"Particularly at around the age of 70 you reach a stage where you have to be very careful. If, at that point, you abandon the work you have been doing, there is a good chance that you will just collapse and drift."

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But space travel can't ease the pressure on a planet grown too crowded not even with today's ships and probably not with any future ships-because stupid people won't leave the slopes of their home volcano even when it starts to smoke and rumble. What space travel does do is drain off the best brains: those smart enough to see a catastrophe before it happens, and with the guts to pay the price-abandon home, wealth, friends, relatives, everything-and go. That's a tiny fraction of one percent. But that's enough.

— Robert A. Heinlein