Not that long ago, scientists thought strikes like the one that shook Chelyabinsk occur once a century, but new research moves that up to once every 50 years, or less, says Jay Melosh, a former geophysicist and asteroid researcher at the U of A who now works at Purdue (where, among his many tasks, he helped build the aforementioned impact-calculating website).
Though the biggest space rocks present the worst threats, the "most likely" impact scenario may be from these lesser bodies, he believes.
"Expect a very strong airburst, something that breaks windows, maybe kills people on the ground," Melosh says. "The civil-defense thing to do is tell people to get away from the windows."
Melosh doesn't expect a relatively small mass to hit the ocean and cause a tsunami that wreaks havoc on coastal cities, something seen in the 1998 movie Deep Impact. Research by Melosh and others, based on previous underwater-detonation tests by the United States, seems to prove that asteroid-caused tsunami waves would break early and lose their energy.
The scariest impact scenario resembles what caused a 110-mile-wide crater 65 million years ago near what is now Chicxulub, a small Mexican town on the Yucatán Peninsula.
The impact rock was thought to have been about six miles wide. The problem for animals and plants worldwide wasn't merely the gigantic blast site, which had to have knocked over trees for thousands of miles in every direction. It was the mind-boggling mass of fiery "ejecta" that rose from the site in a plume, then descended all over the Earth at speeds exceeding 17,000 miles an hour, pummeling and burning everything it touched.
"The [heat] radiation would have been comparable to a pizza oven," he says. "Every animal with a body mass of 15 kilograms or more became extinct."
And not just the dinosaurs were cooked. Ocean animals fared poorly in the boiling water near the surface. "Nearly every species" died off following the event, Melosh says.
Burrowing animals, like small mammals, were among the survivors — and some of them evolved into humans.
Melosh is against the idea of using nuclear weapons to deflect Earth-bound asteroids, and he's wary of a deal inked in September between the United States and Russia that expands research along those lines: "I think some of this is being used as an excuse" to conduct nuclear tests in space."
Still, he's not willing to say NASA has exaggerated the threat of impacts. He just thinks deflection can be done by other means, such as ramming an asteroid with an unmanned kamikaze spacecraft.
NASA has given at least $700,000 in grants to a program led by Professor Bong Wie at Iowa State University's Asteroid Deflection Research Center. Wie and his study team, including scientists with the Goddard center, are developing a system that would punch a hole in an asteroid and then follow up by battering a second spacecraft loaded with nuclear weapons into the mass.
A study plan for the "Hypervelocity Asteroid Intercept Vehicle" was developed in 2011. "Despite the uncertainties inherent to the nuclear-disruption approach, disruption can become an effective strategy if most fragments disperse . . . so that a very small number of fragments impacts the Earth," says a write-up about the project on nasa.gov.
Though only on the drawing board, the proposed system "offers a potential breakthrough or great leap in mission capabilities," NASA says. "The proposed HAIV system will become essential for reliably mitigating the most probable impact threat: NEOs with warning times shorter than 10 years."
Sounds like a cliffhanger.