Present launching techniques are as extravagant as air travel would be if the plane had to be rebuilt after every flight. Spaceflight will only be affordable when it adopts the same techniques as supersonic aircraft. Tourist trips into orbit may then become routine. And wealthy adventurers may boldly go further. If Bill Gates seeks a challenge that won't make his later life seem an anticlimax, he could sponsor the first expedition to Mars.
Some people use the so-called 'insurance policy argument' to advocate a manned space programme.
The craters on the Moon's surface are records of these impacts. An impact on Earth - leaving a huge undersea crater near Chicxulub in the Gulf of Mexico, probably sealed the fate of the dinosaurs 65 million years ago. Indeed, it's higher than any other natural hazards that most Europeans or North Americans are exposed to. The ever-present risk from nature has been augmented since humankind entered the nuclear and biotechnological age. Whether on not humans spread beyond the Earth during the next millennium, we'll still want to know whether we are alone.