Certainly good points.. of course, they may see asteroid belts as more fertile ground for 'terraforming' (astroforming?)'. Plus engineering a small asteroid to be habitable is doable on a smaller timescale than a whole planet, giving far more living space for fewer resources.

@AndrewDodds And when you're a space dweller you won't want to waste resources fighting a planet's gravity well just to build more homes, not with so many less problematic rocks out there. If the OP could come up with a reason why living on planets are required to expand maybe this would work better. But I can't think of a plausible reason to need to do that myself.

There is one thing a habitable planet has that even the best ship can never supply: It's safe to live there. A planet doesn't typically break. It cannot experience fatal hull breaches, or catastrophic reactor breakdowns that endanger everybody. Plus, a planet is huge, providing a lot of place to not meet other people. But, i must admit, your answer is really good.

@Burki You, the happy planet dweller think of planets as safe. But a space dweller would say, we've been in space for hundreds of years with no earthquakes, storms, mudslides, floods, etc. and they'd regard safe as where they were.

Dan Simmons covers this quite well in the last two books of the Hyperion Saga.

@StephenG that still leaves that thing about places without other people. But i guess you might be right, at least for the vast majority of people aboard the ship.

I couldn't see if it was mentioned anywhere but this answer leads to the possibility of having space cities, which could either travel or dock by a planet with tonnes of resources. + Excellent answer

'people who travel anywhere they want in space' -- More like travel anywhere the captain wants to go. Or the ancestors. Or whoever. But there is certainly no individual freedom aboard a generational ship beyond the confines of that same ship. On the planet you can roam freely on a much larger surface. It certainly would have some appeal to a lot of people.

So you assume, Earth-bound humans can have offsprings considering space as their home, but you reject the idea that these space-bound humans can have offsprings considering a planet as their home?

Just like you have people on this planet that would go on a generation ship, you will find people on a generation ship that want to start a planet settlement adventure.

@Holger I think I mentioned briefly that such people were possible, but they'd be a very small minority (IMO) and rememebr that the ship has a relatively small population. So (say) 1% of a billion on Earth is a huge number of people available for a colony ship, but 1% of e.g. 50,000 people is just 500, which is really small for a viable settlement on a planet.

@M.Herzkamp Consider the size of such a possible settlement (see my comment above) and that while they might want to roam far and wide on a planet, the small settlement simply won't have the resources or people to spare (or risk) on such adventure.

The first thing we need to know, is whether the biosphere of a generation ship as small as needed for hosting 50,000 humans can truly serve for an infinite time. Perhaps, it’s predetermined to be stable enough to serve for n generations, but when reaching the target planet, terraforming the planet to build a planet-scale bioshpere before refreshing the ship’s is mandatory. This maintenance may take several generations before the ship can start again. At this point of time, the planet may have a much larger population as well as new native people who do not want to leave…

I think the biggest reason to move on-planet would be to get out from under the draconian family planning policies that must be in effect on the generation ship. Think about the social consequences of hitting the maximum 50k people and allowing no more. If someone gets pregnant would you rather (1) terminate the pregnancy or (2) terminate the most elderly person? Or is everyone kept sterile until they are given permission? Hooray, it's my 50th birthday today and I was just granted the right to have my first child!

Where do you get, "The people you have described have, for hundreds of generations, been a space dwelling people" from? These vessels are traveling at half the speed of light, so your voyages could easily be only a couple generations in length, or even within one.

+1. This has always been my point. Can you imagine being a spacer and walking on even the most Earthlike planet? I'd be looking up at the sky and open space without bulkheads and, in the back of my mind, I'd be thinking: but what if it springs a leak?

@ShadoCat If I am not mistaken, that was a thing in en.wikipedia.org/wiki/The_Caves_of_Steel, except it was backwards: the "spacers" were OK with either but people on Earth were afraid of open sky from living underground for so long.

@MadPhysicist, Maybe. I know I read it somewhere and it stuck, buzzing, to the flypaper that is my mind.

aka Space Amish

The children of people who live in cities often want to live in the country. Many of the children of people who grew up in Europe moved to America. Etc. Just because you find a certain lifestyle preferable doesn't mean that all human beings find it preferable, or that only a tiny number of malcontents and eccentrics will want something else. In this case, as no one has ever built such a ship, we can only speculate what life on it would be like. It would almost surely be cramped and crowded. Perhaps limited variety of food. Etc.

Living on a generation ship would NOT be an adventure. The whole idea of a generation ship is that most people live and die on the ship without ever reaching their destination. Some people find travel exciting because they get to see new and different places and people. But on a generation ship, you never see anything new. You just see the same four walls constantly. Your great-great-grandchildren may get to see something new. I think the excitement of such a trip would evaporate pretty quickly.

@sirjonsnow These vessels are traveling at half the speed of light Where do you get that from ? Have you any idea how much energy is required to get to half the speed to light ? And, much more importantly, how much is required to slow down again at the destination ? The whole point of a generation ship is to avoid these problems. At 0.5c you gain 15% of your rest mass in kinetic energy - that's vast energy which we know of no way to generate on a ship. See the answer by Murphy for a discussion of this and why such high speeds are impractical. And the Q discusses "slow boat".

I don't know why this answer is getting upvotes, is nothing more than a crude straw man argument. The question was "Can people colonise the universe on a vessel travelling less than the speed of light", to which @StephenG replied, "No because the people on board would not want to get off." So assuming the people on board would want to get off, the question still stands, and this answer is entirely void.

@LukeVanIn So assuming the people on board would want to get off Which is precisely that - an assumption ! I am simply arguing that this is not necessarily a valid assumption. And as getting off the ship is rather vital to colonizing a planet (which is specifically what the OP discussed), my point is not only direct it is not a straw man argument. I am actually saying they would be more likely to be colonizing the galaxy without planetary colonization, but that doesn't change the idea that I feel it's unlikely sufficient numbers would wish to transfer from "home" (the ship) to a planet.

@StephenG You are offering you own opinions about whether the travellers would want to be become colonists, while the entire premise of the question is that they would want to, and has nothing to do with their intentions. If the question had asked "...would people want to colonise other planets..." then your answer may have some relevance.

@LukeVanIn Your interpretation of the question is extremely narrow. The only question asked was "Is this a realistic way to colonize and explore the universe? " And I answered to explain why I consider it unrealistic outside of the very narrow view you take.