Since the concepts have been proven, though, getting them to function properly is a matter of engineering, not basic science.
There would appear to be no reason to overcome the engineering challenges until an actually human-habitable exoplanet is discovered, however - even Gliese 581d would have you massing 2.5x your proper Earth weight, in an atmosphere consisting largely of carbon dioxide, orbiting a K-class star (markedly dimmer than our own Sun) on the outer edge of its "Goldilocks zone". Why spend billions (at minimum) figuring out how to cross 20 lightyears of space to get to that? And the other candidates thus far are even worse.
If we want to occupy worlds not terribly habitable for humans, we have plenty of those right here in our own solar system, and we know pretty well how to get to them (Mars, for instance, will become much more useful once the company building the VASIMR coupled-charge plasma thruster manages to get up to about 200 kW output, which could be used for a constant-boost manned ship thrusting at about .001g or so - Earth orbit to Mars orbit in 39 days!).
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Sodium is a metal that reacts explosively when exposed to water. Chlorine is a gas that'll kill you dead in moments. Together they make my fries taste good.