The energy costs of transferring a viable population to an as yet undiscovered new home, or of bringing new resources here, would be, literally, astronomical, and so prohibitive as to be inconceivable.
There is no need at all to assume the Earth will be devoid of resources next century, as P Evans asserts. Money would be far better spent tidying up the planet, controlling population, stopping pollution, ending the spiralling over-production of oil, eliminating hunger, disease and warfare.
Man has always needed something to reach out to, to expand his imagination. Space exploration will and should continue, simply "because it is there".
W K PIPER