Without a major breakthrough (wormholes and farcasters are still purely stuff of SF) we can forget about travel to other systems which are not the really closer ones. I think the Galileo project did not detect habitable exoplanets in them, although it may have been wrong.
nevertheless, presently even the closer system at 4 LY distance may pose unsurpassable problems. This is an excerpt from some material on Astronomy from the University o fTexas.
Basic problems:
1. The speed of light “c” is an upper limit to speeds in the universe, and so
we have no hope of traveling faster than light (as far as we know). And it is
well-established that in order to approach a significant fraction (say 80%) of
the speed of light requires stupendous amounts of energy, more than anyone
has so far imagined from any kind of propulsion system. Even the amount
of energy it takes to approach even a small fraction of the speed of light (say
0.1 c) is enormous.
The energy required to accelerate to a certain speed doesn’t depend on
what kind of fuel you use (see discussion on pp. 300-302), but it does
demand that the mass of fuel you carry will have to be enormous—the mass
needed does depend on the kind of fuel you use because different fuels have
different mass-energy conversion efficiencies (see below).
2. There are designs that could in principle travel at 5-10% of the speed of
light (as discussed in text and mentioned below; most of these are just out of
reach of present technology). But stars are many light years apart, so even a
one-way voyage to the nearest stars would require decades or centuries or
more at a speed of 0.1 c. This will require a multigenerational crew, or
suspended animation, or a very large increase in human lifespan to reach
even the nearest stars. There are severe problems with each of these.
You may notice that the time problem could in principle be solved by
robotic probes. (This assumes that those on Earth would be willing to wait
the centuries required until the robots report home.) One interesting variant
of these are called “von Neumann probes” –they are basically starships that
make copies of themselves and launch the copies, which then replicate again
and launch further copies, etc. A good tactic to colonize the Galaxy! This is
not discussed until chapter 13 of your textbook
Even if we'd be able to overcoem the above basic problems, space travel at relativistic, close-to-light velocities still poses more problems, a simple one outlined here.
http://www.newstatesman.com/sci-tech/2015/04/near-light-speed-travel-increasingly-impossible-according-mathsBottomline: leaving our solar system presently appears to be totally unrealistic.
Right now, the only realistic option appears to be the colonization of Mars, with its terraforming as a target. The moon might be used for some resources, although it would probably be wated time and money.
