It was staring me in the face for years.
Ever since I began reading Atomic Rockets (among other websites) in an effort to educate myself and keep my SF from getting too mushy I kept running into a problem. Why would anyone go colonizing other planets in large numbers?
Judging from our solar system you get a single garden world and the rest of the real estate looks like shit. I'm waiting for more word on the exoplanet hunt but most of them will squash you flat and freeze or fry you at the same time. You could send colonists to the Antarctic or Marianas Trench for a minute fraction of the cost of sending them to Mars and the living conditions are far better (okay maybe the Trench is worse than Mars but Venus still trumps it!)
Don't give me any guff about Belters. Robots could probably do the job just as well. I'm talking an exodus. Not a bunch of engineers herding an army of mining bots.
At the same time most space opera settings have humanity using fusion power and in fact you pretty much need it to have space travel that looks as common and easy as it does. I'm not addressing anti-matter. Anti-matter is a fuel you have to manufacture and that'd take something like ... fusion power to do efficiently.
Fusion power, like space, is hard. We haven't got it yet. It's about five years away and has been for the last twenty years if you get my drift. Don't believe those games that say it's the next tech level. It might be another hundred years away for all we know or more.
There is a way to develop fusion power in the near future of your setting of choice and justify space travel. I propose fusion power generation is made possible using unobtainium. Unobtainium is a very dense element that winds up in the cores of most planets. TO get it you have to troll the asteroids and very volcanically active moons like Io (which turns itself out every hundred years or so.)
Every civilization that uses fun stuff like toasters and laser rifles will need fusion power. Fossil fuels supplies are just too limited. Fission is too dirty etc. People are going to go out to mine that unobtainium. They're going to use some of it for propulsion of course but they should be able to gather more than they expend.
Voila: space travel is made possible and lucrative to find the elements that make space travel possible.
Now I just have to figure out what the unobtainium is exactly. It might be some material containing muons or something we don't even expect. A muon heavy material might have a detrimental effect on computers making it necessary to use organics to mine. There's the ticket: exodus!