I am still puzzled by the original The Planet of the Apes movie from the 70's. How come that sentient species invented weapons, but was unable to develop powered flight, or spacecraft. Thus, the question is:

What could prevent a sentient race from going to space?

Assume an earth-like planet, and human-like species. This race should get through similar history build-up as humans, with one twist: They should not go to space.

What is most obvious factor to prevent them to go to space?

By "space" I mean not being able to pass first orbit, and never launching satellites. But otherwise, I would like to get this race to at least Earth 70s technology level.

This planet has all the resources needed to get to the space (basically think of alternate Earth)

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    $\begingroup$ Maybe it was the lack of easy-to-reach natural resources, having already been depleted. No petroleum reachable by pre-industrial methods. $\endgroup$
    – JDługosz
    Jun 21, 2016 at 8:04
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    $\begingroup$ How come that sentient species invented weapons, but was unable to develop powered flight... If we are just talking about the rifles seen in PotA (and ignore the few millennia that humans have had weapons and not flight), the first really usable human rifles weren't really around until the 16th century, and it would really take until the later 19th century before we start getting rifles that could compare with those in PotA. Powered flight didn't happen until 1903, ~500 yrs after rifles. PotA doesn't have any kind of engine, so they are still pre-industrialized, too early for flight. $\endgroup$
    – n_b
    Jun 21, 2016 at 18:23
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    $\begingroup$ Ask the humans, who still haven't sent anyone past Luna/Terra. $\endgroup$
    – imallett
    Jun 22, 2016 at 0:25
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    $\begingroup$ Lack of motivation? It is terribly expensive work, why bother? $\endgroup$
    – Aron
    Jun 22, 2016 at 1:07
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    $\begingroup$ Absence of moon has been suggested in a few answers, which reminded me of a quote by Krafft Arnold Ehricke: If God wanted man to become a spacefaring species, he would have given man a moon. $\endgroup$ Jun 22, 2016 at 10:38

23 Answers 23


To make sure no space flight, just make the planet a little heavier.

Based on chemical rockets, the cost to LEO is pretty terrible. If you increased the planet mass by a relatively small percentage, creating a chemical 3 stage rocket to deliver a payload (not payroll) becomes prohibitive or even just impossible. 25% more mass for the earth should do it.

Many other lift designs are possible, but we use chemical rockets for a very good reason, the other designs have even less thrust, or have other negative feature (a pulsed nuclear bomb a.k.a. Orion comes to mind).

Consider the retired space shuttle, its payload was 27,500 kg, but the launch mass 2,030,000 kg. I.e., the payload was not much over 1 percent of the total mass., were gravity only 2% higher, the shuttle would not be able to deliver any payload, etc. If you count the entire orbiting section, the mass is about 130,000 kg, which is still only about 6% of the launch mass.

Planes are a much different story, payloads can be a much bigger percentage because they don't have to carry their own oxidizer.

Some designs would be able to work with a heavier planet, but they will be quite expensive to develop. It is unlikely that such would ever happen without earlier experience with chemical rockets.

Recommended article, The Tyranny of the Rocket Equation

Michael Karnerfor does make a good case for balloon launch platforms in that you can at additional expense in development cost and per launch cost have a more efficient launch. NASA has explored this, figuring a possible benefit of about 25% fuel reduction (or conversion into payload). Similar arguments can be made for a number of alternative space launch ideas, such as launching from a plane, launching from the end of a sled on a ramp.

If you are persistent enough, you certainly could launch a small satellite from a balloon even with a 25% increase in planetary mass. You could find that you could only launch very small payloads (balloons are not a very stable platform), and they will be very expensive compared to our launch costs, esp. in higher development costs. Unless the value of being in orbit is well understood, no-one would bother. The lone exception in 1970 tech that I am aware of is using pulsed nuclear bombs, which have their own problems, such as releasing large amounts of radiation into the atmosphere and numerous EMP events.

Quite a few non-rocket solutions have been proposed, many are supplements to chemical rockets. The OP mentions 1970 tech level, most of the non-rocket solutions - including a balloon launch platform do not seem to be 1970 tech level.

Maybe you need a slightly more massive planet to keep the Michael Karnerfors out of orbit with 1970 tech, but the principle is simple, more planetary mass makes LEO much harder and it is already very hard. Maybe you think 25% increase won't stop all LEO at the 1970 tech level, OK how much then 35%, 50% - the principle is the same regardless.

Also note that micro satellites make less sense with 1970 tech under the heavy planet model. You need computers to do smart things to make a micro sat useful for many things. The Apollo guidance computer was introduced in 1966, it required 55 watts of power and weighted 32 kg. Because micro sats would not be useful for a large variety of tasks, the value of balloon and plane assisted launches would diminished as you could not launch large satellites without a very expensive launch platform. Vanguard is perhaps the best example example of what value of non-smart satellite could accomplish.

I believe several people are ignoring something. If the cost is too high compared to the value, no-one will launch things into orbit. For example, if you are willing to be ludicrous, you could launch a LEO satellite from sea level using chemical rockets on a planet with 200% of earth's mass. You would need perhaps 10 rocket stages to put a very small payload in LEO. The cost would be perhaps 1 billion USD per kg or more plus many billions in sunk development costs. It is not just a matter of physical impossibility, just that no-one would do it. The expected value would be far too small to motivate such an action.

So, maybe you think national pride, or a publicity stunt could motivate such a stunt. This seems very unlikely to me giving that the risk of a failure after spending a huge amount sum would still be quite large. Looks at the failure rate of the earlier orbital attempts -- and this stunt would be far more complex. National leaders, or publicity seekers do not like to be associated with huge failures either. I cannot envision anyone doing such a thing.

Just saw this new article on Real Clear Science -- they conclude 50% means we might be stuck here. Thet probably copied my stuff :-)

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    $\begingroup$ The Tyranny of the Rocket Equation applies only if you start from the ground. If you use an airborne launch platform — which is tecnically more difficult than just tacking on more fuel and starting from the ground, but none-the-less feasable — then you drastically reduce the impact of The Tyranny. Also if you increase gravity, then you also increse the density of the atmophere, which makes for more efficient airborne launch platforms. $\endgroup$
    – MichaelK
    Jun 21, 2016 at 11:09
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    $\begingroup$ Actually, if you do the math, you will find that staring from a 10 km or 20 km altitude is not actually a big advantage. Mt. Kilimanjaro would seem like an excellent starting place, yet nobody uses it. I chose 25% earth mass increase because it overcompensates for minor improvements. Sic Semper Tyrannis $\endgroup$ Jun 21, 2016 at 11:48
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    $\begingroup$ Not my intent to insult you but the NASA calculations for launching from even 35 km suggest a 25% decrease in fuel for LEO. I consider this a minor improvement in light of a planet that has 25 percent more mass as high balloon launch will necessarily be micro sats, or an exceedingly expensive balloon launch. We never even considered microsats until we had considerable experience with the advantages of satellites. The initial horizontal velocity component advantage is quite small (less than 1% as you are less than 1% higher). Perhaps we only differ in what is considered a minor improvement. $\endgroup$ Jun 21, 2016 at 12:12
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    $\begingroup$ @MichaelKarnerfors : sorry, but you're wrong. You will not reduce the costs by any significant margin by launching from an elevated platform. Most of the energy required for the orbit is spent achieving the huge velocity, very little of it is used to gain altitude. $\endgroup$
    – vsz
    Jun 21, 2016 at 17:30
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    $\begingroup$ @James_pic -- planes have other advantages too, such as not having to reach orbital velocity, in-flight refueling, acceleration of the ambient air to provide thrust, etc. not carrying oxidizer was meant only to state only them major characteristic that distinguishes from rockets. $\endgroup$ Jun 22, 2016 at 16:48

A bunch of debris in orbit

This is especially flavorful if the world in question has been inhabited for a long time, and the current civilization is built on the ruins of one much more advanced.

At orbital speeds/escape velocity, tiny flecks of paint can be devastating to finer equipment, and large debris would obviously cause catastrophic damage.

I'm not a fan of the film, but Gravity is a good example of what can happen. As is the manga Planetes.

Here's a link, as well: https://en.wikipedia.org/wiki/Kessler_syndrome

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    $\begingroup$ Actually, this is very unlikely to prevent initial LEO launches, as the rubes on the ground probably don't know that it is a shooting gallery upstairs. It is only after they lose a couple of early launches that they will likely realize that something bad happens when you launch. $\endgroup$ Jun 21, 2016 at 19:59
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    $\begingroup$ @GaryWalker I'd say that still fulfills the question's request. It doesn't say it has to prevent them from trying, just succeeding. $\endgroup$
    – ceejayoz
    Jun 21, 2016 at 20:05
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    $\begingroup$ Considering that success is defined as only a single orbit, even with huge amounts of debris, the early launches will luck out and manage a single orbit, esp. given that the higher drag in LEO deorbits the debris with considerable effectiveness. When the value proposition is altered by the relatively common destruction of your satellites you will be much less likely to launch. $\endgroup$ Jun 21, 2016 at 21:29
  • $\begingroup$ debris, or full fledged rings. $\endgroup$
    – njzk2
    Jun 22, 2016 at 17:15
  • $\begingroup$ I like this solution, living on a planet under constant bombardment of cosmic dust would make even a single orbit risky. Considering that there are thousands of micro meteorites that fall hourly the time it takes to prepare a launch above ground might make it a non-starter. Perhaps they dwell underground. $\endgroup$
    – KalleMP
    Jan 8, 2019 at 18:02

When dealing with other societies of human-like species, there's a lot of stuff that you can play with, other than technical. A few other reasons that hasn't been mentioned could be emotionally, culturally or even religiously based.

  1. Old religious texts says that the seed of evil fell from space (or something). While the religion doesn't have a massive hold on all technological advancement, it still has created a collective societal fear of space.

  2. Politicians sometimes campaign on not letting people violate the 'purity of space'. Just as clean water is a human right, so is a pure space above us. Sounds strange to us, but is something that could be developed further.

  3. Due to atmospheric or other conditions - there's no stars visible. No moon either. Space looks like a completely empty void. No use exploring it. Exploring the deep oceans is seen as much more 'scientific' pursuit. People who talk about the void too much are seen as hacks.

  4. Regulation. The current societies that are capable of developing the technology for reaching space are under heavy regulation from whatever Governmental unit. Not that this has to be a political point being made. Regulation could be as a result of conserving fuel, which is used to just barely generate enough power. All excess uses of rocket fuel have to be approved by a very bureaucratic process.

  • $\begingroup$ Yes I was going to mention superstition as well, like earth is flat type :) $\endgroup$
    – rogerdpack
    Jun 21, 2016 at 20:37
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    $\begingroup$ @rogerdpack no one has really believed the earth is flat in the past 2000 years. (except a handful of nut-jobs here and there, but nothing serious). Any sentient society with a few observant people will quickly advance to the point where they understand how it works. (although not seeing stars probably won't hepl) $\endgroup$
    – njzk2
    Jun 22, 2016 at 17:21
  • $\begingroup$ Here's one: no appropriate fuel sources? Assuming they don't figure out atomic, that is :) Or another: inability to see stars and moons, so they think there's nothing out there, or nothing worth pursuing...hmmm... $\endgroup$
    – rogerdpack
    Jun 22, 2016 at 18:56
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    $\begingroup$ Re 3: en.wikipedia.org/wiki/… $\endgroup$ Jun 24, 2016 at 18:10
  • $\begingroup$ This is pretty much what I was going to say. Another slight variation on these themes would be: social/cultural fixation on using science only to create weapons or make money, to the point where space exploration seems a fanciful extravagance rather than a serious endeavor to be earnestly and broadly pursued. (Hey, that kind of describes Earth at this time....) $\endgroup$
    – Wildcard
    Jun 24, 2016 at 19:18

Mankind had firearms since the 10th century CE, and rocket fireworks since the second century before the common era. Still took us until the 1800's to get air-borne, 1900's to get wing-born and all the way to the 1960's to get a human into space.

The short answer: The apes have not gotten that far yet. Firearms or even rocketry does not equate to space-faring.

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    $\begingroup$ Although I agree that the Planet of the Apes example was pretty poor motivation, it was just motivation, not what the question was about. $\endgroup$
    – DCShannon
    Jun 21, 2016 at 21:45

Going to space is extremely difficult. Even to say the human race has been to space is pushing the truth a bit considering how very few of us have actually been there. The chances of any of us who would like to go there getting there is minuscule. (So in a way we're all prevented from going to space)

To answer your question, look at why "we" went to space and take away that factor. It wasn't simply the next logical progression from firearms, you don't just point your gun at the sky and expect to launch a bullet into orbit.

For the human race, there was huge political pressure to do it, between the end of World War 2, where German rocket technology in some form was discovered by both the Soviet Union and the United States, allies in the war who were afraid of each other.

Even then, there was no reason for space travel - it was a ridiculously costly endeavour for no return. But the heads of rocket research on both sides happened to push for it and managed to convince their respective governments that if they didn't, the other would and be able to drop missiles from orbit onto their respective countries.

So take away World War 2 or the German research or the Cold War or even the fact that both Superpowers managed to get access to the technology, and no space travel.

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    $\begingroup$ I'd say the Space Race was worth it simply due to the many hundreds of associated technologies that were developed during it. $\endgroup$
    – SGR
    Jun 21, 2016 at 13:54
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    $\begingroup$ I'm reasonably sure that the ability to drop bombs on your enemy represents a significant return on investment. $\endgroup$
    – Richard
    Jun 21, 2016 at 15:35
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    $\begingroup$ Reconnaissance and mapping were also pretty big reasons for sending stuff to space, even without the tensions. (The tensions did help though) $\endgroup$
    – DanTheMan
    Jun 21, 2016 at 16:16
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    $\begingroup$ +1 for "Going to space is extremely difficult." There's a reason the term "rocket scientist" exists in the modern lexicon as an equivalent to "super-genius." $\endgroup$ Jun 21, 2016 at 18:15
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    $\begingroup$ For the US, the primary motivation at the time was spying on the Soviets, which is why the US was developing launch capability. Eisenhower was actually kind of happy Sputnik has been launched because this gave precedent to having satellites overfly foreign airspace. $\endgroup$ Jul 6, 2017 at 21:08

Hostile Aliens

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If the planet were frequented by hostile aliens who blew up anything that flew more than 20km above the ground, it would make going to space significantly more difficult. A fledgling sentient species would first have to develop shielding capable of stopping a death ray before they could even think about planning a space launch.

Plus, gathering the requisite manpower to conduct such a launch would be difficult when everyone is busy toiling in underground chocolate quarries.

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    $\begingroup$ However the fact that someone is deliberately preventing space travel would increase the motivation to do it in order to beat the aliens. $\endgroup$ Jun 22, 2016 at 11:09
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    $\begingroup$ @TomášZato no amount of willpower will help you slam through a speeding truck and survive. Technology might, but theirs isn't advanced enough. $\endgroup$ Jun 23, 2016 at 5:57

A planet with dense, constant cloud cover might be a strong demotivator. One of the reasons we were drawn to explore the solar system is that we could SEE it. If you lived on a planet with a dense, constant cloud cover, it might not even occur to you that there is anything else up there but sky.

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    $\begingroup$ For example: read about Krikkit in the Hitchhiker's Guide to the Galaxy. $\endgroup$ Jun 21, 2016 at 20:00
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    $\begingroup$ Do note that the only actual requirement is that the stars not be detectable to the species on the planet. The atmosphere could be as clear as glass, but if they have no eyes... $\endgroup$
    – Perkins
    Jun 21, 2016 at 20:59
  • $\begingroup$ By 1970 we commonly had weather balloons and even commercial air travel that were well above the cloud deck. We would have discovered stars well before 1970. Now, if we had no ability to see its a different story. $\endgroup$ Jun 22, 2016 at 5:47
  • $\begingroup$ There are several planets and large moons in our own solar system where the clouds reach almost to the top of the atmosphere. For dense atmospheres, the only way to get above the clouds would be rockets. $\endgroup$ Jun 22, 2016 at 17:17
  • $\begingroup$ @ByronJones But in a dense (but still earth-like) atmosphere, aircraft would be able to fly higher. I haven't done the math, but by gut, I'd expect the ratio of "max cloud height" vs. "max flight height" would be pretty similar independent of atmospheric density, since both are very much dependent on the same factor. $\endgroup$
    – Luaan
    Jun 27, 2016 at 9:18

Lack of motivation

The space race happened at a point in our history when the shooting war had stopped. It was a way of showing technological dominance over the opponent while not actually killing anyone or risking getting killed.

a) the shooting war was still going on
b) there was no challenger in the race
then there's a reasonable chance we would never have put a man on the moon, simply as a result of the massive cost and risk involved, as can be seen by the fact that we haven't been back in a very long time.

In terms of a continued shooting war, there's a possibility we'd be in space a lot more, as another front to the land/sea/air/space, war but it would be a very different use of the low earth orbit to the one we have now.

As importantly there are the industrial scale factors. Automatic weapons can be built by one man in a shed in Pakistan (AK47 clones mostly). They don't need a massive industrial complex. Going to space needs massive manpower and access to a level of technology a world away from even well built assault weapons.

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    $\begingroup$ This is not true. SpaceX designed and built their own rockets in the absence of a shooting war, and it doesn't have any significant competitors in its launch niche. Granted, had the last rocket exploded they would have not succeeded, but all it takes is time and one billionaire to put their mind to it. $\endgroup$
    – March Ho
    Jun 21, 2016 at 22:46
  • $\begingroup$ @MarchHo, that's like saying Dacia also build cars, what's so special about Karl Benz. $\endgroup$
    – Separatrix
    Jun 22, 2016 at 7:01
  • $\begingroup$ @MarchHo I'm not sure what SpaceX's "launch niche" is in your mind, but SpaceX has quite a few serious competitors. $\endgroup$
    – Dan
    Jun 22, 2016 at 18:58
  • $\begingroup$ @MarchHo One thing you need to take into account is that SpaceX is relying a lot on government funding, so there is political agenda to the project latimes.com/business/… $\endgroup$
    – skmasq
    Jun 23, 2016 at 12:42
  • $\begingroup$ Well, the Germans tried to go to space during the WWII as well. The V-2 was already pretty close - it was almost the first sub-orbital ballistic rocket (it typically went to 80km above sea level). "Rockets for New York" would have been full-blown sub-orbital space rockets. Of course, it's not like the Germans historically had a shortage of megalomaniacal projects, but... they still tried to, during a "shooting war". $\endgroup$
    – Luaan
    Jun 27, 2016 at 9:16

A bit OT to your question, but a nod to "The Planet of the Apes" - ie. a civilization built on the ruins of an older one:

The older - "original" - civilization had already used-up some necessary resources or raw-material needed to allow space-flight to happen.

It wouldn't even be necessary for a particular resource/raw-material to be completely used-up, as long as all the easy accessible sources were exhausted... ie. what you could get at with primitive technology.

Nor would it have to be something directly associated with space-flight; it could just as well be lack of resources preventing them to reach a necessary technological level (eg. can't make electronics), or preventing them from making necessary tools (eg. computers or welding-equipment - or even mining/extraction-tools).

One would also expect that space-flight would come pretty late in their technological evolution, so if lack of resources preventing them from reaching prior steps - eg. combustion engines, air-crafts, plastics - space-flight may be difficult to achieve... and perhaps more important, difficult to imagine.


What if some disaster - lets say a new ice-age - devastated our civilization, decimated our numbers, destroyed most our of knowledge, destroyed our machines... Could we start from scratch? With no easy accessible ore of iron or copper... with no easy accessible oil or coal... Remember oil isn't just used for combustion-engines, it's also used to make most plastics. I think it would be difficult to reboot our society - even though we would know it could be done and much about how.

  • $\begingroup$ I thought it was a good argument until I realized that people from the future would just have iron and copper laying around, no need to mine them, and the same with many materials. However the oil would be gone, which would be a big downside. Or they'd use alternatives, as might have happened if we didn't discover oil. $\endgroup$ Jun 22, 2016 at 0:07
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    $\begingroup$ @FranciscoPresencia There aren't really that many alternative to oil, not for all it's applications - especially with primitive technology. There is coal, but the easy-to-find coal has been mined. You can make coal from tree - if there are large woods - but it's lots more work than easily mined coal. As for iron and copper... depending of how long since ti was made, it would've corroded/rusted. It's more difficult to find and mine rust-layers from old junk-yards spread around, than to find and mine iron-ore. And as for making iron, lack of coal could become a problem... $\endgroup$ Jun 22, 2016 at 15:38
  • $\begingroup$ @BaardKopperud Irron ore is rust. In fact, common rust (stick some iron outside and let it go red) is Fe2O3, the formula for hematite (70% iron by weight), which is a high-grade iron ore. The issue is if the corrosion is followed by dispersal and not concentration, not the corrosion. $\endgroup$
    – Yakk
    Jun 24, 2016 at 15:29
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    $\begingroup$ @Yakk Yes, but I was thinking more of the difference between mining it from rocks in places known to have high concentration of iron... and mining - and finding - iron from lots of spread junk-yards, where the iron may be as a thin "rust-layer" down in the soil, and probably in rather low concentrations. $\endgroup$ Jun 24, 2016 at 18:41

Nothing of value in their solar system, not even of scientific or curiosity value. No moon, no planets, no asteroids. Maybe even some kind of anomaly that prevents the stars from being visible. As far as they know, their planet (and I guess their star) is the sum total of existence.

  • $\begingroup$ Welcome to World Builder SE! $\endgroup$
    – Jim2B
    Jun 22, 2016 at 0:32
  • $\begingroup$ See also Planet Krikkit for an example of exactly this scenario being used as a major plot point. $\endgroup$
    – Spudley
    Jun 24, 2016 at 20:43

Eliminate their motivation

(This meant as a compliment to separatrix's answer.)

No fear of extinction

In addition to the Cold War and the space race, we also have the fear of a future cataclysmic event. Some scientists are worried that if the Earth is struck by a big enough piece of space debris, or if we have a global thermonuclear war, our species will cease to exist. Their solution to this fear is to spread the human race to other planets and other habitable places in space.

Earth has a major extinction event in its past. If the planet where your alien civilization is has never had a major extinction event, and has never had weapons capable of destroying the entire population, then they have no reason to fear that their species could be wiped out. This eliminates the desire to spread the species out in order to prevent the risk of extinction.

Stable population size and resources

Another reason we might want to leave Earth is that our population is continuing to grow and consume more and more resources. We may yet be a long way off from using up all our natural resources and running out of room, but we're close enough that we're worried about it being a problem in the future. Spreading out to the stars is a solution.

If your aliens have already developed sustainable energy sources and eliminated all significant reliance on limited resources, then expanding to other worlds to obtain more resources isn't an issue.

If your aliens have reached a stable 1:1 birth:death ratio, then population growth isn't an issue either. Interestingly, humans are trending towards that already.

No large-scale organization

Going to space requires a massive effort across many disciplines and over many decades, even centuries. If your aliens are not given to working together on the required scales, then developing technology to transit through space will never occur.

This doesn't prevent technology from developing, but it keeps any technology requiring large group efforts from developing. Most inventions would be the efforts of individuals and small groups.


One of the biggest arguments against going to space in the first place was and is that it's not worth the cost. One of our motivators was optimism and imagination. We look to the stars with hope.

If your aliens are a bit more pessimistic and risk-averse on average than we are, they might see space as a worthless vacuum with little to offer, and not worth the expenditure of effort and risk to reach it.

Maybe they don't even have enough interest in the stars to have studied them much through telescopes, and they see stars as a flat blanket rather than as distant suns similar to their own nearby one.

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    $\begingroup$ Or, as evidenced in Hithiker's Guide to the Galaxy with the planet Krikkit, just make the night sky devoid of anything interesting. If they never look to the stars, why would they ever go there? $\endgroup$ Jun 21, 2016 at 17:54

Imagine life on the large moon of a gas giant planet. Like Europa, it has a thick crust of ice. An ocean is hundreds of km deep, with volcanic vents at the bottom.

It's in a harsh radiation belt of the planet. We find that hazardous and it damages electronics. But here life depends on it, playing over the ice layer to fix nitrogen etc. The beings far below don't get exposed directly to it any more then earth's core bothers us.

Beings develop intellegence and technology. They must leave their deep habitat and enter areas where the loss of pressure is fatal, as water itself acts differently at such depth! The pressure alone makes ensymes fold differently, and the subtle difference in molecule packing affects the biochemistry.

Then they have to get through the ice. The radiation kills their cells and kills any advanced electronics.

They don't see any sky, and are not drawn to the heavens. Why haven't we reached the Earth's mantle? They have two directions to explore, and don't know that one of them even leads to "space" as a concept. To them it's more like us reaching the core.


Let's consider that of the 200,000 years that we've been around here, we only have been capable of launching satellites during the last 59 years. The fundamental issue here is that a technological society has to be based on modern science, but the scientific method you need to use to do science properly, is very counterintuitive.

For almost all our history, we had to survive in Nature. We needed to trust the information you get from authority figures, your parents, and other respected people who have a lot of experience and knowledge. That knowledge is not in practice reducible to more fundamental basic facts to primitive people living in Nature. So, our brains have evolved to respect authority. Only about 3 centuries ago was the modern scientific method invented as a better way to make progress. But, of course, this only worked because enough knowledge had been accumulated to allow more and more information about Nature to be extracted from experiments and observations.

It may be the case that once a civilization has developed far enough to start to do science and build a technological society, intelligent machines will appear in a matter of a few centuries and that these machines will take over society. Space flight is very expensive, if we were to push for a manned mission to Mars, then it may well be profitable to spend, say $20 billion to develop an AI robot that would do the job as well as humans. So, space exploration may be a natural boundary where AI systems will take over from their biological ancestors.


Intelligent species don't go into space because it's too expensive. Crewed space flight is prohibitively expensive.

An intelligent species will cooperate for the benefit of all. Resources will be shared for sustainable development, to optimise the well-being of all sentient beings in short and long term, based on a utilitarian framework. Perhaps it uses AI to estimate the optimal solution. Because the species are intelligent, they don't destroy each other in warfare. Although they do have a limited use for explosives in mining and road construction, the development of rocketry is not considered.


Apart from near-instantaneous global communication, weather forecasts with unprecedented accuracy, environmental monitoring (Earth observation), progress in science and technology, sheer awesomeness, and peace, what has putting stuff in Earth orbit ever done for us?

  • $\begingroup$ Peace? It seems there are some areas in the world where we need more satellites! $\endgroup$
    – celtschk
    Jun 25, 2016 at 14:07
  • $\begingroup$ @celtschk Plenty of peace-promoting use for space, but one may speculate if the very expensive treshold to get there in the first place had been crossed if it hadn't been for war / cold war. $\endgroup$
    – gerrit
    Jun 28, 2016 at 10:28

You can make a Nuclear War that decimates all population, stopping the technological development, just like the Krogans in Mass Effect.

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    $\begingroup$ Welcome to the site, Giancarlo. Please note that we expect answers be detailed, rather than one-liners. If you would edit your post to indicate why nuclear war would be the best solution to the problem of permanently preventing a species from achieving spaceflight, we would appreciate it. $\endgroup$
    – Frostfyre
    Jun 21, 2016 at 16:29
  • $\begingroup$ Yes it ruined their civilization but they can still go into space and I was not aware that the war stopped their technological progress. $\endgroup$
    – Vincent
    Jun 21, 2016 at 16:58

Deny them fuel.

Leave them a planet in which all the easily accessible fossil fuel sources have already been depleted.

No cheap fuel - no industrial revolution - no technology driven society - no space travel.

Sure. Let's say they learn the secrets of the water pumping windmill, find electricity, develop hydro power, come up with an electrical grid, electric trains. But there is an inherent social consciousness about economy of energy - it is too precious to waste. As are metals, without unlimited fossil fuel to power mining and smelting.

They'd love to use U-235 to achieve a critical mass and breed fissile fuels from abundant U-238 and Th-232. But the found uranium is much more depleted of U-235 than it ought to be, making this almost impossible.

It would be a "slow society" - canals and electric trains. The personal automobile would be an absurdity.

Now someone proposes to launch a single vehicle weighing 3000 metric tons - where every gram of that vehicle is made of an exotic metal or synthetic fuel that can only be made at staggering energy expense. Used once and destroyed! This would not correspond with the society's values.

How do you engineer such a world? Easy. We're doing it right now.

We're mining all the easy-to-get fossil fuel and minerals. What happens if our civilization collapses and another one tries to industrialize 2000 years from now? They're going to find our leftovers - veins we abandoned as uneconomical to mine. That doesn't mean there's nothing there, it just means it's a lot more work, which makes it rather inefficient.

Eventually, their engineers will figure that a previous society mined out those resources before them. That will be particularly obvious with anything nuclear.

  • $\begingroup$ or simply have the geology of the world such that the Carbiniferous period didn't serendipitously leave billions of tons of long-chain hydrocarbons lying around. Maybe with slightly a slightly less reductive (iron) crust, so the O2 % was slightly higher, so the early plants and forests burned. $\endgroup$
    – sbeam
    Jun 25, 2016 at 20:50

A physiology that requires taking lots of liquid with them to survive.

They evolved in the sea. They require taking lots of liquid with them to "breathe", maybe because they need to be continually immersed in it (they breathe through their whole skin?)

Liquid is much heavier than air. Crewed missions to orbit would be prohibitively difficult and expensive.


Religion: A state religion which opposes space flight, but other than that has very little strong statements or controversial opinions, so it's impossible to disprove, provokes nobody into fighting against it.

As an example:

Praise Mother Earth!
For She guided us into existence.
For She is the soil beneath our feet,
the air we breath
and the waters we swim in
For She gives us food and drink
For She is the majestic beauty of the mountains,
the roaring waves of the sea
and green wonder of the jungle
which is our inspiration
For she is the torrid desert,
the biting polar wind
and venomous snake
which proves to us Her might
and teaches us to hold firm together and
be kind to each other in the hour of need
because we are all Her creatures and part of her
She is our one and everything in this
otherwise bleak and empty universe,
our one spot to thrive,
our one spot to live,
out one spot to be!

So keep this in mind,
for now and eternity

Make this the core tenet of the religion of the absolute majority of people on the planet. There will be no reason to separate this from the state or fight against it because really, you need to work if you want to interpret this in an offensive way. So the constitutions will probably also contain it, or the monarch or oligarchs will adhere to it. No one will really fight against it, because the worst you can really say against it, is that it's pointless.

The biggest stretch about this is that people didn't change it over the course of time to confirm their bigotry or help them keep power and similar things.

There is no economic need to (personally) go to space anyway, other than our fascination with it and basic research. Both of which could be directed at the Earth itself too, there is plenty of it which is fascinating and which we know little about. So I don't think they would fight against it too much. At some point they would want to develop satellites for practical and scientific purposes at which point they would probably reinterpret their religion to mean that they can send objects to space as long as they stay out of it themselves and even later reinterpret it again so that they can do manned space travel, but you said you want technological advancement at a 1970 level, so if this buys you a 100 years that would be plenty and it could conceivably give you much more than that.


How about much less ability to withstand acceleration than humans have? That's likely to make the rockets much less efficient when carrying sentients that survive the trip.

  • $\begingroup$ I think that's a pretty good idea, but it would probably have far reaching implications about that species. How much of a crash would it take to kill them? Would walking into a glass door be mortal? What about emergency breaking in a car? What parts of the body would be affected? (I think in humans the eyes are the weakest part) $\endgroup$
    – Nobody
    Jul 4, 2016 at 20:37

Like Giancarlo said, War. But not necessarily a nuclear one.

Say your species have been separated in two clans since millennia, or maybe a more recent rupture because of politics (the Cold War w/o nukes). Both hate each other so much they sacrifice everything to defeat it. The thing is, their powers are equal, so their strikes only succeed in halting technological advances. If this happens before major technology emerges (Middle Ages), well you could end up with a perpetual war freezing a species, making it ignore all but the enemy.

And you get lots of work to implement a cool backstory to explain the war.

  • $\begingroup$ But no V-2 as part of the war effort? No ICBM or quest for higher ground? $\endgroup$
    – JDługosz
    Jun 22, 2016 at 16:05
  • $\begingroup$ I was thinking of a war starting at a technology level comparable to human middle ages $\endgroup$
    – Yk Cheese
    Jun 22, 2016 at 16:06
  • $\begingroup$ So, sadly, no V-2 ;-) $\endgroup$
    – Yk Cheese
    Jun 22, 2016 at 16:07

How hard can it be, it ain't rocket sc... Oh, wait, it is!

Designing rockets is hard. Building rockets is hard. Not blowing them up during the launch is hard. Aiming those pointy things to the correct point in space is hard (space being big and all). And not only is it hard, it's terribly expensive, our bodies aren't build for it and it's mighty inconvenient to stay up there for long.

There's no real reason to prevent a species from going to space. Under normal circumstances I'd say we weren't technologically enough advanced to make it worth it when we started.

However, worth is subjective. The worth increased rapidly simply because of war and the threat of war. See colmde's answer.

Also keep in mind that Planet of the Apes deals with sentient species bigger than ours. Much bigger. So not only is space exploration hard (we have barely scratched the surface ourselves), it's even harder for them. It's also more inconvenient for them.


A race of intelligent jellyfish on a large planet (it needn't have a large surface gravity).

  • They evolved under water and require large quantities of heavy liquid to even exist.
  • Even developing the chemistry and the high energy processes to start thinking about rockets would be exceedingly difficult. Metals aren't abundant or easy to mine, except for some submarine nodules.
  • Physiologically, they can't stand high accelerations (they might die or fragment into "daughter" semi-sentient beings: that's their reproduction cycle - when they're ready they move ashore and are broken into pieces by the waves. The daughter colonies prosper in the rich waters near the shore, and some of them eventually regain the sentient depths).
  • They can't stand micro-gravity at all (even human beings have problems for long periods; their long period is a few minutes).

Such a race might not even be technological unless incredibly motivated (it happens to the pom race in Spinneret), and it would need an external technological kickstart (Milton A. Rothman's Heavy Planet), or some phlebotinum such as gravity fields (Asimov's Victory Unintentional).

Further demotivators could be religious, too. Or the planet might be quarantined by another race (Star Trek's The Abode of Life), possibly for their own "good" (the planet Parval in Weber's Empire from the Ashes).

Or you might think of fancier limitations. For example these beings have evolved to live in mental symbiosis with each other - being out of reach of the supersonic consciousness CB of the colony is first physically painful and mentally exhausting (sort of an Internet Deprivation Syndrome), then causes mental breakdown. You'd need a very rare kind of psychopath, a "loner", to even think about becoming an astronaut.


What if the aliens lived on a planet that's within the habitable zone of a red dwarf? Because it would have to be so close to the star, planets like these would surely be tidally locked. That means that, with one side constantly facing the star, half the planet would be scorched while the other is frozen solid (aside from a narrow habitable band between the two).

A species living on a world like this would definitely have a lot of challenges to face if they ever tried to reach orbit. Spacecraft would have to withstand temperature changes from hundreds (or even thousands) of degrees above the boiling point of water to temperatures well below zero degrees Celsius. Plus, due to the radiation pressure of the nearby star, it would be very difficult and very expensive to attempt to keep a robotic spacecraft in orbit (let alone a manned mission)


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