"Aliens used uranium and plutonium to generate energy for thousands of years. However, by the time they got into space exploration, they've used up every last bit of it. That's why they'll have to try leaving their high-gravity planet with chemical engines."

This sounds good to me, but I fear that there are options I'm not considering. Have I given enough explanation for why nuclear energy can't be used? Are there other ways they could make use of nuclear energy? How can I explain away all the ways they might still be able to make use of nuclear energy to heat up their fuel?

  • $\begingroup$ Er.. Didn't you just said they used up every last bit...? $\endgroup$
    – user6760
    Jan 14, 2020 at 11:29
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    $\begingroup$ Do you have an idea what a time span of "many thousands of years" means in the development of a technological society? For example, there are less than five thousand years the oldest Sumerian clay tablets and the latest Apple iPad electronic tablet... After many thousands of years since then first development of atomic power, why don't they have antigravity? $\endgroup$
    – AlexP
    Jan 14, 2020 at 13:42
  • $\begingroup$ @AlexP Wow. You just assumed my aliens would behave/think/live/age exactly like humans? Why do you assume they'd live in a similar environment? Using a power source and being on a certain level of knowledge with resources to invest into it are two completely different things. $\endgroup$ Jan 14, 2020 at 14:32
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    $\begingroup$ "Using a power source and being on a certain level of knowledge with resources to invest into it are two completely different things": this is . . . unexpected. No, they are definitely not two different things. How could they possibly use uranium and thorium (maybe that's what you meant by "plutonium") to generate energy if they did not have the level of knowledge with resources to invest in it? Making a nuclear reactor does not happen by accident. It requires a highly advanced technological civilization with a very good understanding of physics and resources to do research. $\endgroup$
    – AlexP
    Jan 14, 2020 at 15:03
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    $\begingroup$ @JRE: Not so that they consume every last bit of fissile metals, no. $\endgroup$
    – AlexP
    Jan 14, 2020 at 16:18

7 Answers 7


They can only use as much uranium as they had available, and not all planets will be created equal in this regard. Even something as simple as evolving much later in the life of their planet will give more time for useful fissile materials like U235 to decay into less useful elements. Combined with a lower abundance of fissiles in the protoplanetary disc the world formed from, there's no problem explaining the lack of nuclear fuel.

What will be harder to explain is how they were a technologically advanced race for thousands of years and failed to develop a launch system that will actually work for them, because chemical rockets will not (as discussed in previous questions of yours, here and elsewhere, ad nauseam).

  • $\begingroup$ Couldn't they somehow fuse other atoms together to create new uranium atoms? Maybe I'd have to explain how they already did that and the newer procedures would take a lot more energy than before... Something like that $\endgroup$ Jan 14, 2020 at 11:57
  • $\begingroup$ @justthisonequestion it isn't at all energy efficient. Synthetic elements made in particle accelerators can only plausibly be made in tiny quantities at great cost. It isn't as inconvenient or expensive as antimatter, but it wouldn't be practical. $\endgroup$ Jan 14, 2020 at 12:13
  • $\begingroup$ @justthisonequestion, it's much, much more better to fuse hydrogen to create new helium atoms. And this is hard to deny this reqaction for your aliens. $\endgroup$
    – ksbes
    Jan 14, 2020 at 12:46
  • $\begingroup$ @ksbes I was gonna say something about fusion being perpetually 50 years away being a reasonable fictional handwave, but after a thousand years of technological civilisation someone must have been able to make a laser-triggered nuclear weapon, which gives you Orion drives even in the absense of any other sensible launch system. So yeah... $\endgroup$ Jan 14, 2020 at 12:52
  • $\begingroup$ @StarfishPrime So you don't know how it would be achieved, but you know that it could be used inside of a rockets engine... Is it just plausible to you? Why? Is it something like 'as soon we know this exact part, we can use easily for most things' Please put it into your answer. Thanks for your time. $\endgroup$ Jan 14, 2020 at 16:03


I think you have bigger problems than "nuclear power" if you're trying to force your Thousands-of-Years-Past-Fission Alien Society to use chemical rockets.

Why don't they already have:

  1. Space elevator (AKA Bean Stalk). A geostationary satellite tethered to the planet below. An elevator goes up and down the tether, making it Much Cheaper to get out of the planets gravity. Building one is primarily an engineering problem. That tether has to be Really Strong. Carbon Nanotubes (CNTs) should work, but linking them together well enough or building them That Long in the first place is a Hard Problem.

  2. Teleportation/portals: Aliens that advanced should have come up with something we think is impossible, right?

  3. Ditto for gravity manipulation, reactionless drives, ion drives, and so forth. Even if they only previously used that technology for their equivalent to cars, it should still exist.

Possible explanations

  1. "It's against our religion". Riiight... It was forbidden to "leave our gravity well", or whatever. So what changed? Why are they doing it now? Is the group using chemical rockets a bunch of Heretics?

  2. Apocalypse. Alien Society ain't what it used to be. They still have some advanced tech they were able to salvage, but had to cobble together their rocket out of whatever they had available. Disease, disaster, lost a war...

    2a. Uplift Uprising. A species of animal native to the alien world genetically/surgically modified to be intelligent servants rose up against their masters. They might be just as smart as humans, but not be intelligent/educated enough to understand some of the super-tech their masters used... or they lack some other key ability their masters possessed (telekinesis, "magic", some sense [sonar, color vision or hammerhead-esque electrical sense: hard to use a technology when its controls are invisible to you[by design?]], any ol "deus ex xeno" will do) that prevents them from fully utilizing their masters' technology.

  • $\begingroup$ (1) might not be possible on the alien world. It isn't unambiguously possible on Earth, after all. (2) is magic, not science. (3) ion drives do not belong in the same category as those other bits of probably impossible handwavium. They're not much use at the bottom of a deep gravity well, either. $\endgroup$ Jan 14, 2020 at 13:32
  • $\begingroup$ (1) The requirements might change, but unless they have a severe orbital garbage problem... hey! See: "Kessler Syndrome" (2) Is a catchall for anything we currently believe to be impossible. I refuse to believe that human physics has reached the point where everything we think is impossible is actually impossible. (3) Our current ion drives are useless at the bottom of a gravity well (or in an atmosphere IIRC). Give our ion tech a couple thousand years to improve, and you might end up with TIE fighters (which also use an ion drive). $\endgroup$ Jan 14, 2020 at 13:55
  • $\begingroup$ (1) Even if they could synthesise suitable handwavium then maybe they could build one, only as the OP says, they haven't actually done any space exploration yet, so they can't possibly have a space elevator. (2) Wishing that magical things are real is tenuously compatible with the OP, but wishing really hard isn't a great solution to the given problem. (3) ion-drives are intrinsically high-Isp, low-thrust engines. They will forever be terrible in a deep gravity well. Use an appropriate design. Not everything is a nail, so put the hammer away. $\endgroup$ Jan 14, 2020 at 14:01
  • $\begingroup$ + for 2a. Reading about the Kzinti recently; that was their situation. They took over the tech of their former masters and there was minimal subsequent innovation. $\endgroup$
    – Willk
    Jan 14, 2020 at 14:54
  • $\begingroup$ And Ringo's most of the "Legacy of the Aldenata" races as well. Not necessarily uplifted, but genetically modified subjugated races. $\endgroup$ Jan 14, 2020 at 16:03

Maybe they can't use fission engines, but if they're advanced enough to have depleted all the uranium in their planet after many thousand of years, they definitely have discovered and developed the nuclear fusion. You will need a good explanation on that.


Health on the Homeworld

Their bodies cannot handle radiation as well as human bodies can. Their ecology cannot handle radiation or other pollution as well as Earth biology can.

It could mean that the aliens riding the ship are blinded or permanently made insane by the act of launching. It could mean that the act of launching a ship ruins a city-sized plot of land for generations. It could mean that the act of launching a ship generates mutants which spread across the land terrorizing it. It could make the next generation imbecilic.

Now there's an interesting story - when does the nuclear ship get to take off? Who gets to decide that everyone's descendants for the next hundred years are animals? The next civilization has to pick up from intentional library caches dotted across the landscape. See Niven/Pournell's "Mote in God's Eye" for cyclic civilization , and Niven's "Ringworld" series for indestructible libraries and a world-serving order of selfless family-less librarians.

Perhaps they have political reasons related to it. NIMBY, and no land left in the world without some owner. A long history of garbage dumping on your neighbor.

  • $\begingroup$ Maybe launching the nuclear rocket risks the planet's atmosphere which will have longterm health consequences to all those left behind ? $\endgroup$
    – Criggie
    Jan 14, 2020 at 21:48
  • $\begingroup$ I mean, radiation is natural, it is present everywhere. If their bodies can't handle radiation, they would've died a long looong time ago. Also, in a nuclear engine (a well-built one, atleast) you're not exposed to radiation. $\endgroup$
    – IloneSP
    Jan 15, 2020 at 8:42

Early in their development, think ancient egypt, they got inspired by an Oklo-style natural reactor. This gave them a headstart over civilizations using wood as a source of heat. But it also left them with little naturally fissile material remaining and a strong cultural bias for large scale/low energy density nuclear tech.

Think more in terms of geothermal energy from fission instead of magma rather then our nuclear power plants. You don't want to run a basically unshielded reactor near your settlement, put it under a mountain or pyramid and have the steam & hot water for your central heating come to you.

Make sure your civilisation isn't advanced enough to deal with metastable helium or metallic hydrogen, both superior rocket fuels without the radioactivity.


At some point in their long history, at a time when they’d reached a staggeringly high level of knowledge and technology, their planet was rife with conflict between many opposing polities. These conflicts threatened the survival of all life on the planet.

Political solutions were deemed impossible since no group trusted off of the other groups. And, no one would disarm their doomsday weapons for fear of being vulnerable to attack by coalitions of the other polities.

A great scientist built a machine that generated a planetary scale field which inhibited nuclear decay by modulating the weak and strong atomic forces in ingenious ways. The machine, once started, would become the ultimate doomsday weapon, and destroy the planet if turned off, but made all of the other doomsday weapons and nuclear devices useless.

Faced with a loss of their deterrents, and effectively protected against obliteration, the polities went mad fighting wars using conventional methods — bioweapons, chemical weapons, nano-tech, masers, lasers, blasters. After a thousand years of warfare, they found their own paths to peace, and formed a one world government.

Now, they want to leave their planet but the great machine still operates, preventing nuclear decay, and making nuclear engines inoperative within a few thousand miles of the surface of their planet


There are ways to generate fissile materials, as we do now in small quantities inside particle accelerators . Your folks had 1000 years and couldn't design systems, albeit energy-hungry, to build up reserves of useful fissile materials?

But more important, the total energy content of fission reactors vs. mass is really crappy from a rocket or space ship point of view. Either chemical engines, ion engines, or some theoretical future discovery of a new field, or harnessing "dark energy," will always win.

  • $\begingroup$ Creating fissile material via fusion will necessarily cost more energy than you can get back by splitting it back apart. And if you can do fusion, then you have enough technology to just fuse the vastly more abundant hydrogen and harvest the energy that releases, no fission required. $\endgroup$
    – StephenS
    Jan 14, 2020 at 19:53
  • $\begingroup$ @StephenS A population II star may be old enough that most of its $^{235}U$, half-life 704 million years has decayed. Not to mention that population II stars are likely to be deficient in uranium and thorium in the first place. Thus they could start a project to produce enough $^{233}U$ or $^{239}Pu$ via neutron bombardment to create their first breeder reactor. $\endgroup$ Jan 14, 2020 at 22:45
  • $\begingroup$ @StephenS what you can do on the ground, with lots of "local" energy is different from what you can do after launching, where you have to carry your energy with you $\endgroup$ Jan 15, 2020 at 16:46

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