Two planets rotate each other and are tidally locked. They are where earth is around a star nearly identical to the sun. One planet is much wetter, featuring oceans, swamps, jungles, and forests. The other is much dryer with plains and deserts. Both are about earth-sized and have similar atmospheres. They are close, like, 3,000 miles surface to surface. A moon about the size of ours orbits the two. This system orbits a sun at he same rate as earth. On one lives a race of lizard-men and on the other lives a race of electric eel-men. I'll let you pick which species belongs on which planet. Anyway, they are both extremely sensitive to radiation. Other than that, you can pretend they are human.

Both planets have futuristic technology, so they have all the modern conveniences of earth, and more besides. The lizard people issue an ultimatum. The inhabitants of the other planet must evacuate to their moon colony or face all out war/extinction. They have 1 year, then the lizards will invade their planet and kill everyone. However, the eel-men have prepared for this...

So, how do the inhabitants of one species kill the other ones without destroying the other planet? If one planet is destroyed it throws the other out of its orbit with potentially disastrous consequences. Also, radioactive weapons are impossible to manufacture, because it would kill all the aliens who would make the bombs. Not only this, but these species are similar enough that they are affected by the same diseases. So, how would one civilization wipe out the other and leave both planets habitable? The moon doesn't have to survive, but the planets' orbit must be left mostly unchanged.

Edit - The moon could be unnatural, put there by these alien species. I don't care about the stability of the moon's formation, but there is a sizable body of something that orbits these two planets.

  • $\begingroup$ Let me know if I can improve my question, it is similar to a few others, but slightly different. $\endgroup$ Jan 19, 2016 at 3:06
  • $\begingroup$ Um... Why? If both peoples have access to space, what could be gained by going to war with your planetary neighbor? $\endgroup$
    – Frostfyre
    Jan 19, 2016 at 3:07
  • $\begingroup$ They want the other planet. It takes way too long to find another planet that can not only support life but support their life. They also want the resources of the other planet. $\endgroup$ Jan 19, 2016 at 3:08
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    $\begingroup$ sighs I guess this isn't the time/place for a discussion on reptilian-eel ethics, either... $\endgroup$
    – Frostfyre
    Jan 19, 2016 at 3:12
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    $\begingroup$ 3000 miles and a moon orbiting their barycenter? That's a crude guess from my side, but how in the universe this did come to existence and stayed stable long enough to make intelligent life that happens to be at the same technological level, AND is - even if these are separate evolutionary ways - affected by the same diseases? Is that some sort of superior alien sociology (and astrophysics) student bachelor work? $\endgroup$ Jan 19, 2016 at 6:42

5 Answers 5


The Moon IS the weapon

The Eel people agrees immediately, which confuses the lizard people. 'We're their betters, and they know it', say a Hawk lizard in triumph; the Dove lizards aren't so sure, but can't put a finger on it.

The Eel people asks for more time - there's a lot of eels to move around, you see. given the swift acceptance, the lizards add another year out of 'condescending acknowledgement'. (Or maybe that's a condition: 'If you give us two years, we'll accept your terms.')

The Eel people starts moving to the moon. One day before the 2-year period expires, they issue an ultimatum: The Lizard people must now pay tribute, or face consequences.

The lizard people laugh it off in disbelief. A couple of hours later, their military centers are vaporized. At the end of the day they capitulate, with minimal loss of civilian lizard life. The Eel people now rules the system.

How, you ask?

They cut hundreds, maybe even thousands of large chunks (10-tons or more) of moon-rock and put it in orbit around the Lizard planet with the help of rockets, or a highly precise mass driver build on the moon's surface. Less gravity means less fuel to move things around, and you have two years to position them. Using simple ballistic calculation, they can make them drop over military bases, cities and other strategic points. Once these enormous kinetic projectiles start falling, they can't be stopped.

(Just as a comparison, a 10-ton falling rock from around our Moon's orbit (384,400km) would carry the energy equivalent of roughly 627 Hiroshima atomic bombs.)

Disclaimer: Idea totally stolen from 'The Moon Is a Harsh Mistress' by Robert A. Heinlein.

I used this calculator for kinetic energy evaluation.

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    $\begingroup$ One problem I see with this is that moving massive rocks around in space isn't exactly stealthy, even with detection systems as crude as what we have on earth right now. $\endgroup$ Sep 1, 2016 at 15:19
  • $\begingroup$ @DevilApple227 very true. One possibility may be to store them over the Eel people's planet and then dispatch them via orbit transfer (since both planets are tidally locked.) $\endgroup$
    – OnoSendai
    Sep 1, 2016 at 15:28

The first thing you need to do is change government and apologise. There's no point going into a war having already threatened a war, they're expecting you. It's going to take a good solid grovelling apology, preferential trade agreements and promises of investment to get out of the situation.

The first thing you're going to invest in is a decent spaceport at the nearest point on the other planet. This is to make sure all your other trade agreements go through more smoothly. Buy the land, buy the land around it. Make a point of offering good money for the inconvenience and noise that people living nearby are going to suffer. Promise jobs to locals but make the working conditions so shocking that they'd never take them, fill the vacancies with your own people, you need to run the place after all.

Try to work out the difference between a military base moving millions of tons of goods and a secure spaceport doing the same. Not a lot. You now have a foothold on the planet. You've also created a trade imbalance on the target world. They're going to want more spaceports spread around the planet to allow other regions in on the goods without planetside longhaul freight.

Given 20-50 years you'll have established military facilities near all their major population and industrial zones.

Next make low bid offers to run critical but risky and dirty industry. Power stations and the like. I know you're against nuclear power but it's a perfect option for this, as are water treatment plants and comms infrastructure. It'll take a couple of decades at least before you're sitting on a solid chunk of critical industry.

When you're ready, pick a nice juicy target, like every major city, shut off power water and comms and bomb at your leisure. They'll be used to your ships in the sky and it'll be too late before they work out what's going on.


Option 1: Super Deadly Eel-Killing Plague of Swift Deadliness (Which is Deadly)

Develop super deadly eel-killing plague of swift deadliness. Simultaneously immunize lizards with a vaccine and/or make super deadly eel-killing plague of swift deadliness the super deadly to eel-only plague of swift deadliness to eels but not to lizards. Seriously, there has to be something different about these two species that a deadly plague can leverage! It could be a virus, a bacterium, a prion, or a parasite. It just needs to be highly infectious and highly deadly!

The plan goes like this:

  1. Develop disease and immunize lizards.
  2. Issue ultimatum.
  3. Upon failure to conform, send lots and lots envoys with plague and infect them until most of them are dead. Go to several of their major transit hubs and disperse them there. Eel WHO can only do so much.
  4. Then the lizards make like the Europeans and clean up the rest.

Upsides: Most eels are dead, the lizards didn't do the actual killing of most of them, their armed forces are diminished. Most of the resources are intact.

Downsides: they could develop a cure, and foil this plan. Takes a lot of mad science and lots of secrets.

Option 2: Big Space Rocks

I'll call this the "big space rocks" because you just take an astroid, do some orbital mechanics, and send a modest asteroid at those eels. You'll need to guard the rock, but after a little bit, it'll kill those eels like the dinosaurs.

Upsides: A rock kills most eels, unless they survive in bunkers.

Downsides: Rock can be destroyed prematurely. Math can be done wrong and the asteroid hits the wrong planet, letting the eels colonize the lizard world. Can be survived if eels dig deep enough bunkers. Eel world could be uninhabitable and/or most natural resources are unusable.

Option 3: Brutal, Brutal War

Upsides: you gain the planet with most natural resources likely intact. (Well, as intact as the eels had it.)

Downsides: brutal warfare with heavy losses for everyone. Lizards may lose. Takes forever.

  • $\begingroup$ I never thought of vaccinating the one group. By the way, eels would go underwater, lizards would go underground. Anyway, I like your answer. You were also the only one who didn't force lots of creatures to die developing radiation technology. $\endgroup$ Jan 19, 2016 at 14:25
  • $\begingroup$ I don't think you would necessarily need to vaccinate the attacking race: as long as the disease only affects eels/lizards then you can just wait for all the target race to die, and the disease will die with them. $\endgroup$ Jan 21, 2016 at 9:38
  • $\begingroup$ @MaxWilliams It was an explicit condition that they are affected by the same diseases, so inoculation will need to occur. Really, I would expect there be a difference between them that would make one disease deadly to one while being mostly harmless for the other. Oh well. $\endgroup$
    – PipperChip
    Jan 21, 2016 at 9:44
  • $\begingroup$ My point was that as long as the disease can't spread to animals etc, then when it has wiped out all the inhabitants of the planet, there will be no hosts left for the disease and it will die out. The planet will then be safe,. $\endgroup$ Jan 21, 2016 at 10:03

What do the Eel-iads need to survive?


  • Poison the oceans, lakes, rivers, major aqua-repositories.

  • Turn their water into ice with that instant water to ice formula (forgot the name of that movie...)

  • Vaporize their water. This might be a setback for colonizing the planet by the Lizardians but it'll win the war.


  • Simpsons did it. Matrix did it. Block the sun and laugh your maniacal lizard laughter.


  • Poison/disease that targets the food sources, be they plants or animals (might be safer with poisoning plants, for re-colonization purposes)

Basically, you have a lot of options that largely depend on what you're willing to lose.


The basic way to kill off the other planet is to bathe it in deadly radiation. A powerful beam of charged particles is sent from one planet (ideally from a giant orbiting station, since you don't want to irradiate your own planet) and sent to bath the other planet with ionizing radiation.

There are a few complications in the scenario you have given, however.

Since the planets are tidally locked, only the facing hemisphere will be affected, unless you go to the additional expense of making a mobile firing platform (AKA space battleship) and send it into polar orbit around the other planet to start irradiating the planet below. Here you run into the issue of having a very expensive asset in range of enemy countermeasures. In low orbit, they could be as simple as firing rockets into orbital space and discharging buckets of ball bearings. The enemy ship flies into them with a minimum relative velocity of about 7 miles/second, which will turn anything less than a "death star" into confetti pretty fast.

The second issue is that while the surface will be bathed in deadly ionizing radiation, anyone who burrows underground will be safe from the beam. Submerging under enough water (about 5m) will also attenuate the radiation to the normal background level. While the biosphere above will be destroyed, so long as you have either stored supplies, or a functioning closed life support system, you can remain below ready to exact revenge.

Finally, for the people who want to conquer the planet, there will be a wait ranging from years to centuries as the induced radiation from activated materials on the surface decays. Since you went to a great deal of effort to totally destroy the biosphere of the planet, you will have to essentially terraform the world in order for you to start living on it (several centuries without plant life will change the composition of the atmosphere, for example).

So the risk is pretty high ( they are at the same technological level so they can do the same to you), the rewards uncertain (are there survivors lusting for revenge hidden under the oceans or mountains?) and the end result if everything goes right will be a dead planet that you have to terraform. Is it really worth it?

For more details see: http://www.projectrho.com/public_html/rocket/spacegunconvent.php

  • $\begingroup$ You just destroyed one and a half planets, killed millions of innocent people, and potentially jeopardized the moon base. I hope you're happy! Anyway, you kind of broke the radiation rule. And what is this about ball bearings? That sounds interesting. Try expanding that part of your answer and reducing the radiation. $\endgroup$ Jan 19, 2016 at 4:17
  • $\begingroup$ Throwing ball bearings into orbit in the path of an orbiting satellite or vehicle is a relatively simple means of destroying the vehicle with hypervelocity impacts (the ball bearings spread into the orbital path like a shotgun blast, ensuring at least some will strike). The radiation bath is the only relatively effective and doable means of killing the other planet without destroying the real estate, and since the radiation is being "beamed in", only the surface layers of things will be "activated", so the radiation will fade relatively quickly (unlike radioactive fallout). $\endgroup$
    – Thucydides
    Jan 19, 2016 at 4:31
  • $\begingroup$ How would this affect the other planet? What about orbital bombardment? How would that work? I suppose your radiation beam would definitely be better than nuking the planet. $\endgroup$ Jan 19, 2016 at 4:40
  • $\begingroup$ Orbital bombardment will be much more expensive, since you are accelerating huge masses of impactors from one gravity well to the other, and it will also cause extensive physical damage as well. I said the particle beam should be in orbit or aboard a spacecraft to prevent the owning planet from being irradiated. This is probably the most cost effective way of doing the job. $\endgroup$
    – Thucydides
    Jan 19, 2016 at 5:32
  • $\begingroup$ It's not just a case of waiting for the radiation to subside: the radiation will destroy much, perhaps most or even all of the life on the planet, completely wrecking the ecosystem. $\endgroup$ Jan 21, 2016 at 10:10

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