Simulation of Asteroid 2012 DA14 close flyby of Earth. / NASA/JPL
Asteroids come and asteroids go, but every once in a while one smacks into Earth. What happens on the day when we know one is really coming?
The question is not unwarranted, given the fiery arrival Friday of a meteor that streaked across the Russian sky, causing widespread damage and hundreds of injuries from the air pressure wave, and as Asteroid 2012 DA14 recedes into Earth's rearview mirror after a close call on Friday. It came within 17,100 miles of hitting our planet. That's the closest recorded approach by a space rock that size, one roughly 147 feet across. Now headed for an orbit closer in to the sun, the space rock reached its closest point to Earth as it passed directly above Sumatra on its 17,450 mile-per-hour voyage.
"Something that size would have hit the Earth with the energy of a thermonuclear bomb," says asteroid impact expert Jay Melosh of Purdue University in West Lafayette, Ind., who headed a team that created an online impact calculator, "Impact Earth!" handy for doomsday enthusiasts.
The Russian meteor burst apart in air, creating a pressure wave strong enough to blast out windows across a region of central Russia. Melosh estimates the impact energy of an asteroid the size of 2012 DA14, were it to hit Earth, at about 4 megatons, a bit higher than NASA estimates. Soviet city buster nuclear bombs were about 20 megatons, for context, while the 1945 Hiroshima atomic bomb that killed about 150,000 people was only about 15 kilotons. (A kiloton has the equivalent explosive force of 1,000 tons of TNT. A megaton explodes like 1 million tons of TNT.) NASA's Donald Yeomans compared the blast from such an object to the 1908 Tunguska (tun-GUS-kuh) event in Siberia. "This impact of an asteroid just slightly smaller than 2012 DA14 is believed to have flattened about 750 square miles of forest," Yeomans says.
The La Sagra observatory in southern Spain spotted the 2012 DA14 only last year, as its name indicates (asteroids are provisionally named for their year of first observation and by alphabetical combinations that reflect the date and order of their discovery). "It's worth noting that they only spotted it last year," Melosh says. "If they had calculated the asteroid's orbit and it appeared to be headed toward us, other astronomers would have checked their estimate, released the information, and a media uproar would have ensued" he says.
U.S. astronomers are instructed to report dangerous asteroid sightings to NASA's "Near-Earth Object" program office, says asteroid expert Richard Binzel of the Massachusetts Institute of Technology. International astronomers are instructed to report such sightings to the United Nations Committee on the Peaceful Uses of Outer Space (COPUOS). Both those outfits would face some decisions if they were confronted with an asteroid that appeared to have Earth in its cross hairs, something Melosh calls a 1-in-1,000 year event.
A second relatively close passage last year allowed astronomers to refine their track of Asteroid 2012 DA14 and assure themselves that it would, indeed, miss Earth. But an asteroid its size headed straight for us with only a year's warning might not provide that second-look opportunity, adding to the uncertainty about its path and what to do.
Something that big would have been an 8 on the 0-to-10 Torino Impact Hazard Scale of asteroid threats, says Binzel, where a 10 would be a whopper impact like the one that wiped out the dinosaurs. That asteroid was thought to be 6 miles wide. An 8 indicates merely the threat of regional damages such as the Tunguska event.
A 2010 National Research Council report, "Defending Planet Earth," suggested a number of approaches to a threat the size of 2012 DA14. The most practical space mission that the report suggested for such a small space rock was sending a "gravity tractor" spacecraft to orbit the object. By going into orbit around the asteroid, the spacecraft would alter the space rock's effective mass and hence change its trajectory, which is shaped by the gravitational pull from the sun (more mass means more force pulling on the asteroid, broadly speaking).
A simpler, cheaper and perhaps smarter alternative might just be good old evacuation, says Melosh, who was a panel member for the NRC report. The NRC report criticized Congress for under-funding NASA efforts to spot dangerous asteroids.
The Russian meteor's damage was from the air pressure wave it created as it zoomed into the atmosphere, says Binzel. NASA estimates it was about 49 feet wide and released a 300 kiloton blast. An asteroid the size of 2012 DA14 would break up at about 30,000 feet in altitude if it ran into the Earth, Melosh estimates. Unfortunately this is the "optimal" height in military terms, he says, for delivering the full force of its blast to the surface. "It's tempting to spin out these terrible scenarios where it happens over Moscow or London, which would be devastating, but the truth is that cities are not covering that much of the Earth, and so it is very unlikely that one would be hit," Melosh says. Plus, we would have at least a month to evacuate a city or region that appeared at risk under circumstances similar to 2012 DA14's discovery.
Most of the world, 70%, is covered in ocean, after all, and that is where most impacts hit, Melosh says. "We would clear the shipping lanes and turn the cameras on, and see a heck of a show, most likely."
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