I believe the first step is the colonization of the Moon.
Only if Centaur Upper-Stage successfully hits the moon, and LunarCROSS can find water. If not NASA will have to work out other sites on the moon that may contain water.
It may still be decades after the event that normal, everyday citizens, are able to live on the moon
Virgin are already working on 'space tourism' for the everyday human.
due to its ability to be terraformed
Mars? Very unlikely. There was an asteroid that was found in Antartica (I believe it was called AH84001), which had dead single cell organisms on it, there is nearly no water, and it takes almost two years to reach there making food supplies sparse.
Scientists have also considered the possibility of terraforming Venus
What a ridiculous idea: the surface is made up of volcanoes, the clouds are made of battery acid, the planet is over 400 degrees Celsius. All the previous probes to Venus failed horribly:
Venera 3: Crushed
Venera 7: Lasted 23 minutes
Venera 13: Lasted 2 hours
All built and sent by the USSR, attempting for successful landing failed
horribly, NASA have not even attempted to land there with probes. How you expect natural terraforming is insane. Look at Venus's climate as intense global warming: Venus is quite literally beyond repair, and unless we get very good heat protection there is no way we will successfully land, let alone terraform.
I am not expert, but that is what I believe. Some of this may not be entirely true, I am dragging this from memory, so I do not want too much flame.
Anyway, now for the questions:
*Effect on humans, as a species?
Quite hard to foresee exactly what would happen. Unless terraforming does work in the near future it will be near impossible. If it does work then I believe the planets will be divided into countries, but only 1st world countries: the only people that
can get there. There will be many issues over whom will own the various countries that are split by their borders, but I certainly do not think they will put the entire planet under dictatorship or a selected government. As a race this will be an outstanding breakthrough and besides the wars that may be created will help our expansion and dominance.
As a person leaving this planet would be great. The Ancient Greeks dreamed of having swans fly them to the moon, and in the 1500s people would look to the stars and planets and wonder what it was like: finally we could see what our ancestors had always awed and watched; observed with great amazement.
*Will humans be able to achieve planetary expansion at all? If so, how far do you believe we will get?
I think we could get to Mars: we'll definitely get to the moon. Technology is ever evolving and soon humans might look back on these days and see us as utter
idiots!
After all, the human race should live for a few million years more. I think we will expand to the edge of our Solar System, but after that there is the Oort Cloud and the abyss of dark, lifeless space. The nearest star will be Alpha Centauri which are about to cataclysmically smash into one another, and any planets further than that are invisible at the moment.
Will an alliance be made between all planets? How likely is it that the human race grows apart and eventually disassociates with the others as evolution and time goes on?
Once many planets get colonized I think there will start being tyrannies for the planets. If we stay within the same solar system, then I do not think the humans will disassociate themselves, however if we do make it past the Oort cloud and into distant solar systems: yes it is very probable, especially with genetic mutation in development.
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I am no great expert on this subject, but that's what I believe. Space colonization is a good idea, but I think NASA are planning to go to Europa (one of the Galilean Moons), since fly-by missions have suggested there is the existence of water beneath the surface.
At the moment I have no theories about how we'll get into space colonization, because I cannot guess if the vital technologies required to get successful space colonization will be around, nor do I have enough expertise on this field of science.