An asteroid the size of a 15-story building will zoom past the Earth next week, the closest such flyby in a century and a reminder of the persistent risk of collision.

In a different orbit from outer space, the spinning rock, dubbed DA14, could have struck the Earth with a force equivalent to more than 2 million tons of TNT.

But instead, it will continue on eight times as fast as a speeding bullet back into deep space, averting calamity and thrilling scientists.

“It is a record-close approach for an asteroid of this size,” said Donald Yeomans, manager of the Near-Earth Objects Office at NASA’s Jet Propulsion Laboratory in Pasadena, in a Thursday morning news conference.

On average, an asteroid of

DA14′s diameter — estimated to be about 150 feet — gets this close every 40 years or so, he said. A collision would be expected every 1,200 years.

No “killer asteroids” are headed toward us within the next 50 years — that we’re aware of.

But we live in a cosmic shooting gallery, and we’ve just started looking for threats.

“It’s a wake-up call from our solar system,” wrote Apollo astronaut Rusty Schweickart, founder of the B612 Foundation, which urges planning for the inevitable day a collision occurs.

It is tough to peer into the distant future and accurately predict the intersecting orbits of Earth and asteroids, said Gerald McKeegan, of Oakland’s Chabot Space & Science Center, one of a group of observatories

around the world that tracks asteroids.

Astronomers find several asteroids every week, and an estimated 1 million have not yet been seen. “Sooner or later, one will pop up — it’s just a matter of time,” he said.

If a collision seems inevitable, we would need a decade to successfully deflect one, McKeegan said. That’s because it takes far less energy to throw asteroids off course when they are very far away.

At its closest approach, DA14 will be 17,200 miles above us. That’s far nearer than the moon. The rock — which probably came from the main asteroid belt, between Mars and Jupiter — will fly between us and GPS and weather satellites, which would resemble bugs on a windshield if they collided with DA14. Fortunately, their paths seem unlikely to cross.

Were such an asteroid to hit the atmosphere, it would flatten like a pancake and explode before striking, said NASA’s Yeomans. An impact would level trees with an “air blast,” not a crater, he said.

Just 105 years ago, a comparable asteroid struck Siberia, near Tunguska, and flattened about 840 square miles of forests. A much larger asteroid struck Mexico’s Yucatan area some 65 million years ago, wiping out dinosaurs and three-quarters of all plant

species, cooling the Earth and redirecting evolution to create the species we know today.

DA14′s close call will alter its fate far more than Earth’s because the Earth’s gravitational pull will put it on a less threatening path.

“The Earth will put this one in an orbit that is considerably safer than the orbit it has been on,” said Yeomans.

Contact Lisa M. Krieger at 650-492-4098.

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