The goal is to get from Earth to the Outer Solar System, preferably the Kuiper Belt, as quickly as practical, with a crew of about 12, and a not-set-in-stone dry mass around 30 tons.

The characters have access to something resembling a wormhole, whose other end is in a different universe. Both mouths can be moved without much energy expenditure. For setting-specific reasons, this requires constant attention from a specific crewmember, such that wormhole throughput effectively stops when they do anything else. It is not practical to just build a habitat on the other side and let the rest of the crew wait there, to minimize payload, but using the other side of the wormhole for storage of food, supplies, etc is possible.

I see two ways this could be used for spaceflight: either use the wormhole for fuel-storage to bypass the Rocket Equation, or point the wormhole at a star, and let the star provide exhaust. For character-related reasons, the simpler the end product, the better.

My preference is the Stellar Exhaust solution. However, if they use something like a Coronal Mass Ejection (exhaust velocity potentially approaching 3000km/s), the exhaust would be absurdly hot, include intense radiation, and possibly some other electromagnetic effects I'm less clear on. I don't expect this would be safe on or near Earth, even assuming there's a way to prevent it from frying the ship. The characters have access to handwavium that can mitigate some of this, given constant effort, but I don't know that it'd be enough to make stellar exhaust viable.

Given the fuel storage option, I'm expecting they'd need a high-quality rocket engine, whereas stellar exhaust seems like it could get away with a simple nozzle, if not less.

It's possible I've overlooked something that changes the answer dramatically from either of these.

Given the above, what would be the best way to accomplish the goal?

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    $\begingroup$ as quickly as practical Practical means no more or less than one gee acceleration (or deceleration) so the crew feel a pseudo gravity like Earth. $\endgroup$ Commented Aug 21, 2019 at 1:16
  • $\begingroup$ The Stellar Exhaustion solution might be more feasible. "Coronal mass ejections reach velocities from 20 to 3,200 km/s (12 to 1,988 mi/s) with an average speed of 489 km/s (304 mi/s), based on SOHO/LASCO measurements between 1996 and 2003." Particularly if your wormhole was more selective in its choice of coronal mass ejection. Although plugging directly into a star's photosphere might work as well & be more benign $\endgroup$
    – a4android
    Commented Aug 21, 2019 at 2:15
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    $\begingroup$ "It is not practical to just build a habitat on the other side" so what can you do on the other side? You say yoiu can store "supplies", but what is the difference between a big stockpile of "supplies" and a habitat? What's the limiting factor? $\endgroup$ Commented Aug 21, 2019 at 8:50
  • $\begingroup$ (personally, I'd make the tidal forces in the wormhole sufficiently strong that structures larger than an atom will be torn apart, thuse avoiding the habitat issue, but that does make storing fuel on the otherside somewhat problematic) $\endgroup$ Commented Aug 21, 2019 at 8:51
  • $\begingroup$ Is there a reason they can't just send the ship through the wormhole, shift the end in our universe to the destination and then have the ship go back through ? $\endgroup$ Commented Aug 21, 2019 at 13:33

4 Answers 4


Blow ye winds!

Stellar, schmellar. Too hot and too radioactive! I propose that the far end of the portal be dipped low into the atmosphere of a suitable planet. On the near side, a fierce cool wind will blast forth, laden with droplets of salty water.

Your starjammer will have masts and sails and will sail away propelled by the portal winds. Absent air resistance in space I here assert that the column of air will remain coherent for a long distance, providing propulsive force the whole time.

Arguably a liquid would provide even more propulsive force but then it would be something different than the awesome starjammer I am envisioning.

  • $\begingroup$ If the other mouth of the wormhole were on our universe we could use that to Terraform Venus as well, by reducing its huge atmospheric pressure. $\endgroup$ Commented Aug 21, 2019 at 16:01
  • $\begingroup$ @Renan - yah! Twofer! $\endgroup$
    – Willk
    Commented Aug 21, 2019 at 17:09

It sounds like what you have is more of a portal than a wormhole, but that's just semantics really. You left the science-based tag out so let's see where we can take this. Apart from the portal itself, I'm going to assume that physics is mostly the same.

The idea of dropping the other side into the corona of a star and letting the energy pour out of our side would be great if you wanted the energy for something, but you'd have to do something more to get thrust that way. You can't just point your portal aft of the ship and start thrusting since the energy passing through the portal isn't actually pushing on the portal or the ship itself. It's not like in a rocket where the exhaust gasses are pushing against the rocket motor to generate thrust, and conservation of momentum doesn't really apply since the other side of the thrust is in fact in a different universe.

So instead you'll need to interact with what's coming through in order to derive some thrust from it. Point the portal forward into a set of curved tubes (of some refractory material that can handle the temperatures) that redirect the transferred material backward. Turning the mass around will generate thrust, and you can probably steer a bit if the tubes can be moved or valved in some way.

Or you could use a variation on the solar sail concept where you have a lightweight sail that you direct the flow of matter/energy into. It would probably have to be a bit tougher than the average Mylar sheet though. Perhaps a rigid parabolic or conical structure with the point aimed directly into the mouth of the portal so that it scatters as much of the material to the sides of the ship so that you don't get fried.

Depending on the nature of what's coming through perhaps you could just direct it down a tube with permanent magnets and use magnetic drag to get some thrust. It'd look weird having your apparent thrust pointing forward... but weird isn't necessarily bad.

There might be a simpler way though.

If your portal essentially joins two regions of space in different universes then things like gravity may pass through as well. You could position the other side near a suitably massive object like a neutron star and have a gravity well form on this side. Your ship accelerates in the gravity well, pushing the near side of the portal ahead of it. You control the rate of acceleration by moving the other side relative to the mass you're using. And since your ship would be essentially in free fall relative to the gravity well you wouldn't have to worry about acceleration stress on your crew. Maybe some tidal effects though, so be careful getting too close to the portal.

On the other hand, if gravity doesn't transmit through the portals then you'll have to use the lessons we all learned in Portal 2...

Take two pairs of portals. Place the far ends of both in a gravity well so that an item exiting from one will accelerate towards the other. On the near side put the portal exits inside your ship's engine so that objects cycle through a short part of local space before going back through the loop. Drop something into the 'down' portal and bleed energy off with magnets or something.

You could also push a tube through the 'down' portal and grab the end as it comes back through the 'up' portal. Fill the tube with water or something, which will immediately start to flow through the tube due to the gravity on the other end. Attach both ends of the tube to a machine that will brake the flow, extracting kinetic energy from the loop. Maybe have a turbine in-line that you can use to generate electricity at the same time. Make sure you can stop the flow when you want, and that you can rotate the whole assembly to get directional control.

Now, as long as you let the water flow you'll have both thrust and limitless energy.

Which is why we know Portal is just a fun game. Physics doesn't like it when we break the rules like that.

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    $\begingroup$ Refractory materials ain't gonna cut it. A CME might have a temperature in the hundreds of thousands of kelvin... even the regular boring surface of the sun is hotter than the melting point of anything that might be conceivably made of matter. $\endgroup$ Commented Aug 21, 2019 at 8:43
  • $\begingroup$ Might there be some way to mitigate the heat issue? Ex, if the exhaust is pushing on an electromagnetic field anchored to the ship, or if there was a way to cycle out hot materials for cooler ones without significantly changing the shape of the engine (how fast would that need doing at these temperatures?)? $\endgroup$
    – CAE Jones
    Commented Aug 21, 2019 at 12:29
  • $\begingroup$ Or the answer that Willk provided: use cold gas under pressure. Find a gas giant in the other side and you'll get high-pressure low-temperature gas to push you around. Bonus points for using a second portal pair to drop the used hydrogen into the upper atmosphere of the same gas giant to recycle and keep the universe clean :) $\endgroup$
    – Corey
    Commented Aug 21, 2019 at 23:08

I have answered quite a few questions regarding wormholes by reminding people that those things have mass. Specifically, a one-meter wide wormhole would have a mass comparable to Jupiter.

It is a good thing that you don't want to use the wormhole to go inside it, there would be some complications with humans inside the vessel. Depending on the model, it might be a longer journey than just not doing it, given the relatively short distance (in astrophysical terms).

Since you can move the mouths of the wormhole easily, you can do it like this:

  1. The wormhole can't come close to Earth due to safety reasons, so have the ship go from Earth's gravity well to the wormhole's gravity well under its own power.
  2. Move the wormhole towards the outer solar system with. The ship will follow since it will be orbiting the wormhole.
  3. Once in the outer solar system, have the ship exit the wormhole's gravity well.
  4. Profit!

You can then move the wormhole closer to Earth's orbit to rinse and repeat.

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    $\begingroup$ Doesn't that depend on the type of wormhole? The Ellis wormhole has zero ADM mass and no gravity well. $\endgroup$
    – pregunton
    Commented Aug 21, 2019 at 7:29
  • $\begingroup$ I had considered mentioning the wormhole's apparent lack of significant gravity well, but the description was already feeling clunky. Should I edit it in? $\endgroup$
    – CAE Jones
    Commented Aug 21, 2019 at 12:23
  • $\begingroup$ @CAEJones it'd be better to open a new question, explaining the difference from this one. $\endgroup$ Commented Aug 21, 2019 at 12:52

You have control of a stable worm hole and you want to transverse great distances? I understand one end of it is stuck in a distant universe.

  • you (and your ship) step from Earth into the worm hole
  • move the Earth side to your desired destination
  • step back through the worm hole

Maybe this doesn't fit your story line? You could limit the range that the portal can be moved and still stay under control. This would make the journey many small step-in-step-outs.

This mode of transportation would use very little fuel, be very fast, remove cargo limits, and more, all thanks to worm hole magic.

  • $\begingroup$ There are story reasons it needs to be anchored to a spacecraft, and for the crew to stay on the near-side. There is handwavium that allows the ship to be reconfigured mid-flight, so moving mass around to optimize is doable, but conservation of momentum won't let us get up to cruising speed, then resize the ship. So the step-in-step-out concept can be included to a lesser-but-still-significant extent. $\endgroup$
    – CAE Jones
    Commented Oct 9, 2019 at 15:21

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