If your anchored-to-a-gateway-at-one-end-but-other-end-free-floating wormhole technology has the handwaved ability to match orientation and velocity to the surface of a distant planet, it also has the ability to not match the orientation and velocity of the surface of a distant planet, and if you configure it to be, say, facing downwards and descending vertically at 5km/s, then you have the delivery mechanism for a kinetic bombardment: launch a tungsten slug through the wormhole carefully timed to pass through just as the wormhole approaches the ground, then cut or divert the gateway so you don't get too much shrapnel back through on your side.

One downside to this is that while your wormhole-cannon is firing, it's scooping up a cylinder of the target planet's atmosphere and jetting it back out on your side at the impact velocity. While 5km/s wind in exchange for 5km/s tungsten slug is still an advantageous trade, what sort of damage might you expect such a wind to do on your side? What sort of protective measures would you want to put in place around your gateway buildings to prepare for this 'recoil'?

Assume the gateway is 10m diameter, probably outside on a large hardstanding area (airport runway style). I'm imagining launching the slug on a missile from some distance, since it needs to have enough momentum to travel 'upwind' to the gateway. Total firing time would be 5-10s.

Edit: changed the impact velocity from 20km/s to 5km/s, to be within range of current hypervelocity weapons.

  • $\begingroup$ Hint: what's a 20 km/s rocket exhaust look like? Now subtract the thermal energy that let the (nuclear thermal?) rocket produce such exhaust velocity. $\endgroup$
    – Zeiss Ikon
    Feb 1 at 12:08
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    $\begingroup$ This issue is related to the "Portal Paradox" (youtube link). I suppose in your scenario this is a solved problem, but one could just as well argue the validity of this setup altogether rather than its result. Alternatively, if possible, one could swap the function of the two gateways. This would weaponize some 1570000m3 of air (or water, ground, etc.) into a sizeable cannon and not waste any tungsten. $\endgroup$
    – Plutian
    Feb 1 at 12:20
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    $\begingroup$ why would you simply bombing people with such a highly configurable wormhole, when you could surgically remove all their heads of state, followed by their nuclear reactor shielding and a chunk of the bottom of every dam? $\endgroup$
    – user253751
    Feb 1 at 21:57
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    $\begingroup$ I agree with @user253751. It's hard to imagine any projectile more potentially damaging than the portal itself. Heck, if you shut it off a few seconds too late, it will drill a new volcano. $\endgroup$ Feb 1 at 23:54
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    $\begingroup$ Why not use the wormhole in a more nefarious fashion? Because of the Rule of Cool and the desire to have a scene where everyone's hats get blown off as the attackers basically launch an ICBM at their own installation. Also the wormhole is being used for a more conventional military beachhead operation with lots of aerial drones and military hardware passing through in a 'normal' fashion, with a brief diversion to godrod a particularly stubborn enemy redoubt. $\endgroup$
    – Stephen
    Feb 2 at 10:18

4 Answers 4


This is a Frame Challenge

NOTE: This Challenge was written when the OP wanted 20 Km/s coming through the portal. Changing it to 5 km/s helps, but still... That's an 11,000 MPH wind coming out of a 10 meter diameter hole located at ground level. I believe my challenge still stands. I've seen what happens to the front of my business when 90 MPH wind hits it. I need only look at hurricane damage to see what 200 MPH winds can do. 11,000.... the destruction would be breathtaking.

I originally voted to close for needing more details because the background of the portal is irrelevant compared to the circumstances surrounding the source portal. What's happening to, e.g., the room that's experiencing 20 km/s wind?

Then I converted the wind speed to miles per hour. Sorry folks! I'm an American, and that means MPH means more to me than km/s.

44,739 miles per hour!

The Space Shuttle reenters the Earth's atmosphere at 17,500 mph. The fastest commercial bullet, the .220 Swift, was clocked at 3,180 mph.

  1. I do not believe you can push the tungsten slug through the portal. Period.

  2. I believe what's coming through the portal at the source end has more to do with hot plasma than atmosphere.

  3. I believe with every passing moment your planet (the whole planet) is experiencing a climatological crisis.

And why do I say this?

  • Unless the portal's controls are hundreds of km away, they (or the people pushing the proverbial button) are damaged and/or destroyed in the purported 5-10 second window.

  • It helps if those controls are behind the portal, but not much (the more I think about it, not at all) because of the vacuum that will form behind the portal.

  • If the controls are computer-operated, I don't believe it'll matter. The portal will likely destroy itself due to the heat generated by the passage of all that plasma air.


Maybe you need to rethink the 20 km/s number because unless I've converted it to MPH incorrectly (I'm lookin' at you, Google!), what you've really done is created a way for the attackers to destroy themselves.


  1. Move at a stately 20 km/h. Or even 200 km/h. At that point your wind is relatable to wind tunnel velocities and you need only describe your launch point as being inside the proverbial cold-war era wind tunnel facility to justify/rationalize everything else.

  2. Put the launch point in space. Better still, but the terminus in space, too. That way there's a vacuum on both sides and the launch would be whomping hard for your enemy to detect. If you're using the velocity of the terminus to rationalize the velocity of the slug, then you can move it as fast as you want in space. Put it 10,000 km ahead of the target planet's orbit and release the slug at 20 km/s. Based on Daron's answer, it would hit the ground at 50 km/s.

  • $\begingroup$ ICBM reentry velocity is 6-8km/s, so while we're talking double that, it's perfectly achievable, imo, to push the projectile through the portal. $\endgroup$
    – Stephen
    Feb 1 at 16:34
  • $\begingroup$ @Stephen So we're not talking about a slug... we're talking about something that's powered and using the Earth's gravity to assist with its motion - and we're ignoring the devastation that such a wind at ground level would cause? $\endgroup$
    – JBH
    Feb 1 at 16:46
  • $\begingroup$ I'm talking about a hypersonic missile launched from several miles away and travelling at hypersonic velocity towards the portal, yes. $\endgroup$
    – Stephen
    Feb 1 at 17:00
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    $\begingroup$ @Stephen You didn't say that, and it makes everything worse. Now you have the velocity of the missile combined with the wind velocity. The heat will be enormous. What's the diameter of the missile? Because you're trying to hit a 10m target with a 20 km/s wind coming out of it that's destroying everything around it. You need to update your question and I stand behind my frame challenge. $\endgroup$
    – JBH
    Feb 1 at 17:14
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    $\begingroup$ @Stephen No--an ICBM burns in at that kind of speed in the upper atmosphere. Look at the math for catapult launch from Earth's surface (nobody has been able to figure out how to make one that doesn't destroy itself from the shockwaves) and turn it up still further. $\endgroup$ Feb 2 at 0:36


I suspect the air will not come out the other side at 20 km/s. Much will be converted to heat and the portal will look like a fireball or miniature sun. Or maybe a cone of flame.

The fluid is choked by the small portal size. See the diagram.

enter image description here

The neck of the pipe where it thins represents your wormhole. The red particle and blue particle are moving fast to the right. When they move into the neck, the red particle moves down and the blue one moves up. They collide with each other and with the sides of the pipe. Some of the kinetic energy is converted to heat energy.

Since the portal is so very small and the air starts out so fast, you will get a lot of heat. This can be calculated using the equation:

enter image description here

which I believe gives an upper limit to the speed the air comes through the portal. Increase the speed on the left of the neck any more and the speed on the right stays the same, but the temperature increases.

If the equation looks scary to you, do not worry. It looks scary to me too. You might want to look at answers to similar questions where people work through the equation.

The way I would use the weapon, is put the exit portal on the enemy planet. Point it at the ground. Wait until the planet rotates so the portal is at the "front" of the planet. Remember the Earth orbits at about 30km/s. That means anything dropped in your fixed portal hits the planet at 30km/s. Kaboom.

  • $\begingroup$ Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Worldbuilding Meta, or in Worldbuilding Chat. Comments continuing discussion may be removed. $\endgroup$
    – L.Dutch
    Feb 2 at 20:28

Short, compressed and too fast

Assuming the receiving end of the wormhole comes down at the planet at 20 km/s and the mass that has been transferred via wormhole to retain the velocity relative to the wormhole's other end, you pretty much get a stream of air at normal pressure flying out of the local end of wormhole at 20 km/s. Just the wind, you know. Yet, given that the atmospheric pressure at 100 km height is very small, you will only experience significant wind for the last 5 seconds before the wormhole would cross the ground level. After that, you'll get slammed by a wormhole wide stream of planet matter flying at 20 km/s, potentially eliminating whatever is powering your wormhole with its sheer speed and kinetic energy.

This, however, depends on whether the local end of your wormhole is somewhere at the ground level, with its own local pressure and air. If it would be in vacuum, you would only get a narrow tube of planet matter flying from there at 20 km/s, no matter what kind, wind or rock. If it's not in a vacuum, you'll first experience a suction towards the wormhole as it would be opened into space, then as the wormhole would approach the planet you'll receive a stream of particles coming your way, some of them originating from this side, then your room's air would be rapidly compressed by the incoming 20 km/s air flow which density would increase exponentially over about five seconds, and if you'll not cut the connection, welcome rock train.

If you want to bomb someone with such a wormhole, you should not intersect with the planet

You just position your wormhole's distant end some millions of kilometers away from the surface, facing the planet and located forwards orbit wise, with its speed relative to the star at 20 km/s backwards (retrograde, relative to the planet's orbit). Then throw there an iron orb that'll barely fit into the wormhole - no need to use tungsten as iron is a hundred times less expensive and only three times as light. Then another if you want, or plain feed there a literal train of them. When at the ditant side, the orbs will have a relative speed of 20 km/s plus the speed of the planet relative to the star, and you would have enough time to collapse the wormhole before the intrusion would be noticed. Of course you position it so that when your meteorites collide with the planet's surface, the desired target would be hit.

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    $\begingroup$ ALL HAIL ROCK TRAIN! ALL HAIL IRON ORB! $\endgroup$
    – Willk
    Feb 1 at 14:24

One downside to this is that while your wormhole-cannon is firing, it's scooping up a cylinder of the target planet's atmosphere and jetting it back out on your side at the impact velocity.

That would be true if the wormhole started out, say, near the edge of the atmosphere and rocketed towards the surface at speed. Why would you operate the system like that, though? Open the wormhole about 3m above the ground, move it towards the ground at speed, then cut it off after it moved a total of 20cm. For a 10m wide portal, that'll only suck in about 1.5 cubic meters of air, which shouldn't be a huge problem. The portal will only be open for a tiny fraction of a second so timing would be tricky, but that's a solveable problem.

What sort of protective measures would you want to put in place around your gateway buildings to prepare for this 'recoil'?

If you're planning on operating your wormhole in this manner, you wouldn't put anything in "front" of it to begin with. You'd point the business end towards the largest expanse of nothing that you can find and have all your buildings and equipment safely sheltered at a distance "behind" it. After all, the thing is two-way. If your tungsten slug is leaving the portal at 20 km/s, then the pine tree the portal is accidentally landing on is emerging from your side at 20 km/s.

Overall, though, the answer will rely on how exactly you're planning on handwaving away the ability to have a moving exit portal. Conservation of energy still applies when wormholes are involved. Your projectile is gaining a tremendous amount of kinetic energy, but that energy has to come from somewhere. If you're having to input that energy into the system, then it seems easier to use that energy for destruction directly and leave the wormhole stationary. Or, drop your side of the portal into the nearest star and use the steerable exit portal as the universe's greatest flamethrower.


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