# Interplanetary vs Interstellar travel

I want to be able to have interplanetary travel very cheap and fast, within minutes. Seeing as planets are light minutes away from each other, I suppose even for fast interplanetary travel the tech in ships would need to be close to light speed or lets just say FTL with some type of jump drive.

I do however want interstellar travel to not be possible with ships(but maybe via gates or wormholes).

My question is, does it make sense that tech would allow FTL within a solar system but not between different stars/systems. Is travel speed still a factor with FTL? or is it always instantaneous. Or is there a reason why FTL would have a limited range? I am open to any ideas.

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Saying that planets are "light minutes" away from each other could be considered a bit of a stretch. Take the distance between Earth and Mars when the two are farthest away from each other: almost 2.7 AU, or a little over 20 minutes in a straight line at $c$, and that's our closest neighbor. Jupiter can be up to 6.5 AU away (again in a straight line), which is just under an hour. By the time you are looking at the outer planets, hours at light speed is reasonable; take Neptune at ~30 AU away (at these distances, closest and farthest are essentially the same), a four hour trip at light speed. – Michael Kjörling Feb 4 at 8:55
Thanks for the clarification. It is helpful because travel time is a mechanic I want to implement in a lot of detail... I suppose I will need to research what the average or maximum distances in other solar systems is too. I imagine there are some systems out there with much larger scales. – blackbirdresearch Feb 4 at 8:59
I would expect there to be solar systems with planets at much greater distances, but you have the issue of lower insolation at greater distances (and larger stars live shorter lives). You may want to check out our sister site Space Exploration too, particularly their orbital-mechanics, hohmann-transfer, low-energy-transfer and orbit tags. – Michael Kjörling Feb 4 at 9:06
Even at c travel to the nearest star from here would take over 4 years. Does that help answer your question? – Draco18s Feb 4 at 16:28

Simple: Fuel.

The engines of these ships require lots and lots of fuel. For interplanetary trips it's not a problem, they can refuel once they get to their destinations. But for interstellar travel, bringing the amount of fuel necessary to make the voyage just isn't feasible and there's isn't any place to refuel between stars.

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Also to move the mass of the extra fuel you can need even more fuel. In the real world that's why there's a limit on the size/fuel/speed of rockets. – jean Feb 4 at 17:43
@jean: The rocket hell equations aren't necessarily applicable here. If you're warping space around you, it's more about how much space you warped than the mass of the ship you're warping space around. Similarly, the wormhole would cost energy proportionate to the size of the opening, distance between openings, and/or time the hole is open, with little concern for the mass of the objects moving through it. But it would still plausibly cost several orders of magnitude more to travel 4 light years than 1 light hour. – MichaelS Feb 4 at 23:33
"I want to be able to have interplanetary travel very cheap" in the question, I'm not sure this is compatible with your "lots and lots of fuel", unless you can explain why fuel is so cheap... – hyde Feb 5 at 13:12
The fuel doesn't have to be expensive. Maybe the engines run on element Handwavium which is very abundant on the planets in the solar system, but it's not found in interstellar space. – fiend Feb 5 at 13:25
Why handwavium? Why not hydrogen? It's everywhere in our solar system, but extremely sparse in the space between solar systems. – thanby Feb 5 at 22:41

If you want an easy way around this:

The in-system FTL is 'ground-pumped'.

Human made FTL equipment is really inefficient. Terribly so. So to support an ever expanding need for in-system FTL ability each planet has been equipped with huge base stations that make non-wormhole FTL easier (either though 'pre-warping' the space so only a little power is needed to make the jump, or by pumping power to the local FTL drives via space magic). If these stations stop operating then most ships can't push past the FTL barrier, and the ones that can have limited range before they run out of power/fuel. The advantage that these base stations have over the wormholes is that though they're pretty inefficient they have an entire planetary grid to feed off, which makes individual ship performance much easier to achieve for a lower cost. Your local FTL ships can then fly to anywhere within the field. Oh, and if defence is needed the base stations can be used to the opposite effect: making FTL travel impossible near a planet, so that's a bonus too.

By comparison the interstellar wormholes are much more efficient, but also locally constrained, unpredictable and expensive to maintain. As such no-one would dream of building one for in-system travel where a planetary base station is viable.

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I like this. While not actually a way to travel faster than light using real physics, it gets just close enough to some of the concepts like group velocity to make it feel legit and is bounded... bounded abilities are always easier to accept than the limitless ones. – Cort Ammon Feb 4 at 15:01
@CortAmmon: Group velocity!! That's the term I was trying to remember!! Thanks. :D – Joe Bloggs Feb 4 at 20:12
Ooh, good idea. I've seen some discussions of building interplanetary ships that use large reflective sails, and then a gigantic laser on the ground or in orbit is fired at the sail to provide propulsion. Similar to the idea of ships sailing on the solar wind but with an artifical "fan". Big advantage is that the ship doesn't have to carry ANY fuel: the energy is all put into the laser. But it would limit your range to the distance over which the beam remains sufficiently concentrated. – Jay Feb 4 at 20:53
This answer is great because it side-steps the need to carry fuel (and thus invoke the "rocket hell" equations) but still plausibly limits your range to in-system travel. – thanby Feb 5 at 22:44

Bear in mind that if you really accelerated to light speed on a trip to Mars, circa 150 millions miles, the acceleration would be awesome. Assume you accelerate so that you reach light speed half way there, and then decelerate for the rest of the trip. That would require an acceleration of 230 miles per second per second, or 1.2 million feet per second per second, or 38,000 g's! Everyone and everything on board would be crushed the instant you turned on the engines. Well, maybe you're assuming that if we had the means to travel at light speed, we'd also have some way to cancel the effects of the acceleration.

My first question is: Why do you need to be able to travel around the solar system in minutes? Slow it down to days and it's still a trip that people could make routinely, but it would be painfully slow for interstellar travel. Like if it takes, say, 2 days to get to Mars, that's an average speed of 3.1 million miles per hour. Incredible speed! But at that rate it would take you over 200 years to reach Alpha Centauri. I think the easy answer to your problem is to just slow the ships down to the point that interplanetary travel takes reasonable amounts of time but interstellar is multiple lifetimes.

If you really need your ships to reach another planet in minutes for your story to work, I guess you'd have to posit some reason why they either, (a) can't travel far from the Sun or a planet; or (b) can't travel more than x distance.

RE (a) Maybe they are solar powered. Too far from the Sun, not enough energy, they don't work. Hmm, but why couldn't you put a nuclear reactor on board to supply energy instead? Maybe some hand-waving there. Maybe they need a solar wind for some reason?

(b) is easier: They require huge amounts of fuel. The longer the trip, the more fuel, of course, But it doesn't go up linearly, it goes up exponentially, because for a long trip, you have to carry enough fuel to propel all the fuel for the rest of the trip. Just like present-day rockets require huge amounts of fuel because you have to burn enough fuel in the first few seconds to get the fuel for the entire rest of the trip off the launch pad, etc. You could say that the fuel requirements rise such that for a trip of light-years, the equation becomes unsolvable, an infinite amount of fuel would be required. Ok, I haven't worked out the math if that's actually possible, but at least, an amount of fuel so large that it would take all the fuel you could possibly produce for hundreds of years.

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I think the solar-powered thing is meaningless. A 300 square meter solar array near Earth can only only produce about 550 hp, or about a third of a P-51 aircraft engine. Even if you could create direct thrust like an aircraft (using some external medium), you're not going to get much acceleration, especially on a ship with 300 square meters of surface area. Trying to do it with rocket-type propulsion will be even worse. – MichaelS Feb 4 at 23:43

You could limit FTL travel to places where you've installed beacons or gates for some kind of ghetto wormhole tech and those beacons have to be sent via slower-than-light means, which could have been done in advance of your setting.

You might have a beacon on its way to an interstellar destination, which isn't going to become useful for a very long time.

But if you need to keep wormholes for interstellar travel, then you have to be careful to distinguish the two technologies and explain why the interstellar one works without the beacon (and why you're not just using that one for interplanetary, and why you didn't just send a beacon through the beaconless wormhole technology).

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Thanks for the ideas. Yes I must distinguish them. The way it will work is this, basically the wormholes are not man made, we have no control over them. A wh appears in our solar system which leads to another system far away, interplanetary capable ships jump through and explore/mine/ops and return to our system before the wh collapses, then a new wh appears in our solar system to a new different system. So the interstellar space is always somewhere new and only temporarily availabe, which is the reason no tech is being deployed there... at least this is my current structure. I may modify it. – blackbirdresearch Feb 4 at 7:33
So wormholes can pop up that go any distance but eventually collapse, while humans (or whatever) have only mastered wormholes that are stretched out between two prefabricated gates, and the second gate has to be sent to its final position by conventional travel, then? And trying to push a wormhole gate into an interstellar wormhole only works while the interstellar hole is open (or has other negative effects)? – sh1 Feb 4 at 7:39
ahh. I actually havn't come up with any ideas of human made WHs yet, I did mention gates as a possibility in my original post, but I think the temporary WHs that randomly appear are going to be the main way for intestellar travel. I am not sure how I would implement man made wh gates or how they would work in coordination with the random WHs, but yes conventional travel or the random WHs would be used to set up the gate points. if I implement it, still undecided. – blackbirdresearch Feb 4 at 7:44
Look at the reaction to the claim that the LHC would be "creating black holes". Now scale that up to a wormhole that is actually large enough to allow even a tiny spacecraft to slip through. I can see a huge resistance movement growing in response to the idea of having something like that even in the outer parts of the solar system... press.web.cern.ch/backgrounders/safety-lhc and more generally google.com/search?q=large+hadron+collider+black+hole – Michael Kjörling Feb 4 at 8:58
I will definitely have some type of factions and politics and rebellions happening. I hadn't thought of the reaction to whs specifically.. but I have always liked the idea of humans reacting to advanced tech and splitting off into factions. Though I first thought of this splinter in humanity when thinking of cyborgs, half of humanity would want to augment themselves and others would want to remain pure human. – blackbirdresearch Feb 4 at 14:05

Simply make your handwavium FTL drive require a solar gravity well to function. You can use it within mumble-mumble distance of the sun, but not in the vast reaches of interstellar space.

Edit: Sigh. Noticed that Hohmannfan beat me to it, and even referenced Niven... that'll teach to me to knee-jerk an answer.

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Similar vein, it could require a constant power source, possibly to power a deflector shield to prevent damage from hitting very small particles at near light speed. I don't think we have any technology other than a solar panel in space that can guarantee 100% uptime for tens of years required to travel interstellar at near light speed. – Scott Feb 5 at 12:37
Wait, or does relativity mean that a battery would only see minutes passing, even if an outside observer sees years? – Scott Feb 5 at 12:40
@Scott Don't think too much into it, we're already violating causality here so trying to re-apply it will just give you a headache! – thanby Feb 5 at 22:52

An option that could be considered is the FTL is fine but navigation is dodgy.

So jumping 1 au might get you within 0.1% of where you wanted to go but jumping 1 ly might end up anywhere.

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Something like dead reckoning / inertial measurement navigation drifting over distances in a FTL otherwise interstellar-capable spacecraft? Could give rise to some interesting plot twists... – Michael Kjörling Feb 5 at 9:06
I don't think that would work. I mean if it works like that I would use my navigation system to calculate a series of 1 au jumps to bridge the 1 ly distance. – mg30rg Feb 6 at 17:16
Not if the jump itself was expensive rather than the distance. Imagine your ship only has enough fuel for 3 jumps before refueling. – BevynQ Feb 9 at 3:48

You want interplanetary travel within minutes? Then you do not need FTL, your everyday close-to-c torch ship is enough, and is possible within current physics.

If you insist on FTL though, I propose an opposite scheme to Larry Niven's FTL limitations in his known space series. In his books, FTL only works at a distance from a singularity. You can just use the opposite rue, FTL only works when inside the solar system.

You can also have a type of warp drive that is dependent on local distortions in the gravitational field, e.g the curving of space-time from the Sun and the planets.

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@blackbirdresearch Close to the speed of light, "c", is possible with current physics, so I will bet against you on the realism part :) Ships capable of very large velocity changes are sometimes called torch ships. – Hohmannfan Feb 4 at 7:23
@JoeBloggs At anything except $v \ll c$, I think it's safe to posit something like Star Trek's inertial dampeners. Otherwise, like you say, you indeed have... seat cushion design challenges to overcome, shall we say? – Michael Kjörling Feb 4 at 13:38
Any close-to-reasonable acceleration will exit the solar system long before you get close to c. – WhatRoughBeast Feb 4 at 20:48
@JoeBloggs See my answer: circa 38,000 g's. Turn on the engines, and crew, cargo, ship, and engines are all instantly turned into powder. Unless you have FTL and you also have something to neutralize the effects of acceleration. I guess if you're going to posit one magic technology, you may as well posit two. – Jay Feb 4 at 20:48
FTL only inside a system is a nice cruel joke. Kind of like the short short story "FTA" by George R. R. Martin where hyperspace travel is discovered, but the speed is slower than c in there. – Oldcat Feb 5 at 1:35

Make in-system travel teleporting (sending info about atoms of teleported object via modulated laser, at the speed of light).

This solves the huge accelerations needed for FTL travel, and has other convenient use for travel on the planet itself (about the time someone seriously invented that!).

This cannot work over long distances, because signal is much weaker and error rate would be unacceptable.

Even teleporting over in-system will be more problematic than plain on-planet, errors caused by any random debris, but manageable.

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And this works for interstellar restrictions too. They can put a chain of repeaters to cross vast distances as needed, but the chain would be very expensive to build and maintain, so most people wouldn't be able to get on the list. And it qualifies as "can't get there via ship". Plus, the limited bandwidth effectively limits travel speeds since you can only send a few people at once. – MichaelS Feb 4 at 23:46
"error rate would be unacceptable"... what an epic way to say "Whoops we killed someone in a pretty terrifying fashion" (like accidentally scrambling the neurons in the brain responsible for breathing). Honestly that sounds like an interesting plot for a sci-fi horror story. – thanby Feb 5 at 23:00

You don't need interplanetary FTL, all you need is a sublight warpdrive, which instantly changes the ships kinetic energy temporarily, achieving near-lightspeed for limited amount of time, before needing to restore its original reference frame. By limiting the amount of energy that can be "borrowed” for this, you can set effective maximum speed and prevent speeds too close to c, and have ships with different max speeds, which is usually nice for plot.

Also interstellar travel could be possible this way, but it would take years even to closest stars. If you have some kind of interstellar gates allowing instantaneous travel, nobody would bother with that (except for exploration or military or criminal purposes, which again can be useful for plot).

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There are lots of nice answers here, so this one might not be the solution, but...

How about probabilities? Your warp-drive can funtion based on quantum-physics and have a probability of faliure (like in "you don't arrive at all" or "your particles arrive to the destination but the bonds between them are broken"). You can fine-tune that with a nice limit.

Let's say there is an 1:10^60 chance of faliure on an 1au jump, but an 1:10^26 chance on a 10au jump and an 1:4 chance on an 1 ly jump. Practically nobody would take that risk, so interstellar travel would not be possible with one of those drives without stabilizer gates at both ends of the jump. (Gates, corridors etc.)

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Very outside-the-box, +1! – thanby Feb 5 at 23:03

The best way to approach the problem is to remember that FTL is essentially time travel (if you arrive at a location before the light arrives, you are breaking casualty), and many theories suggest that time travel is ruthlessly suppressed via the physical nature of the Universe. Creating a closed timeline loop with wormholes is supposed to be accompanied by an ever increasing cloud of virtual particles until the two ends of the wormhole are destroyed, for example.

So even if your ships have something like the Alcubierre drive, they will be limited to c, and so travel times across the Solar System might be 8 hrs from "end to end", but you are still stuck with years, decades or even centuries of travel between the stars.

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None of this is really applicable. The whole causality violation thing assumes relativistic equations hold for objects traveling faster than light, but the moment we exceed light speed, relativistic physics aren't really applicable anymore. So if you're handwaving FTL, you can just handwave causality issues. – MichaelS Feb 4 at 23:58
Short answer is no. There are plenty of places you can research the long answer. – Thucydides Feb 5 at 2:58
If you can link me to any info that actually shows FTL requires time travel, I'd be interested in reading it, but so far every single argument I've seen trying to support that claim has made the same mistake. They all presuppose relativistic physics hold true for FTL even though the same physics say FTL isn't possible. Clearly, it's trivial to model a universe where speed has no effect on time at all, so a causal universe with FTL is very plausible. The only place there might be a catch is trying to model a causal universe with FTL where the model accounts for current experimental data. – MichaelS Feb 5 at 5:45

Here's an interesting twist on FTL that you can use to create an added layer of 'rules' or 'disincentives'... allow the traveler to 'jump' through gates either instantaneously or travel between them at a ludicrously shortened time span, with the cost of OUTside time having passed at the normal rate. Basically, the "jump" occurs for the traveler so the distance between stars is traversed in a time span that does not necessitate cryostatis or generation ships, BUT when the vessel gets to its parsec(s) away destination, the system that the vessel left has temporally progressed by p/c time, where p is the distance in parsecs traversed by the vessel and c is the speed of light. (you're just solving for the time it takes light to go 'p' distance.)

Alternatively, you can spice up the danger/difficulty factor of FTL/teleportation travel with known physical phenomena. Maybe gravitational fields impede travel through the jump path... or electromagnetic fields/ interstellar winds of radioactive material "disrupt" the tunnels and cause turbulence or even the chance of a collapsed 'tunnel', so the further you jump, the more Russian Roulette-ish the gamble. If you're unfamiliar with how pre-stellar plasma clouds produce Z-pinch through compounding electical-magnetic field interactions, I'd suggest reading some articles on it. THATS a very cool real phenomenon I haven't seen mentioned much by sci-fi authors. There are some strong theories about how those z-pinch forces may have been just as, if not more, influential in planetary body formation than gravity!)

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Most of this is actually possible. It's just about traveling very close to the speed of light to get relative time dilation effects. Of course, that still suffers from the problems of acceleration as mentioned elsewhere, but if you can handwave FTL, you can handwave inertial dampening. – MichaelS Feb 4 at 23:49

You could argue, that outside the solar system there is too much radiation and for the travel you need better protection from that. The magnetic field of the sun provides you that. It doesn't have to be exactly radiation, but we still don't know too much about interstellar space, so there might be some handwavium that prevents your unobtanium from working there.

This also gives you nice "Currently teleportation isn't working" options from solar rays and the such.

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I don't think this really works. There's actually less radiation the farther you get from the sun, and magnetic fields don't affect radiation. There are charged particles out there, but they're being repelled by the charged particles streaming out from the sun, so I don't think you'd see much difference there either. The idea of solar flares interfering with jump drives and such is interesting though. – MichaelS Feb 4 at 23:52
@MichaelS We still don't know too much about interstellar space, so perhaps you could use the sun as a protector from some "handwavium", that prohibits our unobtanium drive to work there. – akaltar Feb 5 at 12:26

The distances between neighboring stars are tens of thousands of times the distances between planets in a solar system. Even the outer planets will be only billions of miles apart, while the nearest stars will be tens of trillions of miles part. Other stars in the same galaxy will be hundreds, thousands, tens of thousands, and hundreds of thousands of times far away.

So if your space ships accelerate to a speed, coast at that speed, and then decelerate at the destination, a journey to even the nearest star will take tens of thousands of times as long. Thus for every minute an interplanetary trip takes, a trip to even the nearest star is likely to take several times 6.944 days. If you can think of a reason why it is perfectly feasible to spend minutes in a space ship traveling in interplanetary space but not feasible to spend a few weeks traveling in interstellar space your premise makes sense.

Maybe the people of that society are so used to instant communication with the outside world (at least with other people on the same planet, if not between planets) that they can't stand the thought of going here it takes years to communicate with home.

If interplanetary trips take days, interstellar trips to even the nearest star tens of thousands of days, or nearly a lifetime to the nearest stars.

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