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I am currently trying to make a world that has casual interplanetary commutes with near future technology (within one century); so, to find a reasonable top speed for crafts in my interplanetary world, I -

  • found the farthest major body technically orbiting the Sun (in this case, Sedna)

  • calculated its distance from the Sun in Astronomical Units (940 AU at aphelion - its farthest point from the Sun)

  • converted that into time taken at the speed of light to reach x object (in this case, 6 light days to travel)

  • and divided it by the upper bounds of how long a person would spend traveling to one destination during the early modern period (600ish days) - giving me a total of 1.57 AU per day, or 1% of the speed of light

Given the technological constraints of a near future civilization and time parameters, what propulsion technologies would allow the citizens of such an civilization to travel at 1.57 AU per day?

Important Notes -

Survivability is not an issue, assume everyone is a trans-human

Cost is also not a issue, I can handle the space-economics

Speeds reaches should be relative to an outside observer

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    $\begingroup$ I'm not saying there is something wrong with your question, but sometimes people have not considered certain things: The method of propulsion isn't really the limiting factor here. Of course it would be nice to be super quick, but that won't be possible in only 100 years. There are other problems, for example: There is nothing on those planets that interests us = those travels just consume money for no reason. You might not be able to return to earth's gravity after some time in space. Landing on another planet does have a horrible success rate. That list goes on. $\endgroup$
    – Raditz_35
    Sep 13, 2017 at 8:59
  • $\begingroup$ A six hundred day trip, one-way, isn't a commute. It would take a propulsion technology that we are unlikely to develop in the next century for 600 day trips to Sedna to be possible. Ion or plasma drives are plausible in the next century, but they have low rates of acceleration, i.e., one centimetre per second squared. They would take ten years to attain one percent of lightspeed. A trip to Sedna will take 8.1 years or 1479.86 days. $\endgroup$
    – a4android
    Sep 13, 2017 at 13:16
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    $\begingroup$ 600 days isn't a commute, 30mins is a commute, 2 hours is an excessive commute. $\endgroup$
    – Separatrix
    Sep 13, 2017 at 13:16
  • $\begingroup$ Determining 'casual interplanetary commute' by how long it takes to fly from Earth to Sedna is like determining 'casual inter-city commute' by how long it takes to drive from DC to Moscow. The farthest actual planet is Neptune at an average of 30AU- isn't that a more useful baseline for intra-solar-system travel? $\endgroup$
    – Catgut
    Sep 13, 2017 at 18:49
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    $\begingroup$ Survivability isn't an issue anyway, it would only take about 88 Earth days to get there at 1g accel/decel. Slightly less for the travelers as they'd get to a bit over .12c, but I don't think I even want to try to calculate that. $\endgroup$
    – Kevin
    Sep 14, 2017 at 0:21

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A nuclear saltwater rocket could get you up to 3.6% light speed tomorrow (technologically speaking) and fission-fragment rockets will do 5% light speed (also with today's technology). That gets your "commute" down to 30 days at top speed but they cost the Earth to build and still more to fuel and at maximum drive system accelerations you'd arrive as a thin paste on the internal wall closest to drive and most of the designs only allow for "on" and "off", maximum acceleration or nothing.

Sorry but I don't think there's a theoretical drive yet spawned, even FTL, that cuts interplanetary travel down to the "minutes of travel for a few dollars" level that you need for commuter travel.

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  • $\begingroup$ Which G forces are we speaking about with these drives? $\endgroup$
    – lijat
    Sep 13, 2017 at 18:50
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Increasing the speed and lowering the cost is what would make it casual just like it did in every form of travel on Earth.

For the better part of our history, traveling from one city to another was not considered casual travel. Heck, most people didn't travel more than a day's walk. Then trains and cars made travel both faster and cheaper.

Travel across the ocean use to be only for the rich and those desperate enough to be loaded like cattle in the bottom of cruise ships (as my ancestors were). That travel was only casual for the very rich. Increasing ship technology put such travel into the hands of more people.

Finally, the airlines made travel around the world fairly casual. It is both fast and relatively cheap. Granted it takes a bit of money for that travel to be casual but only the very poor could not get from one end of the US to the other in less than a day.

So, the cost of space travel needs to go down and the speed needs to go up (which will happen as the cost goes down). I think that some kind of reactionless drive would be a good starting point but it isn't necessary. The biggest cost of space travel is getting out of the gravity well. So anything that reduces that will make space travel cheaper. With current tech we could do that with:

  1. Space Cannons: either chemical or electromagnetic cannons that fire you into space. If the barrel is long enough, the acceleration will not squish you.
  2. Laser Launch: build an array of lasers that shine on a block of ice under the payload. The steam produced gives the thrust to get into orbit. Jordan Kare was a big proponent of this.
  3. Airship to Orbit: Fly the payload out on a lighter than air lifting body. John Powell
  4. Space Elevator (arguably): This isn't doable with current tech but it may or may not be too far away. According to Arthur C. Clarke, it should happen in about 30 years since people stopped laughing about it 20 years ago.
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  • $\begingroup$ One of the words for the old English districts was Riding, apparently, I'm not sure how accurate this is, this was because they were literally a day's ride from the lord's manor. $\endgroup$
    – Ash
    Sep 13, 2017 at 17:55
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    $\begingroup$ @Ash, Yes. Inns tended to be spaced a days ride out from each other (where there were any at all). Also, from your comment note that only the lords had horses. So that travel wasn't casual for most people. $\endgroup$
    – ShadoCat
    Sep 13, 2017 at 18:07
  • $\begingroup$ You said it best. Oh and you forgot the orbital rings, skyhooks and possibly......wait for it: Project: Orion (though this last option is technically illegal under the UN's Partial Test Ban treaty so.....). :( $\endgroup$
    – Future Historian
    Sep 13, 2017 at 19:28
  • $\begingroup$ @FutureHistorian, I'm actually a big fan of Project Orion and NERVA drives. That doesn't make me popular among the PC crowd. I figure that orbital rings and skyhooks would happen after we had the tech that could build space elevators and I had to draw the line of reasonableness somewhere. $\endgroup$
    – ShadoCat
    Sep 13, 2017 at 20:41
  • $\begingroup$ @ShadoCat. Same..... $\endgroup$
    – Future Historian
    Sep 13, 2017 at 22:15
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The technology to propel objects to different locations don't really matter, what limits travel of humans in space in this case is the acceleration you can apply. You can't expect normal people to undergo acceleration of more than 3-4G for short time frames and you definitely can't have a constant acceleration of much more than 1G for a long trip. More than 1G means you feel more weight and if you do 2G, it would be like weighing 2x as much.

The physical limitations of space travel really means you need to constantly accelerate once you are off the planet to get to your destination. Once you reach half way, you can decelerate at the same rate. This is really the only physical way to move large distances in space without "teleporting". People will die if you accelerate/decelerate too fast and your ship will fly right past your destination if you don't decelerate. Since there is very little friction in space, you won't decelerate naturally. The mechanism to do this the simplest way would be some sort of rocket with some sort of fuel and the propulsion pointed in the direction you need to accelerate/decelerate, since current chemical fuel is unlikely to improve to densities required, you'd probably need some form of nuclear fuel. Another way is to use lasers from earth/orbit to accelerate and lasers at the destination to decelerate. This would require a ship built to handle large amount of energy being delivered with lasers.

Another thing is that when you are traveling for a long time in space, the time on your ship would not be the same as the time on Earth or your destination. For you might only experience the travel to be a day or so, it could be a week for someone at your destination. If you were commuting, this could be a real problem. The basics is that to you, you can constantly accelerate past the speed of light if you want from your perspective on the ship but to anyone looking at you from outside, you won't ever go faster than the speed of light and the faster you go, the more energy you need to get ever so slightly toward the speed of light. This is simply time dilation meaning that time moves slower for you than an observer as you move closer to relativistic speeds.

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  • $\begingroup$ Agreed. In fact, casual interplanetary travel is less technology and more costs of getting an infrastructure to make it possible in the first place. $\endgroup$
    – Future Historian
    Sep 13, 2017 at 19:25
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I think the most reasonable technology is the Alcubierre drive. Assuming that a energy density lower than that of vacuum can be created, this proposed warp drive bends space-time around the craft to effectively travel as fast as you want, even superluminal speeds.

The most interesting thing about the Alcubierre drive is the fact that it doesn't violate any physical law. In the spacecraft reference frame, its speed would be zero in relation to the local reference frame of the bubble. The movement would be created by contracting space-time in front of the craft and expanding it behind, effectively transporting the objects inside the bubble.

From a purely mathematical and physical point of view, the Alcubierre metric (from which the drive was proposed) is consistent with Einstein's General Relativity field equations. However, it has some caveats, mainly the fact that to contract spacetime, you need negative energy. And to get negative energy you would need some kind of exotic matter.

Recently, though, it was shown with the conformal gravity formalism (which is kind of an extension to general relativity) that it could be possible to build such a device without negative energy and, hence, without exotic matter.

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You specified that everyone is a trans-human. What kind of modifications are we talking about?

You could solve your issue altogether if you accept an "uploadable" brain. Space travel would then be just an issue of reliably sending data across planets and download it into another body.

One could just keep its own planet-bound duplicates in a storage until he needs them, then book a planetary transfer, deactivate its body on Earth and upload its consciousness to his second body on Mars in a few minutes/hours (depending on connection speed).

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