When I was a kid, I was a big supporter of manned space exploration. New frontier! Push the boundaries of the human race! Escape this limited little planet! Onward and upward! Give humans some bigger future to hope for, etc. etc....
But now I read articles like this one and find myself nodding:
Consider the enormity of an effort to send astronauts to Mars. When Mars is closest to Earth, the distance is still about 200 times that between Earth and the moon, which means it would take several months to reach Mars. The amount of food, water, oxygen and other basic supplies necessary for such a journey would require a far larger spacecraft than anything built yet. And it's by no means certain that humans could survive the trip.The difficulties of manned missions to Mars are only part of a larger truth: manned space exploration is now too expensive to justify economically. Orbital travel will be viable as tourism, but missions to the other planets - whether for science, or for some commercial venture like mining - can be done almost as well by robots, at a tiny fraction of the cost in money and lives.
The astronauts would be exposed to cosmic radiation and other dangers when in outer space or in the Mars environment for two years...
And the physical issues are enormous. Even with vigorous daily exercise, will an astronaut be able to walk on Earth after two years under no gravity? Will the astronaut's digestive system operate properly? What of the heart and other organs? What if there is a medical emergency? Finally, upon arriving on Mars, astronauts would find blood-freezing temperatures (more than 100 degrees below zero Fahrenheit at night, even at the equator) and a suffocating atmosphere of carbon dioxide and no air.
And the logistics are overwhelming, from the massive solar arrays that would be necessary to provide constant electric power to the challenges of resupply and refueling.
But let's consider the question: what would justify putting humans all over the Solar System? As I see it, it would be worth going if we could live there indefinitely in large numbers. Assuming we don't break Einstein's laws, and are thus stuck in this Solar System, being able to live offworld would require either:
A) the ability to terraform other worlds, or
B) the ability to adapt humans to be able to live indefinitely in zero-G.
(A) is going to take amazing new energy sources (i.e. fusion); for that matter, building the fast, reliable, safe spaceships to ferry us quickly back and forth between our newly terraformed colonies would take awesome energy too. I'm not talking about total energy (which, with solar power, is unlimited), I'm talking about energy density, i.e. the energy a spaceship can carry on it. The best thing we have right now is fission, which, even if we made it safe (by using thorium), involves taking a very limited resource from an overpopulated and energy-hungry planet. Until we get fusion or its equivalent, factories in China and farmers in the Midwest are going to be able to outbid space exploration a thousand times over.
(B) is an interesting possibility, since all it takes is really spiffy biotech. If we could design humans to live in zero-G, we could conquer space with only slightly better technology (mostly radiation shielding) than we have now. Of course, those humans wouldn't be quite the same as the rest of us, so it's not clear if people would support creating a race of spacemen if that left the rest of us stuck in this gravity well. But it's certainly a thought.
If we don't get at least one of those technologies, manned space exploration will have to wait until Earth is no longer resource-constrained - until the human race either improves its efficiency dramatically or decreases in number significantly, or both.
In the meantime, we should remember that explorers didn't cross the Atlantic in oared galleys. They waited until they had lateen sails, compartmentalized hulls, compasses, astrolabes, etc. You and I may not live to see the golden age of manned space exploration. We'll have to comfort ourselves with the thought that it will come...someday.