From the ground, it didn't really look like an explosion. Standing at a press site about 6½ km 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 startup 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.