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In a universe with no faster-than-light travel, the Solar System has largely been settled, and has been for at least 200 years.

I envision populations to look something like this:

Earth: 8 billion;
Moon and Earth orbit: 2 billion;
Mars: 2 billion;
Asteroid belt: A couple million, mostly blue collar workers.;
Jovian moons: 2 billion, mostly spread over the Galilean moons;
Saturnian moons: 2 billion;
Outer planets: A half billion, mostly weirdos.

I'm willing to hand-wave gravity differences (I'm not super interested in spinning spaceships, so artificial gravity) and terraforming processes. For the larger moons, we develop an easy process for refining oxygen from the gas giants and the residents out there have implants, etc. to allow them to breathe.

Basically, Earthmen, Mooners, Martians and Gassers can easily live on each other's celestial bodies.

However, I want a journey to Titan to take around 9 months. How can such a long travel time be compatible with settlement and movement on the scale of billions?

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    $\begingroup$ Bigger issue is the "why". Earth has amenities such as air, gravity, protection from lethal radiation, easily accessible water and other volatiles. It would take a very good reason for billions of people to live in space. The travel time is not really relevant in comparison to that. $\endgroup$ – Ville Niemi Nov 21 '15 at 2:56
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    $\begingroup$ I would be much more concerned with why the travel times are so short. $\endgroup$ – Quiquȅ Nov 22 '15 at 1:59
  • $\begingroup$ @VilleNiemi I'm considering legal (population caps) and religious reasons. $\endgroup$ – Azor Ahai Nov 24 '15 at 1:01
  • $\begingroup$ The travel times are short or at least way more realistic than in most science fiction. So I wouldn't worry too much about making a justification for them. And they're not too long for settling, not at all. Obviously even in any case most people would just live their full lives in one spot and not dart around the solar system all that much. $\endgroup$ – Lassi Kinnunen Jan 8 at 8:54
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How can such a long travel time be compatible with settlement and movement on the scale of billions?

So far most of the answers have tackled the question from the direction of "how can I justify long travel times?" I'll approach from an alternate angle: "given that travel times are long, how can I justify extensive settlement?"

(As a matter of fact, I would try to justify why the travel times are so short, given that real-world trajectories take 6 years or more to get to Saturn!)

I would probably analogize to the colonization of the New World (the Americas). In the heyday of sailing ships, it took on the order of two months to sail across the Atlantic. However, many people made the (dangerous) journey; why? What were their motivations? Simplistically, we can break it down into a couple of categories:

  • Escape: from war or persecution.
  • Land: the one thing they don't make any more of. If you run out of space, you need to expand somewhere.
  • Money: for workers, this means new sources of employment. For entrepreneurs, this means new sources of profit (including natural resources).

I think that these can be readily adjusted to suit your situation. For example:

  • The Moon: a (relatively) convenient place for Earth-dwellers to expand to; a good source of raw materials; an excellent location for providing launch services to the rest of the solar system.
  • Mars: probably the planet most amenable to terraforming.
  • Asteroid belt: another excellent source of raw materials.
  • Outer planets: you get the idea; take your pick.

As for the large populations, remember that they don't all need to travel there. It's likely that after a couple hundred years, most inhabitants of the solar system will have been born on the planet they live on, and you may be able to justify very fast population growth.

If that's still not enough, extended lifespans or stasis/cryosleep technology will make the long journey seem shorter.

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  • $\begingroup$ Hohmann orbit transfer to Saturn takes 6+ years. Trajectories using gravity assists take many years longer. Minimum energy (Interplanetary Transportation Network) trips take many decades. A 9 month trip time to Saturn implies a brachistochrone (constant acceleration aka fast trajectory). $\endgroup$ – Jim2B Nov 21 '15 at 6:45
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    $\begingroup$ @Jim2B thanks, I made a serious arithmetic error; should be corrected now. $\endgroup$ – 2012rcampion Nov 21 '15 at 6:51
  • $\begingroup$ Thanks for your answer - but since I find it hard to justify ~1 year travel times, 6 years seems outside of the realm of possibility for my setting. $\endgroup$ – Azor Ahai Nov 24 '15 at 0:58
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For short travel times like this you could use very refined solar sails (theoretically an advanced sail could go from the Sun to Pluto in as little as 3 years) or massively powerful fusion drives. Either way you are now looking at extinction level events moving through the Solar System, since the kinetic energy of ships moving at such speeds makes nuclear weapons somewhat trivial.

This would make people on the terraformed worlds very nervous, since accidents that cripple the drive or navigation system mean there is a possibility of something arriving on a collision course with gigatonnes of energy.

Based on your conditions, I could assume that initial settlement was by more conventional ships taking years to arrive (still moving very rapidly in terms of kinetic energy, but "only" in the kilotonne to megatonne range), so settlers have been aware of the potential danger since "day one" 200 years ago. As technology is refined and higher velocities become possible, I suspect that every inhabited body in the Solar System will take effective steps to prevent or minimize the possibility of high energy impacts. Essentially, there would be a solar system wide "ATC" to control the arrival and departues of spacecraft (no "tramp" ships anywhere, ever), and a powerful "Space Guard" or even Space Navy capable of vaporizing any potential threats to inhabited colonies.

Colonies themselves would be heavily built (initially to be sealed off from hostile environments and provide radiation protection), and I could see a very paranoid civilizations arising, with colonies being built from multiple small shelters which can independently survive should a nearby strike or other event cripple parts of the colony.

So we have billions of very paranoid and cautious people inhabiting the Solar System, looking grimly skyward as ships approach on high energy orbits, keeping their fingers hovering over the triggers of a planetary defense system, just in case....not exactly the environment for large scale peace and collaboration. The Solar System could accidentally become a very restrictive and tightly controlled place.

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  • $\begingroup$ Still, isn't the chance much higher that they would miss the planet? $\endgroup$ – Azor Ahai Nov 24 '15 at 0:52
  • $\begingroup$ Indeed, but given that you would have a truly Earth Shattering "Boom" if you were wrong, would you take that chance? As well, just because it missed you does not mean a rogue spaceship isn't dangerous to someone, somewhere. $\endgroup$ – Thucydides Nov 25 '15 at 2:08
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Travel is cheap because you are moving around mostly by means of skyhooks and catapaults (on airless bodies)--mass ratios are low so you don't have to throw away your rocket every trip. Since the boost period is very short you are seriously limited in maximum velocity because of the lack of human tolerance of acceleration. You also have to wait for launch windows and most flights are to the next planetary (or asteroid) orbit even if your final destination is farther away.

You can also get situations where the trip is very asymmetric--one end of the trip has a big catapault (wrap a linear motor clear around the moon and you can boost to anything from sun-grazer to an escape orbit) but at the destination you shed your speed in an aerobrake and have no monster catapault for the return voyage.

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They could easily have settled even with the long travel times. Humans are (usually) quite patient, especially if they have a excellent reason to get somewhere.

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In 150 years from the American Revolution to WWII, about 60 million people left Europe to head for other territories, in the Americas, South Africa, Australia, and elsewhere. This is by far the largest mass-migration in Earth's history, and it completely reshaped the planet.

This did not happen in the age of instantaneous travel. The sailing vessels which did most of the transportation at the beginning of the time period took 6-12 weeks to cross the Atlantic, Europe to America. Sailing times to South Africa or Australia were longer. Despite the time investment, many people still thought it was worthwhile to go, and many people I mean around 10% of the people that lived in Europe during that time, left.

The transportation times in your setting aren't that different from those in the Age of Sail. 9 months to Titan is comparable to the time it took to sail around the Cape of Good Hope from Britain to India. If you assume 10% of all the people on Earth over the next 200 years want to leave, that means we are looking at 1-3 billion emigrants, depending on how fast population growth on Earth is. That is plenty of people to populate a whole solar system.

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A few ideas:

All interplanetary transportation needs to go through a shady Spacing Guild like in Dune. You travel on their terms and their schedule, or not at all.

How bureaucratic are the governments? Does most of the solar system treat Jupiter like North Korea or Iran? Obtaining a travel visa could take months, or you might have to go through alternative channels like with a charitable organization that is on a tight budget.

Do all trips need to take nine months, or just the one in your story? Maybe the ship blew an engine and no one was willing or able to help, so the ship had to hobble along at a very slow speed the rest of the way.

Does it matter if only one of the parties (either the travelers or those who are waiting for them) experiences the nine-month wait? At that level of technology, you probably have the means to fly a ship fast enough for time dilation to become an issue.

Sickness/civil unrest/bad weather (Jupiter's Great Red Spot) force incoming ships to stay in a holding pattern for a long time.

The ship is really old and obsolete, and the passengers can't afford a better ride.

The ship is passing through enemy territory and has to hide behind a slow-moving asteroid or move at a snail's pace to avoid being detected.

That might help you get started...

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  • $\begingroup$ Thanks, but I was looking for a reason that this regularly undertaken journey would always be around 9 months. $\endgroup$ – Azor Ahai Nov 24 '15 at 0:53
  • $\begingroup$ Correct me if I'm wrong, but my travel time is roughly 0.0002c, that hardly seems fast enough for time dilation? $\endgroup$ – Azor Ahai Nov 24 '15 at 0:59
  • $\begingroup$ @PressTilty Having a constant travel time from Earth to Saturn is really unlikely. It should at least vary with +- 1 month. wolframalpha.com/input/?i=distance+to+saturn $\endgroup$ – Taemyr Nov 24 '15 at 8:39
  • $\begingroup$ @Taemyr I definitely realize that. $\endgroup$ – Azor Ahai Nov 24 '15 at 19:37
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Due to some handwavium, ships are slow but really cheap. So pretty much anyone who want to can travel, although life support for a journey that long won't be cheap. To address this, you might also invoke a really cheap cold sleep (hibernation) process.

With that said, you've got a real problem with habitat construction/terraforming. 2 billion on Mars suggests that something really bizarre happened to make the surface livable. The alternative, that 2 billion people live in artificial habitats, simply does not bear consideration. The expense would be fabulous.

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  • $\begingroup$ I was looking for the passengers to be awake during the journey. As for the artificial habitats, I'm willing to handwave those. The cost of New York City, if you built it all at once would also be fabulous. $\endgroup$ – Azor Ahai Nov 24 '15 at 0:54
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Well, I think your timeline is a bit off. I did a little rudimentary math and here's what I got. My assumption is that out of the 2 billion people traveling to the Saturn area, only 1 billion needed to be brought there. I'm open to comments on that one, since I did absolutely no research on it. There are 2400 months in 200 years. If one trip was made from Earth every month, that trip would have to be made by 416,666.7 people! Call it 416,667 to make it less gruesome. Obviously there are a lot of things not considered here, like the fact that you can't just leave Earth at any time you want to get to Saturn with today's technology without great cost. This is just to get an idea of how easy or how hard this would be. Clearly it's very hard. You would either need a very large ship or a large number of ships to do this. And don't forget this is zero time spent figuring out how to live on a moon of Saturn or constructing the habitat. It seems like it would be easier to either make the trips shorter, which I gather is not something you want to do because it's part of your plot, or to extend your timeline out. Other alternatives might be some found alien technology. Maybe some kind of teleporter that requires multiple hops and some recovery time or something like that.

In short, in a realistic universe, I don't think such large populations are compatible with lengthy travel times.

Hope this helps.

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    $\begingroup$ The way I read OP the 200 years number was not how long it took to settle the Solar system. It's how long the solar system have been settled. So your math is a bit off, there will be more time for the population to grow, so less people will have been brought from Earth. Also more time to bring the people. $\endgroup$ – Taemyr Nov 23 '15 at 8:44
  • $\begingroup$ @Taemyr is correct - and you don't factor in birthrates, in 20 years a population of 400,000 could easily double with metered breeding. $\endgroup$ – Azor Ahai Nov 24 '15 at 1:01
  • $\begingroup$ Oops. It does say that. My mistake. I did factor in 1 billion births, however. $\endgroup$ – ozone Nov 24 '15 at 1:53
  • $\begingroup$ Personally, I still don't think it's compatible with the longer travel times. I don't think the population would grow exponentially because, except for Earth none of the locations can easily support the large populations. You can't just give birth to a billion people. You have to build habitats, grow food and provide all the services people need. That would certainly slow the process. I think this would take tens if not hundreds of thousands of years. $\endgroup$ – ozone Nov 24 '15 at 1:58
  • $\begingroup$ @ozone Once you get local production facilities, and you will need to if you are to get the population into the millions, building hapitats and farms will be an exponential process. (It will grow either with the population, or more likely with the automated constuction facilities) $\endgroup$ – Taemyr Nov 24 '15 at 8:36

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