From the ground, it didn't really look like an explosion. Standing at a press site about 4 miles from the launchpad, amid the rippling, crackling sound waves generated by the rocket's chemical propulsion — a disturbance so great it sent fish leaping from the river in front of us — all systems seemed to be go.

There was a puff of white smoke overhead. A lengthy silence. And then a NASA rep on the PA, befuddlement in his voice, pronouncing what had happened a "non-nominal" event. For SpaceX, the aerospace start-up that had been supplying the International Space Station without incident for some time, the explosion of its Falcon 9 rocket was surely a shock — all the more worrisome because the company intends to start ferrying humans to space come 2017.

"It happens," said our bus driver, distilling the essence of the event. It does happen: Of all our scientific pursuits, perhaps none is more prone to spectacular failure than space travel. Yet the impulse to explore seems to endure. The occasional tragedy is the cost of the larger triumphs.

SpaceX embodies that grimly intrepid ethos. As Bloomberg's Ashlee Vance details in his new book on SpaceX founder Elon Musk, the company's short history brims with failure and redemption — a microcosm of the longer history of spaceflight. Its efforts to build workable rocket engines ended in flames many times. Three iterations of its Falcon 1 rocket blew up between 2006 and 2008. Then, out of options and nearing bankruptcy, the company finally nailed a do-or-die launch from the Kwajalein atoll in September 2008. NASA contracts, public esteem and profitability soon followed.

Since then, SpaceX has had a nearly perfect record in its commercial launches. And it has made steady progress in modifying its Dragon capsule for passengers, with all of the most sophisticated safety enhancements. If a crew had been sitting atop the rocket that expired June 28, the company's president reassuringly said afterward, they would have escaped easily.

Yet history still advises caution. Another calamity — an unforeseen problem, a counterintuitive failure — is always lurking in rocketry. Even if SpaceX can build the Volvo of spacefaring, there will be more non-nominal events, more anomalies. And yet the pursuit will go on, farther and farther into the galaxy. Musk, SpaceX's enigmatic spirit guide, wants to go to Mars. He wants to die on Mars, in fact. NASA plans to get there, too, sometime in the 2030s. Russia and China still want to get to the moon.

It's hard to say why traveling to space — risking so much to reach a lonely, irradiated vacuum — remains so alluring. But I think Tom Wolfe was probably on to something in "The Right Stuff," his book about the Mercury astronauts and the early days of the American space program: "Everyone who felt the spirit of NASA at that time wanted to be a part of it. It took on a religious dimension that engineers, no less than pilots, would resist putting into words. But all felt it."

After the launch, with a late-afternoon Florida thunderstorm gathering, I stopped at the Astronaut Memorial at Kennedy Space Center. It's dominated by a shimmering granite edifice divided into 90 panels, two dozen of them bearing the names of American astronauts who died in the line of duty. It's a pretty stark reminder that exploration has always been a death-haunted pursuit. But the monument's most arresting statement, the one that stuck with me long after leaving, was implicit. There are an awful lot of empty panels on that wall, awaiting more names.