THE BRAVE ASTRONAUTS who lost their lives Saturday were pressing the literal limit of what separates the earth from all else that is. In dying, they remind us of the primordial truth that human beings exist to press such limits, even knowing that the results are often tragic. Space flight has been tamed no more than the human project has itself been purged of risk. When women and men consciously defy that risk for the sake of the universal impulse to know and to find, and when they then die doing so -- we, the rest of their kind, rightly respond as one family, honoring them as exemplars of human nobility.
The vast blue in which the Columbia crew died has always drawn the human gaze in the quest for something more. Surely that is why from the beginning, humans have populated the sky with angels, gods, and heroes. In our own time the sky has gone from being the place out of which fire falls -- the realm of bombs and missiles -- to the place in which the earthly borders over which wars are fought become invisible and therefore meaningless.
That, after all, was the ''Earthrise'' epiphany, the picture taken by the moon walkers a generation ago of the blue-green ball hanging in the void, our fragile planet as an oasis of life and hope in an indifferent cosmos. Then, marvel of marvels, didn't that borderless dream of one earth become institutionalized in the joint Russian-American space station, which Columbia and other shuttles regarded as an outermost home?
Space exploration defined the Cold War at its most dangerous -- from Sputnik in 1957, announcing the Soviet capacity to rain H-bombs on the United States, to JFK's retaliatory race to the moon, which sparked a final American military dominance. Ultimately, a counterimpulse rooted in common humility before the vast unknown led Washington and Moscow to cross an even bolder space frontier -- into a realm of cooperation. Transforming ''throw weight'' and ''force projection,'' the enemies became partners in the very enterprise that had most endangered the earth.
That partnership is enshrined in the space station where even now a Russian and two Americans face perilous uncertainty after the Columbia disaster. Their common plight reminds us that in nothing was the reversal of the Cold War more absolute than the transformation of space from nationalistically demarcated battlefield to transnational field of human investigation. That good tradition was being honored last week by the presence of Israel's lone astronaut aboard the Columbia, a successor to participants from other nations.
One of the things that makes the sky newly dangerous is the resurgent temptation to elevate armed borders into the air and beyond, a reiteration of the ancient trumping of the human with the tribal. That impulse is reflected in initiatives sponsored by Defense Secretary Donald Rumsfeld toward a militarization of space under the so-called US Space Command.
Defying a nearly unanimous UN consensus, the Pentagon is staking claims on the ''high frontier,'' a corollary of current US strategies toward global military dominance. The Bush administration's missile defense program is the first stage of this expansion. Space-based laser weapons, ''sentry satellites,'' orbiting ''kill vehicles,'' plutonium-powered space probes -- all an appalling, if little-noted, coming of age of Star Wars. Nothing would more thoroughly betray the humane spirit of the Columbia astronauts than a retreat from international space partnership for peace in the name of one nation's space-based hegemony. That it would be America's only makes the very thought of such betrayal more grotesque.
Today, in the formal memorial service for the lost Columbia astronauts in Houston, and in the coming days of mourning and reflection, our nation's heart will be full. Again and again, we will see, in the broadcast image, the blue sky cut by the white arc of sudden devastation. We will see the faces of the dead men and women, of their bereaved families. We will see faces of newly stunned Israelis.
And what will all of this prompt in the American heart? Will we come out of this grief more alive to the fragility of all human life, and therefore to its preciousness? Will we recognize in the world outpouring of empathy a signal that international commonality must now transcend every narrow notion of ''national security?''
When our eyes drift skyward today, what will we see? What about the refusal of the very air, not to mention outer space, to define itself by anything but the color blue? The dense, deep, endless blue in which, as our noble astronauts keep telling us, the exquisite planet Earth hangs -- hangs there without even a thread to hold it up.
All that this lovely sphere hangs by in the otherwise indifferent void is human courage. May those who have just taught us this again rest in peace.
James Carroll's column appears regularly in the Globe.
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