We can already do, and have done, a great deal in space. We have scouted all the major planets, landed on the Moon, Venus, Mars, and Titan, and dropped among the clouds of Jupiter. We have passed through the heliopause into interstellar space.
The International Space Station has shown that crews can live and work aboard a spacecraft for years, with no emergency requiring evacuation to Earth or urgent support from Earth.
This is the primary requirement for human interplanetary travel. At a fundamental level, add a drive bus and you are good to go. Nor is any really major handwave needed for a solar or nuclear electric drive capable of reaching Mars in 2-4 months. (If Mars leaves you cold, adjust for the destination of your choice. It will probably be colder.)
All you need to wave is a check for $200 billion or so, to pay for developing your vehicle and mission from conceptual design to flight testing and human spaceflight certification.
Do not expect to get there for much less than that. The Airbus A-380 and Boeing 787 Dreamliner - commercial products of private industry, working in a mature kindred technology - each cost some $15 billion to develop. Such projects simply require an enormous amount of costly engineering work and one-off fabrication.
SpaceX and Scaled Composites do not prove otherwise. They prove only that the ecosystem has a place for small, agile skunkworks that can underpay top talent to work on exciting projects. Even technologies like 3-D printing won't really change the equation, because initial space costs are mainly engineering costs, and engineering at the cutting edge remains a craft trade.
But for a trillion US dollars or equivalent, give or take, you could probably build yourself a decent start on the classical rocketpunk midfuture: a second generation station with spin hab; outposts on Luna and Mars with ships to serve them; a human mission to Jupiter; altogether up to a few hundred people living in space. Call it the Clarke-Kubrick vision, though it could equally well be called the Ley-Bonestall vision.
The specifics are all freely subject to change. Commenters have challenged such rocketpunk-era verities as an orbital station as transfer point, and of course there are debates about where we should actually go and in what sequence. But this infrastructure, or something comparable, is the trillion dollar admission ticket to everything else.
A trillion dollars is a lot of money. More precisely it is a staggering, awesome, incomprehensible amount of money. It might end up being more than we are willing to spend on space travel in this or any century. But it is not impossible money. It is comparable to NASA's cumulative budget from its beginning to the present day, and about twice the cost of another public transportation system of similar age, the Interstate Highway System.
I am very doubtful that the private sector can or will take us into deep space on its own. But the largest corporations have revenue and market capitalizations of a few hundred billion dollars, so - given persuasive enough reason to believe that it would be profitable - it is not utterly out of bounds to imagine a global commercial consortium raising a trillion dollars.
The time scale of space is really, in large part, the money scale of space. If space spending in the later 20th century had remained at Apollo levels we might well have had the Clarke-Kubrick vision on schedule in 2001. At the levels of space spending and resulting space progress that we have seen over the last 35 years it is about in line with what we might expect for 2101.
It could as easily be 2071, or 2171. (Or never.) Given a sufficient (hand)wave of great power muscle flexing it might be 2031. But I will use 2101 as my conservatively optimistic benchmark. This presumes that we continue going into space in the same rather muddled, low-keyed, but persisting way we have since those heady early years.
So. At the start of the 22nd century, or broadly comparable date of your choice, we have regular interplanetary travel, but still very little of it, and what there is is very expensive.
Production jetliners, as I've often mentioned, cost about $1 million per ton at the factory ramp. But commercial jets can be sold for that price because Boeing and Airbus expect to build several hundred of them, spreading out the development cost and permitting semi-mass production efficiencies.
The first generation of interplanetary ships will be handbuilt prototypes. The second generation will still be largely handbuilt, though modular construction will begin to allow limited production runs of standard hab pods and the like. So a ship capable of carrying 10-20 people on an interplanetary mission, with departure mass of 1000 tons, dry mass 500 tons, gross payload 200 tons, might cost $5 billion assembled on orbit and ready for loading.
Adjust ticket prices accordingly. Suppose that your ship can make 10 round trips to Mars in a design service life of 25 years, so charge each round trip $500 million up front. Add another $500 million for 500 tons of propellant lifted from Earth - don't expect launch cost under $1 million/ton at the modest traffic volume of the early interplanetary era. And don't expect to get it from anywhere else, not at this stage.
So (simplistically!) $1 billion for our ship to make one round trip to Mars. It carries 20 people in transport configuration, so that will be $50 million, please. For a first class ticket $100 million - not for the caviar and steaks but the chef and stewards.
I confess a personal weakness for Pullman class interplanetary travel. The Realistic [TM] space travel alternative of doing basic preventive maintenance on microgravity toilets for 200 million miles would get old even faster.
But a note of practical caution to my libertarian minded readers. A world with so much loose money sloshing around the economic elite that it can send a Pullman car full of billionaires to Mars every two years is also a world with thousands of Paris Hiltons. Never mind poverty and social injustice. At some point sheer annoyance will bring out the guillotines.
Ahem, back to the point.
What happens after the early interplanetary era, in the second century or two of space travel, is much more conjectural, even by the necessarily naive standards of this essay.
Personally I think that by far the most likely human space future, through the 22nd century and well beyond - in short, through the midfuture - is far more like Antarctica than Heinlein: a chain of scientific and technological outposts, gradually extending outward.
Space is remote, costly to reach, difficult to live and work in, implacably indifferent to human life, and filled with things that fascinate us.
It is probably not filled with McGuffinite.
The human Solar System may well belong to artists, not writers. A deep space effort like this provides all of the lovely images - dawn on Mars, gliding through the rings of Saturn, everything Chesley Bonestall imagined and more - but not many plot lines, and certainly not the favorites among this bloodthirsty audience.
Which might be a feature, not a bug: a Solar System touched more by our aspirations than our failings.
That is probably not what you want, but I will save other possibilities for another post.
The image of an ESA concept for Mars exploration comes from a Romanian space website.