A large comet is quickly approaching my fantasy world. Its impact will spell doom for all life on the planet. Fortunately, there are mages on this fantasy planet who possess a magic capable of shattering the comet. But, magic isn't free, energy has a cost....

At first blush you may think I am asking a question about magic. I am not. I'll take care of the magic when I write my story. What I hope to discover is the most efficient means of shattering the comet. From this perspective, I'm looking for a science-based answer.

  • Use the Swift-Tuttle comet as the basis of your analysis.
  • Do not worry about how the effect takes place or is delivered.
  • The best answer will explain the lowest energy method of shattering the comet.
  • Do not worry about the physics of what-happens-next to the comet. I'll take care of that, too.
  • The comet must be shattered, not deflected or stopped (I have plans for the remnants).
  • The remnants of the comet must still be headed for or already orbiting the planet.
  • Please consider the orientation of the comet to the world as unimportant.

Example effects:

  • Vaporize the liquids in the comet.
  • Create a temporary wall the comet impacts, thereby shattering the comet.

If the answerer can demonstrate that vaporizing the liquid would require less energy than absorbing the impact of the comet using brute force, it would be selected as the best answer.

Once again, it does not matter (and is irrelevant to this question) how these effects are delivered to the comet. since the delivery mechanism is magic you need not worry about rockets or any other delivery consideration. The magic can be thought of as "bringing the energy needed for the effect to bear."

Question: Given the conditions and examples above, what is the lowest-energy method for shattering the comet?

For future viewers, while I've marked the answer, the answer supplied by Shadowzee provides several viable options (as well as several less viable, but still funny, ones).

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. If you're talking about voting to close or reopen, I'd advise looking through the conversation in the room. I've moved it there because the comments were getting highly unwieldy. $\endgroup$
    – HDE 226868
    Jul 10, 2018 at 0:31

7 Answers 7


Boil the ice inside the comet, the trapped steam causing it to explode. This would be pretty efficient since the energy used to change the ice to steam would also be used to shatter the comet. As well, there would be less heat lost to space because the center is insulated by the outer layers of ice.

  • 2
    $\begingroup$ This seems like it would create large shards, rather than fine crystals $\endgroup$
    – bendl
    Jul 9, 2018 at 20:30
  • $\begingroup$ Are comets entirely encased in dirt/rock though? I thought the tail of the comet was created by exactly what you describe - vaporizing ice - escaping through cracks in the surface. If there are ways out, I don't see how the pressure can build up enough to shatter the comet. $\endgroup$ Jul 9, 2018 at 20:31
  • 1
    $\begingroup$ This is the right answer, because OP wants comet blown up into little pieces, & applying energy all at once to a small volume of water near the center will blow it up with the least energy given the magic available $\endgroup$ Jul 9, 2018 at 23:32

Your comet is made of ice. Ice happens to be melted by heat. It also happens that fire is a magic that is pretty easy to learn.

So, instead of inventing a new branch of physics, study the orbital dynamic of a suitable kinetic bullet to hit the comet, and also figure out what a kinetic bullet is, take a mage, or a bunch of mages, and have them focus a fire spell on the comet.

The comet will start sublimating, more or less like it would do when getting close to the sun. Continue applying until the comet is gone.

If you are able enough to focus the spell on a specific place, you can even use the ejected gas to "steer" the comet around, deviating it from its crash route.

  • $\begingroup$ Would the melted ice then reform into 'chunks' (assuming the spell stops), thus getting the desired effect? $\endgroup$ Jul 9, 2018 at 16:44
  • $\begingroup$ @ThomasMyron, given the small mass of the comet, the water molecules will easily reach escape velocity and will hardly be able to aggregate again. $\endgroup$
    – L.Dutch
    Jul 9, 2018 at 16:50
  • $\begingroup$ What if the comet is question was Swift-Tuttle? $\endgroup$ Jul 9, 2018 at 16:58

The lowest energy solution for your mages is to make use of the powerful source of energy you already have in the solar system: the sun.

Instead of having your mages pump energy into the ice themselves, have them paint the comet black. Fairly little work for them, just a bit of surface coloration, and then the sun does the heavy lifting.

Use a magical paint that absorbs 100% of all incoming (light/heat) energy, and paint it in strategic patterns so you get contrasting/asymmetrical heating, some parts melting (or boiling) while others stay frozen, and you should be able to make it crack or shatter.

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Given your stated result you need to hit the comet very hard with something harder than it is, like a really big rock. The DnD spells meteor strike and meteor swarm would be the best existing matches I can think of. This will cause it to shatter rather than evaporate as it would if hit with an energy spell like a fireball or lightening bolt.

  • 1
    $\begingroup$ Many 9th level magic missiles would do the job. Plus it's force damage so it wouldn't cause the meltage that the OP is trying to avoid. $\endgroup$
    – James
    Jul 9, 2018 at 21:53
  • $\begingroup$ @James True, you can tell I don't play a magic user very often. $\endgroup$
    – Ash
    Jul 10, 2018 at 10:31

The biggest issue I see is that your comet is going to be a ball of ice and rock roughly 26Km across. It has more than 300 times the energy of the comet that killed the dinosaurs so if you have a magic system that can stop this comet your going to have people who are capable of wiping out all civilization without a second thought. That being said here is a list of ideas you could possibly apply:

  • Super heat the core of the comet causing the ice to vaporize and expand outwards shattering the outer shell of the comet
  • Throw an equally large stone at it using magic
  • Create a plant that uses water/ice to grow, launch or teleport it to the comet and have it grow all over the surface using up the ice until only the rock is left. When the comet hits the atmosphere it heats up killing the plant and leaving the shattered remenants raining down onto earth
  • Just create a super shield or force field that the comet can collide with
  • Create a reverse gravity field around the comet causing it to break apart
  • Create a black hole in the path of the comet to break it apart
  • Move everything into the shadow realm temporarily when the comet hits and come back later
  • Move the comet into the shadow realm
  • Just teleport it into tiny pieces
  • Use holy magic to summon god to stop it
  • Warp reality so the comet just turns into bubbles
  • do 100 push ups and sit ups a day with a 10Km run everyday and once you become bald just punch it
  • Summon an eldrich tentacle horror to stop it
  • Use laser beams to break up the comet into smaller sections
  • Time freeze the comet using a elaborate ceremony with a ceremonial sword of a hero used to enforce the spell. The comet becomes part of the planet until someone removes the sword causing it to smash into the ground.
  • Cast a shield that bends physics and perfectly reflects all energy applied to it
  • Awaken Gaia who will stop the comet for you
  • Crush the comet using gravity magic until the insides of it super heat then dispell the magic causing the comet to explode
  • $\begingroup$ Some good ideas. The non-serious ideas were too funny to complain about. :) $\endgroup$ Jul 10, 2018 at 0:36

Contract out the job

You Mages simply need to be able to communicate with Mars a few months in advance of impact and negotiate a price for Martian assistance.

The Martian Interplanetary Gun can rapidly place a crew of Martians on the comet, and their Heat Ray will quickly reduce the threat to ice blocks.


Go antimatter

You could magically inverse the charge of electrons and protons in some proportion other than 50% of the mass in the comet, if its around a few pounds it will instantly annihilate into pure energy, it would blow up in the range of a 100 megaton hydrogen bomb, more than enough, do you want something more spectacular?, make the comet 99.99% antimatter leaving only a few pounds of normal matter, again it would blow up like the hydrogen bomb but the fragments of the comet would be pure antimatter, the instant something its touched by them they annihilate it in a fiery blaze.


In any of the options mentioned there would be huge consequences or none depending on the distance of the comet, if its around the distance of the moon, the explosion would be microscopic and the radiation would disperse harmlessly, if it where to happen closer, say the van Allen belt there would be huge auroras all over the world, if you choose the pure antimatter chunks and they orbit close to the planet there would be auroras 24/7 for months probably, as the space close to planets with atmosphere its not really that empty and particles of different gases would annihilate with the chunks all the time creating lots of radiation each time, from the ground it would look like mini suns crossing the sky

Not asked options but still cool to mention

A 50% percent conversion would be something to be seen, it would annihilate a 27km meteor into pure energy creating ionizing radiation far beyond anything or anyone could imagine, if we make the comet a perfect sphere and made entirely of water for the sake of simplicity it would have around 82448000000000 cubic meters of water, the amount of energy created would be 3218769920000000000 megatons, just for a little perspective the tsar bomba was "only" 50 megatons, this amount of energy would create so much radiation that it would sterilize basically the whole solar system and if it was anywhere close to a planet it would blow all of its atmosphere and cook it until there was nothing else but a sea of radioactive lava, it would be close to setting off a small supernova.

here if you want to play with the numbers


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