Humans will reach the planets in this century; at least there is a rather good chance that we will, without ever needing to be explicit about Step Two. The inherent coolness of space travel, along with national vanity and parochial economic interests, has turned out to be sufficient for half a century. There is no inherent reason why this should not remain the case into or through the midfuture, as our steadily growing capabilities carry us outward.
What this highly plausible space future does not have room for, however, is most of our favorite space tropes. Last post I made a comparison of space to Antarctica. The popular literature of polar exploration is tiny, and most of what there is deals with the early days. (Amundsen and Shackleton are the two names I remember off of hand.) Real space travel may turn out very similar. People will work very hard and spend a great deal of money to see to it that dramatic adventures do not happen in space.
Nor does the human scale of the thing lend itself to space opera. In the early interplanetary era - and, in all likelihood, for a long time after - there may be hundreds of people in space, but probably not thousands and certainly not millions. There will be a space economy, but no economy in space: the ships will be transports, not liners, and certainly not tramp freighters. (Sob!)
For story purposes this is not what we want. We want a lot of people in space. We want the outposts to grow into bases, then towns, then cities, and of course we mostly want them to end up fighting space battles with each other. For this we need a justification.
At least in 'Murrican science fiction, the profit motive is enshrined as probably necessary and certainly sufficient reason to go into space on whatever scale is desired. This attitude is not just confined to the libertarian-minded; Evil Megacorps in Space are a variation on the same theme. (I am not sure how it is elsewhere. Clarke's space midfuture, at least in his earlier stories, seemed not unlike the 'realistic' vision I portrayed above.)
The most popular profit motive has been mining. This is only natural. Mining fits the broad Western trope, and it does take people to the most Godforsaken places.
You have to make a few friendly assumptions to get space mining for terrestrial use to pan out (so to speak). But the subtler problem is then what? Suppose we learned that the rings of Saturn are full of McGuffinite. There is not going to be a rush of would-be Belters heading out to be Ringers instead.
Instead there will be some very big consortium formed, or a handful of them, probably with more than cozy relationships with existing national or para-national space agencies. A very focused program will develop the technology to do one thing: Go to the rings of Saturn, extract McGuffinite, and bring the stuff back to Earth. This effort will not go anywhere or do anything else. (With a limited but potentially important exception I'll get to below.) It will involve the necessary minimum number of humans in space, especially Saturn space; from every operational perspective the optimum is zero.
And once in place, beyond Earth orbit the mining operation may scarcely interact with other space activity. Mining transports headed for the Rings do not stop off at Mars or Titan. The experience of developing countries is that resource extraction infrastructure is not very helpful. The rail line runs from a seaport to the mine, and even the seaport is chosen for access to the mine, not its potential as a general trade entrepot.
Resource extraction is an economic monoculture, and like other monocultures it does not support a rich ecosystem.
The most popular political McGuffinite, a great power arms race, has a rather similar problem. As earlier discussions here have shown, great power warfare scenarios offer little role for space cruisers in whatever form. Only for laser stars that may well be robotic, and kinetic killer buses that will certainly be.
A somewhat different matter is resource extraction in space for use in space, such as the popular lunar shipbuilding industry. This is not McGuffinite, because it is not a reason to go into space. It is something you do only when you are already in space, and in a big way.
And of course there are other complications. Building spacecraft requires an enormous industrial base, and every pipe wrench has to come up from Earth unless you set up a pipe wrench factory or at least a fab. The lower energy cost of orbit lift from the Moon can evaporate quickly when you consider all the front end and operating costs.
It will be a long time before we have production industries in space.
An exception could be propellant, because space travel uses so much of it, and it is fairly simple stuff. Once we are regularly going somewhere with accessible volatiles, there will be consideration of obtaining propellant from them. This is not as simple as it is often made to sound. For example, all the ice on Mars is no use to deep space craft unless you lift it to Mars orbit, a major spacelift operation even if you can do it with a one stage vehicle. And there is no space infrastructure on Mars but what we take there.
I would say that the early interplanetary era ends on the day that a ship makes a routine burn in Earth orbital space using propellant that did not come from Earth.
Propellant production differs from McGuffinite mining in one crucial respect: It is inherently tied to the rest of the space infrastructure. And space travel is no longer entirely geocentric; for the first time, some of what happens in space stays in space.
Still a long ways from the Solar Confederation versus the Planetary Union, but everything has to start somewhere.
If I were working out a future history with a serious illusion of plausibility, I would stay away from McGuffinite. It is an ancient, overused crutch, and not a convincing one. Given worlds enough and time (and both are available), exports to Earth may well arise, as consequences rather than cause of space exploration. These may be - almost certainly will be - entirely unexpected and counterintuitive.
To quote myself, from last year's 'A Solar System For This Century':
Someone will find out that burgundy grapes grown in a Martian greenhouse have a distinct flavor. Pretty soon they are shipping back little airline size bottles that sell for $500, with just enough for a toast, and 'robustly Martian' ends up being used to describe burgundies from lands where Charles the Bold once ruled.Unlike McGuffinite, Martian burgundy doesn't have to be globally profitable - paying back the cost of going to Mars in the first place, or even the cost of the transport system. It only needs to be locally and marginally profitable ('marginal' in the formal economic sense, not precarious). Those little bottles only need to pay for themselves, their contents, and the extra propellant to get them to Earth. Ivan Q(ing) Taxpayer already paid for the transport ships, though you'd never know it from the collected works of the wine industry council.
Multiply such serendipities and, gradually, the human space ecosystem grows more complex. Oenologists now have a place on Mars, bringing a body of specialized knowledge and also an outlook on life and civilization.
This sort of thing takes time, probably lots of it, because it cannot be planned, it can only evolve. It may not happen. Indeed it probably will not happen, even in a future of interplanetary travel, because space travel is inherently so difficult.
But it is the one most likely path to get you from Earth to space opera.
The imagined image of Cassini, as so often, comes from Atomic Rockets.