I am creating a world where there is an interplanetary network that allows planetary commuting to and from all planets and major moons in the solar system with 0 propulsion being supplied by the craft itself . Is such a system possible? If so, how would it function and is it:

  1. Scalable

*Can this system be feasibly expanded across the solar system with the tech of a futuristic civilization?

  1. Time efficient

How fast can the system transport people? If it is too slow, it might not be practical for large scale commuting between planets.

  1. Available to the public

Does the system allow people of all ages to use or is it restricted to physically fit humans in their prime and with lots of training?

  • 2
    $\begingroup$ You need to ask about a single issue in a given question, I count at least four separate questions here as this stands. $\endgroup$
    – Ash
    Feb 22 at 6:10
  • 2
    $\begingroup$ Only a comment because I can't/won't back this up with numbers: I'd look into solar sails. The "propellant" is provided by the sun and should be plentiful enough for quite a lot of crafts. The resulting acceleration is low but constant. Can't think of any health impact that are caused by this kind of drive itself. $\endgroup$
    – ooak
    Feb 22 at 8:52
  • $\begingroup$ @Zetrox the simple answer is “yes” to the basic title question about “planetary commuting to and from all planets and major moons in the solar system with 0 propulsion being supplied by the craft itself,” but the rest of the details need to become separate questions. You just need tugs moving your vessels around in complex orbits. It’s not actually a bad design because the propulsion is scalable: Just upgrade the tugs as tech moves on. $\endgroup$
    – Vogon Poet
    Mar 7 at 23:25

3 Answers 3


In a sense, your Interplanetary Transport Network already exists. That plus stasis or hybernation plus the appropriate societal changes fulfill your requirements, because ITN is slow.

Otherwise, you're talking about significant energy requirements, and that energy has to come from somewhere - but more than this, that momentum has to come from somewhere. So you might not need fuel, but you still need propellant (i.e., reaction mass).

Either that or something like monofilament spiderwebs around planets, and maybe that's where the technology might come into play. Or some way of efficiently recovering the energy of a speeding capsule over very long deceleration distances (billions of kilometers). Or, even more far-fetched, transfer momentum through space (so-called "tractor beams"), with nearly perfect efficiency.

However you do that, there's the matter of accelerating people.

The semimajor axis for Neptune's orbit is around 4.4 billion kilometers or 4.4 trillion meters; a constant acceleration of 9.81 m/s2 allows covering that distance in 11 days (terminal velocity is about 9300 km/s), unless I made some mistake. So basically you can get around any two points closer than Neptune in 22 days or less, using Earth standard conditions. Less time than that requires more acceleration, which is harder on the heart (every time you double the acceleration/deceleration - for the whole travel - you reduce travel time by just 30%).


With zero propulsion from the ship; you need a thrower and a catcher.

You might be able to use a powered magnetic rail gun (or coil gun) as both a thrower and catcher. They can fire projectiles 5000 mph in just 30 feet; in space you can make them as long as necessary to get up to speed in a survivable acceleration; even hundreds of miles long if that is what it takes. A pittance in space. And (like the coil gun) the rail sections do not have to be continuous; You can have manageably sized stages of acceleration, say a quarter mile each. Even thousands of them. Of course, I'd presume you have unlimited solar power to provide the transfer; you can locate as close to the sun as is safe and comfortable for power. Or you can use some sort of fusion engines to power the stages.

You can probably accelerate at 1G for as long as it takes to reach your desired speed.

The rail gun, with clockwise and counterclockwise magnetic fields, exerts a propulsive power on a projectile in single direction. So the same kind of gun can be used for deceleration as a catcher, it is just that coming in the opposite end, a propulsive power going "out" provides deceleration power for a projectile coming "in", i.e. traveling through the gun in the opposite direction. You don't even need to reverse polarity.

We might have a power problem far from the Sun, I'm not sure if recovering the momentum on deceleration can capture the power; but I'll presume not. So fusion engines at the destination might provide said deceleration power.

It would also occur at a safe 1G, in the same length of space. Of course power can vary at either end, depending on the mass of the ship, to reach a consistent final speed. Even if that is just turning on/off some number of stages in the rail guns.

Your only real problem is running into stuff, or missing the catcher for some reason; but otherwise there should be no drag in space.

In extant big railguns typical projectiles are about 6 pounds and accelerate to 5000 mph in 30 feet. I don't have the math at hand, but multiply both the mass of the ship by tens of thousands, and the length of the railgun by a hundred thousand (57 miles), and I think something in there would work. Recall that your ship is pretty much 100% life support for several weeks on end, no engines, etc.

Although there is zero on-ship propulsion, the ship might want navigational power, thrusters and such for docking with space stations, landing on a planet, etc.

And of course the destination "receiver" rail gun needs to be built in the first place! Builder ships might accelerate out by a railgun, but with no receiver would likely require fuel to slow down to their destination. But again, fusion engines.

You could even call it a "railroad", and like that, tracks must be build to a new destination, but the materials to do that can be carried as far as possible on the existing railroad.

And this is much like a railroad; accelerate to high speed, slow down to zero, and transfer to other rails to change directions.


Solar sails require only a very small amount of energy to trim the sail, as long as everything is already in an orbit.

The downside is, like all other extreme low energy transportation schemes, they're very slow -- they make ion engines with thrust of a few Newtons look like dragsters; the pressure of sunlight on even a sail a kilometer across is minuscule.

As with other answers, this could be combined with some form of suspended animation to counter multi-year transit times, coupled with the extreme low energy orbital resonances labeled as the Interplanetary Transport Network -- or augmented with an external energy source, fixed to planets, moons, or asteroids: solar powered lasers.

With a powerful enough laser boost, a solar sail is capable of generating accelerations of a tenth of a G or so (compared to fractions of a milligee with sunlight alone), as well as being able to accelerate at right angle to the sun's light or even directly toward the sun. Using a laser to launch and another to catch, solar sails could give most of the advantages often cited for ion engines -- very greatly shortened travel time, and more advantage the longer the trip -- without requiring even the tiny amount of reaction mass or onboard energy generation those engines do.

For reference, 0.1 G constant boost from Earth to Mars at closest approach (via lasers on the Moon and Phobos, for instance) would only take a few weeks, and even Neptune is only about a year away. I wouldn't call it commuting, exactly, but it is accessible to anyone willing to take the appropriate nanites or whatever to maintain bone mass and muscle tone in prolonged low gravity.


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