A planetary "all clear" sounded late Sun day night. A potentially hostile celestial visitor - a half-mile wide ball of primordial minerals named 2004 XP14 - whipped past our blue island on its way to what we all should hope is an eternal journey.

We should hope the journey is eternal, because if it stops here, so, most likely, would much of what we consider to be civilization.

Consider: Meteor Crater, the Arizona landmark many people visualize when they think of an asteroid impact, was the result of a relative interplanetary pebble. That chasm, nearly a mile across and more than 500 feet deep, was the result of impact by a space rock only about 80 feet in diameter.

But the Apollo-class asteroids - those whose solar orbits, like that of our most recent unwelcome guest, take them across the orbit of Earth - include far larger threats. Like the three-mile rock that blasted the 62-mile-wide Manicouagan impact structure in Quebec more than 200 million years ago - one of the largest impact craters still visible on the planet's surface. Or the six-mile space bomb believed to have eradicated the dinosaurs 100 million years later.

Scientists tell us that such truly catastrophic hits come every 100 million years or so, which makes us roughly due, the way such things are measured. But not to worry, they say; as far as they can observe and predict, nothing really nasty is headed our way. Of course, the length of that view is the cosmic equivalent of an eye's wink. We know of some 150 Apollo objects up to five miles across. It's likely that there are thousands we haven't tagged yet.

When the one with our name on it comes, they say, we'll know it - for a moment, at least. A land hit would obliterate thousands of square miles, followed by 13-magnitude earthquakes, volcanic eruptions and dust clouds that would blot out the sun for months. An ocean hit would produce mega-tsunamis up to three miles high, not only eradicating coastal life, but flooding the interiors of continents as well. The atmospheric steam cloud would alternatively turn Earth into an equatorial hothouse, then an icebox.

No, we would not survive as a civilization, most likely not as a species. That's why those who watch for and plot the orbits of these doomsday rocks are doing such important work. And why those who dream of thwarting the cataclysm dream such important dreams.