A fun but thoroughly unrealistic read. A while ago, I tried my hand at predicting the future of space, mainly from the perspective of its population level (which currently stands at 6). The trick is that while new technologies may come and go, the actual expansion of humanity into space is going to be driven by a stepped series of economic concerns (not explicated in the scenario below, although I suppose it would be fun to do so) which will always go at a fairly deliberate pace. Space technology is not like internet technology: even the most radical breakthroughs can't be promulgated overnight, but require extensive periods of finance and R&D before they become a significant force. Anyhow, predicting the future is a fool's game, but I suspect that it might go something like this:

2010s : The ISS continues to operate with a crew of six. In the middle of the decade, it is joined by a small Chinese station, raising the average population of space to 8. By the end of the decade, a small Indian space station has been built, and one or two small private space stations are in orbit, and the average population of space has risen to 17. Throughout this time, suborbital tourism becomes increasingly commonplace. Although this has a very significant effect upon the total number of people who have been to space, it has little impact on the average population of space. Meanwhile, the world population passes 7 billion people.

2020s : National and commercial space stations continue to be launched and expanded. By the end of the decade, the average population of space has risen to 40. A handful of these are workers in governmental lunar outposts. Meanwhile, the world population passes 8 billion people.

2030s : Continuing incremental expansion of national and commercial space habitats. This decade sees the first tentative missions beyond cislunar space. By the end of the decade, the average population of space has reached 80 people. A few of these are wealthy retirees who now consider themselves to be permanent residents, although they are still completely dependent upon Earth for material resources.

2040s : Continuing habitat growth in cislunar space, including growing numbers of "settlers". Further expeditions to asteroids and Mars, although no permanent habitats yet. By the end of the decade, the population of space has passed 150 people. Meanwhile, the population of earth passes 9 billion people.

2054 : The world population peaks at just over 9.2 billion. The same year, the first baby is born in space. The total population of space reaches 215 people, about 1/3rd of whom are permanent residents. This means that 0.00000078% of the total human population lives in space.

2061 : 100 years after Yuri Gagarin's spaceflight, 340 people are living and working in space at any one moment, including 120 permanent residents, 8 of whom were born there. The first continuously-crewed habitats have now been built on Mars, home to a few dozen people.

2077 : The population of space passes 1,000 people, including 500 permanent residents. Meanwhile,the rapidly-aging population of Earth has fallen to 8.5 billion.

2100 : The population of space passes 3,500 people, including more than 2,000 permanent residents. About 20% of the residents are native-born. Meanwhile,the population of Earth has fallen to 7.6 billion.

2119 : The population of space passes 10,000 people. Almost 90% live in cislunar colonies, but over 1,000 now reside in habitats on Mars. Owing to advances in micro-manufacturing, these settlements could be nominally self-sufficient, but are in fact are largely dependent upon Earth.

2161 : 200 years after Gagarin, almost 100,000 people live in space. Over 15,000 live beyond the cislunar system, in settlements which are increasingly driven by internal economic activity rather than trade with Earth. Emigration from Earth has increased substantially, with over 4,000 people making the one-way journey every year.

2189 : Net emigration from Earth peaks at almost 20,000 people per year. The population of Earth has now fallen below 4.9 billion, and easing environmental / economic pressures are making it a more attractive place to live. Some reverse immigration from space is now also occurring, although in very small numbers.

2213 : The population of space passes 1 million people, approximately 23% of whom are native-born. 75% of space residents are part of the cislunar socio-economic sphere, but a number of fully self-sufficient societies are now established on Mars and certain asteroids. Meanwhile, the population of Earth has fallen to 4.3 billion.

2230 : For the first time, native births in space surpass net migration from Earth.

2247 : The population of space passes 2 million people.

2310 : The population of space passes 4.5 million people. Native-born spacers now out-number immigrant Terrans.

2382 : The population of space passes 10 million people. Settlements have been established in the Jovian system, engaged in mining the He3 fuel which now powers the solar system's economy.

2461 : 500 years after Gagarin, more than 22 million people call space home. Many of these have never seen Earth's orb with their own eyes. Meanwhile, the population of Earth has reached a steady-state condition at just under 2 billion people.

2612 : The population of space passes 100 million people. More than half of these live beyond the cisulunar sphere. Much of the socioeconomic activity in space is now entirely independent of Earth. Immigration from space to Earth is becoming more tightly regulated, to ensure net zero population growth.

2846 : The populaton of space passes 1 billion people. The inner solar system still comprises the vast majority of the non-terran population, however hundreds of new worlds have been established in the Kuiper Belt, with populations ranging from a few thousand to a million apiece. These artificial worlds are made from the vast hollowed-out interiors of nanotech-fortified, spun-up Kuiper Belt Objects, with artificial suns floating at their centres.

2911 : There are now more people living in space than on Earth.

2961 : 1,000 years after Gagarin, 62% of the human population lives off-Earth. Some of the self-sufficient trans-Neptunian worlds have decided to become generation ships, and are accelerating at a low but continuous thrust through the Oört cloud on their way to other solar systems.

3047 : The total population of humans and human-derived species once again reaches its 21st-century peak of 9.2 billion. This time, more than 70% of that population lives in space.

3313 : The population of space reaches approximately 100 billion people. More than half live beyond the orbit of Neptune. A few hundred million live on worlds which have passed the halfway points and are now decelerating towards other suns. Less than 2% of the population lives on earth.

"Our remote descendants, safely arrayed on many worlds through
the solar system and beyond, will be unified by their common
heritage, by their regard for their home planet, and by the
knowledge that whatever other life there may be, the only
humans in all the universe come from Earth.

"They will gaze up, and strain to find the blue dot in their
skies. They will marvel at how vulnerable the repository of
raw potential once was. How perilous, our infancy. How humble,
our beginnings. How many rivers we had to cross before we
found our way."

-- Carl Sagan