Assume there are many inhabitable planets in the same solar system. What new technology would make traveling between these planets cost-effective and (relatively) fast?


This question asks what technology would be useful for traveling (1) to the edge of the solar system (2) ignoring economics and safety. I'm instead asking what would allow travel (1) between planets in the goldilocks zone that is (2) cost efficient and safe for humans.

With our current technology, we could probably send people to Mars, but it would be very expensive and not have much benefit. I'd like to design a solar system where travel between planets is used frequently for many purposes.

Factors to consider:

  • a trip doesn't need to be cheap, but it should be within reach of a normal person (i.e. not a billionaire)
  • a trip should be short enough that it's not just a once-in-a-lifetime trip (probably a week or less)
  • planets should be able to trade with each other (as long as the resources are valuable enough to send)

What technology would solve these problems?

  • $\begingroup$ "between planets in the goldilocks zone" you realise that for the most optimistic opinions that only includes Venus, Earth and Mars right? less optimistic of opinions might exclude both Venus and Mars, so depending on who you ask the answer could be that 'no inventions or discoveries are needed, only one planet is in that zone and you can travel between it and itself by walking' 😁 $\endgroup$
    – Pelinore
    Jul 26, 2022 at 14:59
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    $\begingroup$ more seriously, given the reasonably short (as these things go) distances involved we probably don't really need anything new we can't already build for travel between Earth and those other two that wasn't entirely incomparable to the journeys of early European settlers travelling to the Americas. $\endgroup$
    – Pelinore
    Jul 26, 2022 at 15:03
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    $\begingroup$ VTC: The rules in 2017 were different compared to the rules today. This question is too broad and too opinion-based. To give you an example, in 1989 Rocwell International published a poster called the "integrated space plan". I have one hanging on my wall. It's uber cool. It also proved to be dead wrong. If they couldn't do it.... $\endgroup$
    – JBH
    Jul 26, 2022 at 18:28
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    $\begingroup$ Worse, as written this is asking for an off-topic infinite list of things. The 2017 question might survive today because it's asking for an on-topic finite list of things. $\endgroup$
    – JBH
    Jul 26, 2022 at 18:29
  • $\begingroup$ @JBH the 2017 question asks what propulsion technologies would allow travel WITHOUT economic and safety constraints and this question asks what propulsion technologies would allow travel WITH economic and safety constraints. How is one infinite and the other not? $\endgroup$ Jul 26, 2022 at 19:02

10 Answers 10


I'd say you want the following bits of technology:

  1. Mature non-rocket spacelaunch system. I'm a fan of something like StarTram. This is a big, expensive, high-tech project, but I'm of the opinion that without some kind of way to get into space without a Big Dumb Booster it'll always be too expensive and potentially too risky. An economical non-rocket system (which might include space fountains, launch loops, orbital rings, space elevators, but probably not skyhooks as they seem to alarming to regular passenger use) will let you get large amounts of stuff and people into space in relatively short order, which is a pre-requisite for serious development of space, and regular and not-too-expensive interplanetary trade and travel.
  2. Compact and powerful nuclear reactors for spacecraft. You sorta get this for free with some designs of nuclear rocket, but not all.
  3. Very powerful rocketry. Your time limits (a week!) are pretty tight, and in order to meet that you need very powerful rockets. High-end fission and fusion systems are probably the order of the day. Orion drives probably don't cut it... you'll need working z-pinch or ICF systems that are at least one order of magnitude more powerful than any current proposals.


  1. Powerful and effective beam propulsion systems, such as a super magbeam or maybe some kind of laser ablative propulsion, though the effective range of the former may be too low and the effective specific impulse of the latter is probably too low as well.
  2. High quality radiation shielding systems capable of warding off a wide range of dangerous particle radiation. A combination of passive shielding (water and polyethene and boron fibers) and powered systems (electromagnetic) will probably be required.
  3. Effective and comfortable artificial gravity systems. For all but the most outrageously powerful rockets, you'll be either using a rocket whose thrust is much less than 1G, or simply coasting between your boost and brake burns without any acceleration at all. This is perhaps the easiest bit of the whole list.

I'm always happy to talk at interminable length about rocketry, interplanetary travel and spacelaunch infrastructure, but this answer is already too long. Feel free to ask further questions about specific aspects of this answer, though, and I can respond separately and in detail! Note that you don't necessarily need something that can thrust continuously at 1G for the whole trip, or thrust for the entire trip (just at the beginning and end), or even spacecraft that have their own propulsion system at all. There are plenty of options!

a trip doesn't need to be cheap, but it should be within reach of a normal person (i.e. not a billionaire)

a trip should be short enough that it's not just a once-in-a-lifetime trip (probably a week or less)

You are spoilt by modern rapid transit and the assumption that you can fly just about anywhere in the world in under 48 hours. Here's a wonderful map from 1914 showing isochrones... how far you could travel from London in a certain number of days.

Bartholomew's Isochrone map from 1914

(source: An atlas of economic geography, public domain, whole book available on archive.org, this map on page 94)

It might be hard to see the text, but the red area is "less than 5 days", pink for 5-to-10 days, yellow for 10-20 days, green for 20-30 days, pale blue for 30-40 days and dark blue for more than 40 days. Sure, you might not be paying a visit to your equivalent of the Australian Outback every couple of years, but honestly most people don't even do that these days when they could get there much faster. (I'd relax your "1 week" requirement, if I were you. It makes the tech-levels required for your rockets much easier to attain.)

That's from a little over 100 years ago. The cost of travel wasn't cheap, but neither was it entirely unreachable... according to this site (of unknown provenance), a 3rd class transatlantic ticket on the Titanic would have cost £7, equivalent to ~£870 in today's money (~1050 USD). That's a bit cheaper than a modern flight from, say, New York to Sydney, but it is in the same ballpark.

Getting from Earth to Mars or Venus in under 10 days is tricky, but not impossible. You of course get to choose how much handwaving you do here, but be aware that inner system flight times of days instead of months are seriously high-tech and potentially hazardous, given the power levels involved.

planets should be able to trade with each other (as long as the resources are valuable enough to send)

Most resources won't be, and resources that are valuable enough to trade might be easier to build, grow or mine in space, thus removing the whole "lifting out of the gravity well" bit.

People and post? Sure. Scifi-classic tramp traders shipping random break-bulk? Not so much.


One exceptional condition (not a technology, but a property of the planets) would be if the "home" planet were part of a double planet system. Earth and Luna are sometimes called a double planet, but Earth's mass is about 80x that of Luna -- if the planets were close enough to the same mass, and orbited near enough to each other to be tide locked, it might be possible to travel from one to another with less effort than it takes us to go to our Moon.

Various past questions have established that a shared atmosphere isn't possible for planets as heavy as Earth (so no 747 trips from one to the other), but it would still be very possible for two such planets to orbit close enough to tide lock to each other, requiring barely any more energy to transfer between the two than the minimum to reach orbit.

This puts the transfer (one way) within reach of mere millionaires with the technology of the 1990s (Soyuz or Space Shuttle); large scale use of ships like Falcon Heavy or Super Heavy/Starship could make this cost less than a million dollars per passenger by mid 21st century levels.

Obviously, civilizations on both planets must be close to the same level of technology, be in communication, and share some basic ideas (value of goods, for instance), lest your travelers become stranded on the "other world" where the natives aren't willing (or able) to provide a return launch.

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    $\begingroup$ In the limit, a binary planetary system could have (carefully) conjoined space elevators. No 747 to the other planet, but you could catch a train. (relevant space exploration post) $\endgroup$ Jul 26, 2022 at 16:35
  • $\begingroup$ @StarfishPrime Makes sense. Geosynchronous height would be at the neutral point between the two bodies, even if much lower on the other side. $\endgroup$
    – Zeiss Ikon
    Jul 26, 2022 at 16:38
  • $\begingroup$ ...lest your travelers become stranded on the "other world" where the natives aren't willing (or able) to provide a return launch. is a good point for this question, but would make a pretty awesome story on its own $\endgroup$ Jul 28, 2022 at 13:12
  • $\begingroup$ @KlausHaukenstein That thought occurred to me when I wrote the answer. $\endgroup$
    – Zeiss Ikon
    Jul 28, 2022 at 13:23

Sounds like you're in the market for a brand new torch ship! Step right this way...


Winchell's site goes over various definitions for a torch ship, but it boils down to any rocket which has both high thrust and high specific impulse (fuel economy). In general this means getting away from chemical rockets and moving over to rockets which use some kind of nuclear reaction.

Ones we might be able to build in the near future are mainly based on fission - Orion drives and nuclear salt water rockets. These are every bit the radiological hazard you might expect them to be, so you wouldn't want to operate them in a planet's atmosphere. Not one that you cared about, anyway.

More speculative versions are based on fusion. Some of these (deuterium-tritium rockets, for instance) are still pretty dirty. Others (proton-boron and proton-proton) are much cleaner, but you still wouldn't want to be anywhere near the exhaust when the engine was running.

Naturally, the cleaner reactions are much harder to initiate and sustain than the dirtier, more neutronic ones. Thanks, universe.

All of these have the potential to get you to a closer planet like Mars in pretty short order. The exact amount of time will depend on how much delta-V your ship's engine and fuel fraction gives you, but think in terms of days instead of months.

This has a lot of advantages. For one thing, shorter trips mean less radiation exposure for passengers and crew. Engines that aren't as mass-limited mean that you can push more cargo and carry more shielding. The spacecraft is reusable, which keeps costs-per-trip down. You can reach the asteroid belt in only a little more time than it would take to reach Mars, so you can potentially extract raw materials from there to maintain your space effort, instead of having to lift them out of a gravity well.

It's not all roses though.

If you're using an engine with a highly radioactive exhaust, you'll probably only want to use it in space. That means that you'll need a more conventional way to escape Earth's gravity well to get to your interplanetary craft.

Fortunately, that falls into the realm of reusable rockets (now not actually science fiction any more) and spaceplanes. Check out the Wikipedia page for SpaceX for a breakdown of their current and future reusable launch vehicles, or Skylon for an idea of how a spaceplane might operate.

Project Rho is a veritable gold mine of engine ideas, as well as in-depth discussions on any other facet of spacecraft design you can think of. All of it's are based in real physics, too. Scroll down to the bottom of the page for a full list of subject areas.

Everything listed represents a significant engineering challenge, but none of it is physically impossible.


Not sure how to tackle the 2nd point better than the answer by @L.Dutch, but I'd say Reusable Rockets and Space Elevators.

  • A trip doesn't need to be cheap, but it should be within reach of a normal person (i.e. not a billionaire).

Looking at the most expensive parts of making rockets, as well as what folks like SpaceX are trying to save money in is fuel and engines by making them reusable. If launching from Earth, and depending on your tech level, you might need something that can propel upwards with relative ease and that it's mainteinance can also be rather cheap, as putting cargo in a railgun might not be the best idea cost-effectively.

Perhaps using a space elevator for Cargo and leaving Rockets for people might work as access to an elevator mightn't the cheapest way to get people to orbit and private, reusable spacecrafts may exist alongside planes by then. After that, a 'Metro Station' from orbit (or within the space elevator itself), might just make the cut.

  • Planets should be able to trade with each other (as long as the resources are valuable enough to send)

The Moon, for example, has a lot of valuable materials in the form of impact craters and lunar regolith. If rocket fuel is easier to produce in outer space, or people manage to make Helium 3 fission reactors that we can harvest and refine from lunar dust, sending it off in simple propulsion vessels into this 'Metro station' in orbit may work if you have a a fraction of the gravity of earth and little or no air resistance.

If not, you could always install an elevator in a particularly productive enclave or colony and have them put materials in orbit for you. You'd basically litter your systems with easily accessible docks or airports that way.


I would have to boil it down to simply one thing. Just efficient and cheap fusion technology. If you can attain this, you have energy. With that energy you can find ways to get you cleanly into orbit, like ablative laser propulsion. And quickly to other planets with numerous types of torch ships. Id posit fusion if not the ultimate form of energy propulsion is still a necessary item to any greater form of energy production.


The Mars Cycler

Everyone wants to build better rockets. While useful, it isn't financially practical for trade. You want to put a self-sufficient space station on a trajectory that oscillates between the two orbits on a schedule. This is the public transit model taken into space.

The really difficult part of getting to Mars is accelerating enough mass for a self-sufficient environment that can keep people alive for the trip. If you only have to accelerate a small shuttle up to intercept speed, then you make the whole process a lot cheaper, safer, more efficient and faster.

The practical use of cyclers involves having numerous cyclers for each planetary pairing, very much like numerous trains running on a metro line. Shorter transit times are usually performed at the expense of a longer overall orbital time, where the station would spend most of its time waiting for its next cycle to come around. A 75 day transfer time can be achieved, but any one station would only be useful for 1/25th of its life.

Apologies for throwing a monkey wrench in the works, but even with sustained 1G acceleration, you aren't going to get practical 1 week trips between planets. It would take most of a week just accelerating and decelerating, not to mention transferring between transport modes.

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    $\begingroup$ Cyclers are a nice solution for all sorts of things, but there's no practical way for them to help attain the (unreasonably!) short travel times the OP requested. $\endgroup$ Jul 26, 2022 at 16:34
  • $\begingroup$ Yea, I updated it based on this observation. OP is requesting speeds that ignore the vastness of space and break physics in a "one big lie" manner. If they do that, then physics doesn't matter. $\endgroup$ Jul 26, 2022 at 16:37
  • $\begingroup$ You should run the numbers yourself before claiming impossibility ;-) a ship running a 1G brachistochrone can cross 6AU in a week, with a top speed of 0.8% of C... a velocity and delta-V achievable without even having to resort to antimatter rocketry. The spacecraft would be obscenely, outrageously powerful (specific power in excess of 14MW per kilo) and there are an enormous number of technological hurdles to before we could build such a thing, but it is far, far from "one big lie" type soft-scifi magitech. $\endgroup$ Jul 27, 2022 at 10:03
  • $\begingroup$ There's been a lot of math on the possibility of "torch ships" and it all comes down to needing some kind of magic to keep the reactor from melting the ship it's attached to. That puts it into "one big lie" territory. I didn't say impossible, I said impractical. Once in a lifetime trips don't last for two years, so we can't presume shortest distance. 1G transit time varies between 1.7 and 4.6 days. Travel US to Japan is about double the actual plane time, and you still have to account for ground-to-orbit time. $\endgroup$ Jul 27, 2022 at 21:19
  • $\begingroup$ No, not really. But I'm not hear to discuss how quickly goalposts can be moved, so I'll leave you to it ;-) $\endgroup$ Jul 28, 2022 at 8:01

It will need to be a technology which allows keeping 1 G acceleration for prolonged times without adding the burden of the additional propellant mass needed for it with conventional rockets.

Constant acceleration is notable for several reasons:

  • It is a fast form of travel. When ergonomics are considered, it is the fastest form of interplanetary and interstellar travel.
  • Constant acceleration creates its own artificial gravity, potentially sparing passengers from the effects of microgravity.

As detailed in this answer on SpaceExploration.SE

To travel half the distance to the moon would take about 1.75 hours. The other half distance spent decelerating would take the same amount of time.

Using Days and AU (astronomical units) we can see 3 days will get about 2.5 AU (halfway to Jupiter). 4.5 days will get you 5 AU (halfway to Saturn). 9 days will get you 20 AU (more than halfway to the Kuiper belt)


I note that you migh twant to design a star sysem where interplanetary travel is as easty as possible.

Part One: Robot Characters?

If the characters in your story are all robots who don't need to breath, they can live on asteroids in a vast asterodibelt orbiting their star. The energy costs of launching from an asteroid and landing on another one would would be tinty fractions of those for launching from and landing on habitable planets.

Or you might wantbiological characters and thus want to design a star system with as many habitable planets in its habitable zone as possible.

Part Two: Scale of Science Fiction Hardness.

If you can accept a low science fiction hardness score, you could just go with imagining there are as many planets in the habitable one, and they are as easy to travle to and from, as you want.


Part Three: The Circimsellar Habitable Zone.

But since you asked your question here, you probably want to write a reasonably plausible and possible story. So you want a situation with as many habitable planets in the habitable zone as possible.

To find out how many orbits for habitable planets there can be in your star system, find the luminosity of the star compared to the Sun, and so multiply the inner and outer edges of the Sun's circumstellar habitable zone by the square root of the luminosity ratio.

The problem is, nobody kows for cetain how wide the circumstellar habitable zone of the Sun is.

Here is a link to a list of about a dozen estimates of the inner or outer edges, or both, of the Sun's circumstellar habitable zone.


Note the vast differences in how wide or narrow the Sun's circumstellar habitable zone is in different estimates.

So my advice to a writer who will be content to have only one habitable planet in their solar system is to take the luminosity of the star and use it to calculate the distance at which an orbiting planet would reeive exactly as much radiation from that star as Earth gets from the Sun. I call that distance the Earth Equivalent Distance or EED. And they should put their planet at the EED of their star, or within one or two percent of that EED, in order to have a highprobability that nobody will calculate that a planet in theorbit chosen could not be habitable.

But writers who want several different habitable worlds in their solar system will probablyhave to risk making their star's habitable zone wider than the narrowest habitabable zones which have evern been calculated by scientists and hope that future calculations will support wider habitable zones.

Because there are physical limitations to how close two planetary orbits around a star can be.

Part Four: How Close Can Planetary Orbits Be?

Astronomers have now discovered a number of palnetary systems with two or more planets orbiting the same star.

According to this list:


Kepler-36 b and Kepler-36 c have the smallest ratio between their orbital semi-major axis, a difference of 11 percent. Tehorbit of Keplar-36 c has a semi major axis 1.11 times that of the orbit of Kepler-36 c. I believe it is actually about 1.1127 times, making it a little worse. The starshould be billlnsof years old, and the planetary orbits should have been stable for billions of years.

I note that according to Hart et al in 1979, the outer limit of the Sun's habitable zone is only about 1.048 as far as the inner limit. So if the relative spacing between the Kepler 36 planets is the closest physically possible, it would not be possible to have two consecutive planetary orbits within Hart's habitable zone around any star.

Habitable Planets for Man, Stephen H. Dole, 1964 has a discussion of plantary orbital spacing.


On pages 49 to 52 Dole discusses the "forbidden regions" around each planet's orbit, where the gravity of that planet will destabilze the orbits of any other astronomical bodies. According to Dole the size of a planet's forbidden zone can be calcuated from The mass of the star, the mass of the planet, the semi-major axis of the planet's orbit, and the eccentricity of the planet's orbit. It seems that in our solar system the forbidden zones around planetary orbits occupy about half of the distance from the Sun to Neptune & PLuto.

The more massive a planet is, the larger its forbidden zone will be, the less massive a planet, the smaller its forbidden zone will be.

Dole estimates the mass range of planets habitable for humans on pages 51 to 58.

Estimates of the mass range for the more general question of habitability for lifeforms that use liquid surface water are in this paragraph from this article:


A minimum mass of an exomoon is required to drive a magnetic shield on a billion-year timescale (Ms ≳ 0.1M⊕, Tachinami et al. 2011); to sustain a substantial, long-lived atmosphere (Ms ≳ 0.12M⊕, Williams et al. 1997; Kaltenegger 2000); and to drive tectonic activity (Ms ≳ 0.23M⊕, Williams et al. 1997), which is necessary to maintain plate tectonics and to support the carbon-silicate cycle. Weak internal dynamos have been detected in Mercury and Ganymede (Kivelson et al. 1996; Gurnett et al. 1996), suggesting that satellite masses > 0.25M⊕ will be adequate for considerations of exomoon habitability. This lower limit, however, is not a fixed number. Further sources of energy – such as radiogenic and tidal heating, and the effect of a moon’s composition and structure – can alter our limit in either direction. An upper mass limit is given by the fact that increasing mass leads to high pressures in the moon’s interior, which will increase the mantle viscosity and depress heat transfer throughout the mantle as well as in the core. Above a critical mass, the dynamo is strongly suppressed and becomes too weak to generate a magnetic field or sustain plate tectonics. This maximum mass can be placed around 2M⊕ (Gaidos et al. 2010; Noack & Breuer 2011; Stamenković et al. 2011). Summing up these conditions, we expect approximately Earth-mass moons to be habitable, and these objects could be detectable with the newly started Hunt for Exomoons with Kepler (HEK) project (Kipping et al. 2012).

So if Dole's formula is still considered to accurate, a writer could design an imaginary star system with the masses and orbits of the various bodies and then calculate whether the planets in the habitable zone were within each other's forbidden zones.

But as I said, there is still a lot of uncertainty about the limits of the Sun's circumstellar habitable zone.

Part Five: Designs for Star Systems With Many Habitable Planets.

Wouldn't it be great if there was site by a professional scientist devoted to designing imaginary star systems with as many habitable planets in the habitable zone as possible? Thaht would be great for science fiction writers who want more than one habitable planet in their star systems.

Well, Sean Raymond's blog PlanetPlanet does have a section for designing imaginary systems with as many habitable planets as possible.


So maybe you should find a system there which has as many habitable planets you think that you need for your story, and then try to some expert in space travel to decide what methods would be necessary to go from one planet to another in that system in a year, or in a month, or in a week, or in a day, and describe to you the methods necessary for those varying speeds and travel times.


Even though this is mentioned, I don't think it is detailed well enough.

Space elevators and orbital stations. Space elevators have the advantage of carrying cargo without problematic rockets. In some designs you could power the entire thing using solar panels at the other end. Best part, if it is long enough, you only let go of the elevator. You don't need extra trust to get where you want. For instance, with a space elevator of 55000km you could easily get to Moon, L1 and L2 points without any propulsion. Once at Lagrange points, a spacecraft could use high impulse drives to navigate to its destination with minimal fuel.

You might also make the elevator much longer. a 150000km elevator would be enough to send materials out of the solar system if gravity assist is used. Length of the elevator may increase the cost but the type of the material needed to construct it stays the same. We are a few decades from constructing a space elevator. We could currently build space elevators on the Moon or on Mars.

Economics of this whole thing will still be expensive. Current cost estimates are about 200 USD per kg to low earth orbit. 16000 USD just to LEO per person is not a number that everyone will be able to afford regularly. After LEO elevator will keep ascending by itself, but if you are to travel to say Mars there will be additional cost of spacecraft. Still, you don't even need to be a millionaire to get to other planets. Also there will be cheaper launches when the demand is low. I think it will be a good estimate that a person could travel to Mars using Economy class ticket about 20k$.

However, travel times will be much longer than what you have asked. It might be possible to speed these up, but at a cost. Current designs seem to be running about 300km/h. At these speeds, it will take a week or more to get to the release point. After which you will travel slowly towards your target which could take months if going for Mars. However, these speeds could be modified easily. Since after 100km or so, the air will be thin enough, you could attain any speed as long as friction between cable and elevator is managed. Also travelling faster means deceleration will be needed on the other end. Meaning even more cost. So realistically speaking, even at the increased cost, Mars will be taking a few months.

To sum it up, with space elevator, it will be possible to get nearby planets at a manageable cost. It will take time but nothing impossibly long. If the demans is high enough, large space craft departing about once a year from Earth to Mars and back could carry many passengers with living and working spaces that will allow these individual to contiue to function during the voyage. This type of ship will never land, it will hop between stations at the elevators, letting elevators to transfer people and cargo. Finally trade between planets will be possible as long as a kg of the material will worth more than several hundred dollars. This include rare minerals and metals as well as microchips. Best of them all, you could use this system as early as 2050 and it will be a plausible scenario.



Fission or fusion highways involve placing a line of fusion fuel pellets (frozen DT ice) or fissile material in long 'highways'. This can be done by solar sail linelayers or other passive propulsion means. By impacting these pellets at extremely high speeds, you can generate immense thrust without having to actually carry any onboard engines freeing up room for other things. Using this method, you can reach Saturn in only 7 days assuming extreme optimism, and space travel becomes ridiculously easy if the infrastructure is in place.

Read up further here... (math and much more in-depth science beyond my short summary)



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