The space travel is limited by some qualities of the host planets (size, mass, etc.).
Another limit is the inhabitants’ biology, since the acceleration is inseparable form rocket launch. It is hard to believe that if a planet’s features provides an opportunity for space travel by chemical propellants, then its dwellers necessarily strong enough to survive the start. Thus it seems to be possible, that some intelligent beings unable to leave their planet by rocket.
But it would be a mistake to overrate its importance. In the absence of chemical propellants, we can use atomic energy; if acceleration is unbearable for our body, then we can try to build a space elevator, etc.
Although it is not totally indifferent the date when a civilization can leave its planet, technology seems to be a panacea for this problem. Probably there are cultures who can leave the surface at an early time of their technological development, and there are others, to whom it is a more difficult problem, but this difficulty is surely not a solution for the Fermi paradox.
Similar statements can be done about living in space. A broad scale of living beings from bacteria to tortoises, dogs and human can survive a launch, but the majority of them would die quickly without the aid of technology. It is not a surprising, after all, evolution adapted them to survive in a totally different environment.
This raises some questions.
Whether an intelligent being can live in space as easy and naturally as in Central-Europe? Notice we neither can live on the ISS nor in Budapest for a longer time without the help of technology (including buildings, warm clothes, etc.). This can be interpreted as only a difference of degree, not as a substantial difference. All in all, humans are technology dependent beings, and we have a reason to believe that every intelligent race is formed by their evolutionary past and uses technology to survive harsh environments. So it is likely, that unless their original environment wasn’t the outer space, they would be adapted to it.
What follows form this?
The Strong Anthropic Principle states that our Universe is “fine-tuned” for our existence in a certain way. There are three possible interpretation of the fact, that our cosmic environment is hostile to us.
- · It is possible (although it seems to be improbable), that there is no a better solution (Universe) for an intelligent being. The “fine-tuning” perhaps takes into consideration not only our biology, but our technology, as well. In this case this Universe is not “fine-tuned” for us, but for us with our technological capabilities.
- · There is a superior being somewhere in the Universe, whose “natural environment” is the whole Universe, which is fine-tuned for him/her (it is theoretically possible, too, that we will be that superior being in the distant future, but it seems to be a far-fetched explanation).
- · Universe is not fine-tuned, since it is fundamentally not life- or intelligence friendly, as it turned out.
This interpretation is rather prosaic, but I vote for this.