There is only one recent impact of a space rock so large. In 1908, an asteroid or cometary fragment plowed into Siberia. The “Tunguska Event,” named for the unlucky but unpopulated locale, flattened millions of trees across nearly 1,000 square miles.
Meteor Crater in Arizona — nearly a mile wide — was also probably gouged by an office-building-size space rock about 50,000 years ago.
Moving up the scale of destruction, a “planet killer” about five miles wide slammed into the Gulf of Mexico 65 million years ago, most scientists say, triggering a planet-wide cataclysm that wiped out the dinosaurs. That scenario, once heavily debated, appears more certain after a study published Thursday in the journal Science found that most dinosaurs went extinct relatively soon after that collision. With the dinosaurs gone, mammals thrived, eventually evolving into people.
“I think it’s fair to say, without the dinosaurs having gone extinct, we would not be here,” Paul Renne, director of the Berkeley Geochronology Center and leader of the study, told the Associated Press.
Much smaller fragments — leftovers from the dawn of the solar system — skip into our atmosphere every day, sometimes streaking spectacularly across the sky as meteors. Car-size rocks arrive at a clip of about once per year.
Asteroids as big as 2012 DA14 are expected to hit Earth much less frequently — about once every 1,200 years, Yeomans said.
What to do if scientists detect an asteroid headed for Earth? Given enough lead time — say, 30 or 40 years — deflecting the threat will be feasible, Lu said. “If something’s going to hit, we’re going to pull out all the stops.”
A Hollywood favorite — nuclear bombs — might work, as might gentler means such as parking a spacecraft near the asteroid so that its gravity pulls it to a safe orbit. Last year, the European Commission launched the NEOShield project to start planning such missions.
“The hard part,” Lu said, “is finding the million or so asteroids this size or larger and getting advance notice.” If 2012 DA14 had been on a collision course for next week, the year-long lead time since its discovery would have been too brief to launch a deflection mission, said Lu.
The B612 Foundation has raised a “few million dollars,” spokeswoman Diane Murphy said, but it needs $30 million or $40 million a year to build their telescope, which would scan the entire sky for city-killer-size rocks.
Lindley Johnson, who heads NASA’s $6 million annual program to find near-Earth threats, acknowledged that the agency could do better. “We still have a lot of improvement to make in finding all of the hazardous asteroids,” he said.
Case in point: The sky-scanning camera that first spotted asteroid 2012 DA14 was purchased not by NASA, but by a small nonprofit group, the Planetary Society.