Those who were not yet born or old enough to share in the wonder of the first moon landing can’t imagine the thrill.

Worldwide, 500 million people gathered around television sets to witness a human accomplishment that up to that time had been the stuff of science fiction. Generations of Americans grew up thinking the moon to be an unreachable destination.

And yet it was reached by the Apollo 11 crew 40 years ago Monday, then five more times by different Apollo crews. And it may be again when NASA rolls out its Constellation mission to return to the moon as a stepping-off point for manned missions to Mars and beyond.

But even as those plans go forward, the chorus that has echoed for decades since space exploration began cranks up again, asking: Is it worth the money? How do we justify spending billions to send people and machines into the sky when we can’t fix homelessness, disease and poverty?

With the federal budget deficit topping a trillion dollars, with Medicare and Social Security facing diminishing returns, with an economy still sputtering, with so many out of work or uninsured, how can anyone propose spending more on rocket ships?

It’s a valid question, and always has been. But the answer is just as valid: Because we are human. And as long as space remains a mystery unsolved, we must try to solve it.

Our fascination with space began in ancient times, beginning with ancient peoples who saw gods in the star clusters. For centuries, we have explored our Earth, reaching the far ends of the planet. There is more to be found here, but the vast ocean of space offers ever more answers to the riddles of our planet’s origins.

Yes, we have other priorities, human needs at home. Those have been around forever as well, and will remain. We still should address those, but surely we can do so while still pursing the scientific knowledge that may solve human concerns in the long run. Americans can perform multiple tasks at the same time; it does not have to be just one thing or the other.

The planned mission to the moon and Mars will take mankind on a similar journey to what some witnessed in the 1960s, when we first ventured out past earth’s gravity. What we find on Mars, for instance, could answer key questions of our own planet’s past, and more importantly, its future. If Mars once held life but now lays dormant and lifeless, could we suffer a similar fate? And if so, what do we do about it?

The exploration of space may be key to our survival as a species. Who knows when the next asteroid or comet sighted through the Hubble telescope will be found on a collision course with Earth? The ability to learn about such bodies and their tendencies might save us from cataclysm.

And what happens centuries from now if and when our planet becomes uninhabitable? Could we travel to new worlds to start fresh? Perhaps that’s merely the stuff of old science fiction movies. Then again, space travel to the moon was considered a similar fantasy at one time.

Space exploration, like any scientific venture, is not the voluntary choice some think it to be. Since humankind first became civilized, we have sought to explain our world, to probe the mysteries of life, of our planet and worlds beyond. We can no more stop the clock on exploration than we can put back all the knowledge that has been gained over the centuries.

Putting the scientific quest for answers on hold is not an option, and never has been. We’ve conquered many of the dangers of our world by leaving ignorance behind. Expanding the human mind, in fact, is the real final frontier, the last mysterious world to be discovered. We can only do so by pushing forward.

To do so will, no doubt, take a lot of money. But that is a human-sized problem compared with traversing the vastness of space. Public-private partnerships, international coalitions, more resilient and affordable materials — all these can be accomplished with the right goals in mind. All we have to do is summon the will.

That challenge is how we first made it to the moon. President John F. Kennedy, just four months in office, threw down that gauntlet when he launched the quest to reach the moon by the end of the 1960s. Though fledgling NASA had just recently put satellites into orbit, the wheels were set in motion. Teams of engineers toiled day and night to sketch out the logistics of a space program that would, in incremental baby steps, lead to a lunar landing.

JFK’s promise was made good, with five months to spare, by a country that has always strived to do big things. Despite the turmoil of the times — the Vietnam War, the fight for civil rights, assassinations, protests and riots in the streets — Americans got busy and figured it out. As Kennedy knew when he issued his directive, no one can stop Yankee ingenuity when it is focused on a tangible goal.

We have many such goals that can and should be reached, be it creation of renewable energy or ending the scourge of terrorism. But concurrent with those needs is the desire to look past our daily problems and do something of lasting importance. No one will celebrate the 40th anniversary of a new omnibus transportation bill or reformed health insurance, despite what benefits they may bring to us now.

We need to do more than fix small things that are broken. We need to create something larger than ourselves through our dreams and the efforts to reach them.

As Kennedy said, "We choose to go to the moon and the other things ... not because they are easy, but because they are hard."

There is nothing harder than sending men to other worlds. And that is reason enough to keep doing it.