to become populated and adanced enough to send out colony ships of their own, a world should colonize all the stars within 10 light years within about 200 to 11,000 years after being colonized.

Thus humans would be able to spread across the galaxy at the rate of 10 light years every 200 to 11,000 years, at an average speed of 0.000909 to 0.05 times the speed of light, and thus spreading at an average rate of 0.000909 to 0.05 light years per year.

Since the farthest parts of the galactic disc are probably much less than 100,000 light years from Earth. at that rate it should take less than 2,000,000 to 110,000,110 years to colonize the entire galactic disc of the galaxy.

But it would be impossible to colonize those parts of the galactic halo where the density of stars is low enough that most stars are more than 10 light years from their nearest neighbors. Thus it would be impossible to reach most or all of the globular star clusters and most or all of the small satellite galaxies of the Milky Way Galaxy, let alone more distant galaxies in the local group like the Andromeda galaxy, let alone galaxies tens or hundreds of millions of light years away, let alone galaxies billions of light years away.

Part Two:

Assuming much faster and less probable space travel.

But if humans are able to built spaceraft which can travel much faster than 0.001 to 0.1 times the speed of light, and/or are able to build spacecraft capable of deliving human colonists to distant stars after voyages much longer than 100 to 10,000 years, humans could spread much faster and reach much more distant locations within a single voyage.

Thus humans might be able to spread from star to star even among the widely separated stars in the halo of the Milky way Galaxy, and thus reach globular clusters and even the small satellite galaxies.

Part Three:

Assuming the space travel capabilities in the original question.

And if humans develop spacecraft which can travel at almost the speed of light, so that time dilation slows down time aboard the ships to a tiny fraction of the passage of time in the outside universe, a ship could travel 100,000 or 1,000,000 light years in only a few decades of ship's time, and humans could conceivably spread from galaxy to galaxy. Thus humans could conveivablye reach every galaxy which is not so far away that it is receding faster than light, or faster than the very close approximation to the speed of light which such ships could achieve.

So humans could probable colonize every galaxy that is now no more than a few tens of billions of light years away.

Part Four:

Assuming FTL

And if it ever becomes possible to build ships with Alcubierre drive or some other form of faster than light travel, the sky will be the limit, as the saying goes.

  • $\begingroup$ If humans develop spacecraft which can travel at almost the speed of light (part 3) we're still talking generation ships aren't we? Unless they also have hibernation technology, decades worth of time still needs to supply the passengers with a livable habitat(?) $\endgroup$
    – Len
    Apr 25, 2021 at 3:56
  • $\begingroup$ @Len Decades is not the same as generational spaceship (though livable habitat would indeed be necessary). Travel time is manageable (from ship's perspective): Space Travel Calculator $\endgroup$
    – Alexander
    Apr 25, 2021 at 4:10
  • $\begingroup$ @Alexander, but say it was five decades of travel time? Added to anyone's life time that would basically be a life sentence. $\endgroup$
    – Len
    Apr 25, 2021 at 4:15
  • $\begingroup$ @Len - true. But this does not necessarily make it generational starship - same people who started from Earth would live to see the destination. $\endgroup$
    – Alexander
    Apr 25, 2021 at 4:23

Adding to what M. A. Golding and GrumpyYoungMan said, there's reasonable reason to believe that we'll never even leave Earth, let alone colonize space. If you're talking about exploring the entire universe we could set up a sleeper ship with a theoretical total conversion rocket and just go in any one direction, but the thing about colonizing space (as well as long-term space exploration) is you have the massive baggage of caring for squishy, organic tissue, which has to be fed, watered, and oxygenated.

Even assuming total efficiency of recycling organic matter and water from feces and urine, the problem is eventually the food will run out of stored energy because there is no external source of energy to catalyze the conversion of carbon dioxide and water to sugars. On Earth there is an external energy supply in the form of the Sun, but this may not be available in sufficient amounts, especially in deep space. You are constantly losing energy from the system due to heat output from human bodies. Most of the space travel technology we have now depends on rationed food and capturing CO2 via artificial methods such as zeolite rather than creating a true closed ecosystem.

On top of that, we are rapidly finding that most planets have some really funky geochemistry that make them untenable for colonization. Mars is often cited as difficult to colonize because it has little atmosphere and no magnetosphere. What is becoming more apparent is that the Martian soil is actually toxic to Earth life due to lethal levels of perchlorates, so even if one were to create a totally enclosed bubble on the Martian surface it wouldn't be possible to grow food. So one would have to bring soil from Earth as well as water and the proper atmosphere. The expenses of maintaining a Martian outpost would be astronomical.

On top of that, there are a number of problems with human physiology that make us very unsuited for space travel. Such as the bone loss issue, even with copious exercise astronauts frequently lose non-trivial amounts of bone mass while in space, and they never get it back. Other concerns include the negative psychological effects of long-term isolation or confinement, or what long-term exposure to radiation might do in deep space where you only have a small thickness of metal to protect you from radiation exposure. If humanity is going to travel to space, they would only do so via such copious degrees of genetic engineering that they wouldn't be Homo sapiens to begin with, physiologically and possibly even psychologically.

It might be possible to land humans on the Moon, Mars, or even extrasolar planets using a sleeper ship, but maintaining an active colony there would be extremely expensive, next to impossible, and provide next to no benefit to whatever society decided to send a mission. Even the whole "Noah's Ark" argument, that humanity or a group of humans should set up a colony elsewhere to avoid extinction or preservation of their culture (i.e., the Puritan argument), would be completely stymied by the fact that even planets with life would likely be incompatible with human biochemistry at best.

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    $\begingroup$ soil is not required (hydroponics is a thing), and soil (basically coarse silica powder, plus trace elements) can be manufactured $\endgroup$
    – Jasen
    Apr 25, 2021 at 12:11
  • $\begingroup$ @user2352714 I'm going to disagree, on several accounts as systems when can perfectly recycle waste are more than possible given a constant source of energy(example, earth, the sun) and you don't need a planet or a moon or a rock to be openly habitable with a breathable atmosphere to settle it either. If we can settle the solar system, then we inherently have the technology to settle galaxies as well, barring propulsion. As for the changes, that seems a bit extreme, but reasonable, and while they might no longer be the same species, they'd still be human, they'd still be of the genus homo. $\endgroup$ Apr 27, 2021 at 18:37
  • $\begingroup$ @user2352714 meaning the only real technological constants are waste recycling, which is solvable as well as propulsion. Medical problems can be solved by uhh medicine and even if they end up being unsolvable, there's this thing called centrifugal force, which is really centripetal force when the observer is considered stationary relative to their own perspective $\endgroup$ Apr 27, 2021 at 18:42

I understand that your question is for today's capabilities but a ship would travel for long distances needing to repair part and upgrade along the way.

Even if a ship is limited to the knowledge and capabilities of those and the technology on the ship and limited by their slow speed of travel, information from other colonies is only limited by light speed, so they can be updated on new ways to increase their performance.

This goes against your question, so I only put this out as extra information but it is futile with our current technology to expect to reach anywhere past our local stars without major damage or wear and tear to the ship and the need to fix parts or replace parts. By that time a ship which had left 50 years after yours with more advanced technology could have caught up and passed you.

So I suggest thinking about leaving to create outposts and thinking about how the future will involve AI bodies and minds, ones that can be sent on tiny pea sized ships that can be propelled close to the speed of light, meaning those AI minds can reach at least 2 billion light years away and upon reaching the giant superclusters like Horologium-Reticulum and beyond the Great Sloan Wall they can have mechanical bodies built for those minds that have been sent.

Not the answer you were after but an option to consider when thinking about the future of human capabilities with technology and the very furthest distances in the universe that could be reached.


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