First part here.
Second part here.
Third part here.
There's also another reason why it doesn't matter whether we destroy our solar system or not: it'll be utterly annihilated anyway, so every harm we will do in the process of expanding will be erased from existence in the long haul. Considering this fact and claiming that we need to think now about strategies to cope with it could sound awkward but, again, it's just a matter of facing reality and planning ahead. A matter of perspective. We are tiny, our species is not: it transcends us, it'll go on without us. What's wrong with taking necessary actions to prevent a disaster that surely, not probably, will happen in the future to our descendants? In this regard, I think that our short attention span is a critical problem and we should tackle it with all our strength, the sooner the better. Our lives are short and, having always been unable to effectively control our population, often brutal and miserable. Only a little percentage of individuals in the whole history of humankind has been able to engage in something more intellectually challenging than mere survival. The number of such people has never been so high, but this is because we are nearly 7 billion (and we spend nearly all our free time on FaceBook, but that's another story). The percentage is still low. They are still only a tiny fraction of the living. We need to prolong everyone's lifespan considerably, keeping growth in check in the meantime, if wte want to have a chance to grant the survival of the species.
Because Sol is dying, like any other star. It's happening in this very moment. Given sufficient time every single, fierce atomic furnace in the galaxy will become a dense pebble a few kilometers wide or a black hole, depending on its mass, and will be eventually swallowed whole by the swirling monster at the center of the Milky Way. It's an imperceptible process, but we know for sure that in 5 billion years Sol will change dramatically: the hydrogen feeding its fusion process will have been nearly all converted to helium. The energy generated by hydrogen fusion, pushing Sol's surface outward, will drop low enough to succumb to the strong gravitational pull of the Sun's mass. The nucleus of our star will rapidly shrink. The shrinking will concentrate helium and generate friction and heat, until the reaction is ignited again by sheer pressure. Helium will transform into carbon and Sol will become a red giant with a burst of expansion. It'll swallow Mercury and Venus, vaporizing them. Maybe Earth will not be completely destroyed, but its oceans will evaporate and its atmosphere blasted away by intense heat. If we don't get out of here that will be the end not only of humanity, but of all life on Earth. We know it'll happen. Why aren't we studying a way to escape?
And where will we go from there?
There are, once more, many choices.
The nearest star to Sol is Proxima Centauri. It's a red dwarf about 4.2 light-years from here, in the constellation of Centaurus, and it's part of a system with two other, much more massive stars, Alpha Centauri A and B. We still don't know for sure if there are planets orbiting this complex triple system, but we surely should take a closer look. Other suitable targets for interstellar travel are, in order of distance from the Sun, Barnard's Star, Sirius, Epsilon Eridani, Tau Ceti and Gliese 581. The latter is particularly promising because it has planets in the so-called “habitable zone”, at the right distance from the star to receive roughly the same amount of energy Earth receives from Sol. Liquid water could be almost certainly available there.
The problem is that even the shortest travel would require 4.2 years at the speed of light (from now on I'll refer to it using “c”). To give you an idea of the distances I'm talking of, the unmanned probe Voyager 1 – launched on September 5, 1977 – is about 17 billion kilometers away from Sol, that is... only 0.002 of a light-year. Not so far, uh? Well, it's the most distant (and still active!) man-made object. In four to six years from now Voyager 1 is expected to cross beyond the helioshere, the outer layer of the bubble around our solar system that is composed of ionized atoms streaming outward from our sun. Once beyond our heliosphere, Voyager will venture into the interstellar medium, which fills the space between stars, thus becoming humankind's first interstellar spaceship. It will come in proximity of the first star on its path (missing it “only” by 1.6 light-years) in 38,000 years.
You surely realize that the engineering challenge is staggering. We just can't imagine now a feasible way to travel fast enough to make similar voyages practical. There are theoretical workarounds such as the ones often used in science fiction novels, like wormholes or the Alcubierre drive (that is maybe more feasible than the former and could reach velocities much greater than c), but building a ship able to reach and maintain such speed is beyond our actual capabilities.
Fortunately we can do with only a fraction of that speed, if we get smart and plan our voyage right.
Using only available technology, a ship equipped with nuclear pulse propulsion could reach a cruising speed of 8%-10% c (it was a serious project: this is BBC's "To Mars by A-bomb" documentary, or click here to take a quixk look to some working models of the spacecraft). The same stands for propulsion based on a magnetic sail powered by a massive laser residing in our home system: such a ship could potentially reach even greater speeds, because it would not need to carry its own reaction mass.
If we can get good enough in building closed-cycle life support systems and we find a way to prolong our lifespan and we develop a permaculture, we'll be able to build generation starships with a real chance of success. We will not see our new home among the stars, but our descendants will. Once again the problems are huge... but this is the very reason why we ought to start working on them now.
(to be continued... )