YARN
Fleet Captain
The very expensive Biosphere 2 project failed. That is partly due to design flaws ignored in the interests of financial economy. Even the cheap version was horribly expensive. The lesson thus far is, we don't know how to build a self-sustaining habitat. A generation starship would qualify as such, and any crew and descendants would be sentenced to slow death. Any such habitat or ship is in the same category as a cheap method of interplanetary travel: Scientific speculation, good for science fiction but no practical use.
If people can't survive an interstellar journey, could frozen embryos? Currently we don't have enough skill to make machinery that could reliably function through several centuries of extreme cold and radiation exposure. Nor do we have the slightest idea how to program robots and computers to raise children, even if we had artificial wombs, which we don't.
Interstellar travel would only be feasible for potentially immortal artificial intelligences, when such things are devised. Whether such strange creatures would want to go to other stars is impossible to predict, however. People with indefinite lifespans could aspire to interstellar travel I suppose but they could develop serious cabin fever along the way.
Blue sky speculations about an end run around special relativity are fun, and good enough for scifi, but they aren't to be taken seriously. Interplanetary travel is feasible but financially impractical. Notions about using other planets and comets and asteroids for resources usually involve some far-fetched assumptions about the ability to gather and process miscellaneous stuff without cost and with the very convenient ability to make pretty much anything you want out of the stuff at hand.
For example, a comet can, supposedly, be mined to produce greenhouses and grow food. Chlorophyll requires magnesium, however, and I don't think every comet can be expected to contain unlimited amounts of magnesium, even in the trace amounts needed.
Waste products from the manufacturing process seem to be ignored in these scenarios as well. In space it takes energy to throw stuff away but having junk floating around can be downright hazardous.
"Long-term" does not necessarily mean "interstellar," it just means that you are traveling for a long time.
We already have the technology for long-term space travel.
An ORION space craft traveling at a small fraction of the speed of light could make it to Pluto in a matter of days. ORION is technology that already exists -- you would just have to build the thing.
We have the technology to send humans on a tour of the entire solar system, if we so desired.
How long do you want them out there before you are willing to call it long-term?