Image: Asteroid
This artist's conception, prepared by NASA, shows a meteorite blazing toward Earth. Scientists say that objects mere tens of yards (meters) across could not create a global catastrophe, but they could still wreak more damage than an atomic bomb under certain conditions.
By Alan Boyle Science editor
msnbc.com

Many scientists believe an asteroid or comet killed off the dinosaurs, but the impact of cosmic collisions on human history is a matter of sharper controversy. Some researchers say computer simulations and historical reviews may shed new light on how common such impacts were in the past — and how much of a threat they could pose in the future.

In recent years, most of the concern about cosmic threats has focused on the hundreds of space rocks and iceballs bigger than 0.6 miles (1 kilometer) in diameter that have a chance of someday colliding with Earth. Scientists say objects that size could hit with the force of hundreds of hydrogen bombs, having a global environmental impact and potentially killing off whole species.

NASA estimates that such collisions occur every 100,000 to 1 million years, based on analyses of lunar impacts going back to the early 1980s, said David Morrison, who heads the astrobiology and space research directorate of NASA’s Ames Research Center.

“There is no ‘theory’ involved,” he told MSNBC. The figures are based on observations of crater densities on the moon, estimates of the current number of near-Earth objects and the assumption that the average impact rates have held steady over time.

These figures account for near-Earth objects as big as the one that scientists say hit Mexico’s Yucatan coast 65 million years ago, followed by the demise of the dinosaurs. But they don’t include less deadly objects much smaller than a kilometer in width.

“The short-term threat is more from the smaller objects than the larger objects,” said Benny J. Peiser, an anthropologist at Liverpool John Moores University who specializes in the social implications of the asteroid/comet threat.

A computer simulation indicated that more than 1,000 fatal impacts could crop up in a 100,000-year period, Peiser said last week in Washington during the annual meeting of the American Association for the Advancement of Science.

The computer software, devised by the University of Arizona’s John Lewis and run by Michael Paine of the Planetary Society Australian Volunteers, generates scenarios based on assumptions about the distribution of near-Earth objects. Thus, the results shouldn’t be taken as predictions of what would actually happen. However, Peiser said the simulation pointed up some factors that may not be fully considered in official risk estimates.

Without a trace
Most of the fatal events in the 100,000-year scenario were airbursts over land, analogous to the 1908 Tunguska fireball in Siberia. On the basis of eyewitness accounts and the blast pattern left behind, scientists say a 60-meter-wide (200-foot-wide) object blew up above a remote Siberian forest with the force of 15 million tons of TNT.

Other blasts occurred in or above the ocean, setting off tsunami waves that inundated coastlines.

Only 3 percent of the scenario’s fatal events left a crater. “Land craters (on Earth) are therefore a very poor indicator of the hazard due to comets and asteroids,” Peiser said.

Casualty counts
In this and other long-term scenarios, at least one catastrophic event turns up. The 100,000-year scenario, for instance, includes the potential for an impact involving a 3-mile-wide (5-kilometer-wide) asteroid that touches off a climate catastrophe, wiping out the human race.

That “Armageddon” was excluded in figuring out the fatality count for a typical 10,000-year period — one-tenth the total span for the computer scenario. Paine and Peiser came up with 13 million deaths traceable to asteroid or comet impacts, assuming a constant worldwide population of 5 billion.

Historical record
How does this square with human history? Here, scientists may review records left in the language of folklore. For example, in China’s Shanxi Province, 10,000 people were said to have been killed in the year 1490 by a hail of “falling stones” that some astronomers surmise may have been triggered by the breakup of a large asteroid.

Bruce Masse, an environmental archaeologist with Los Alamos National Laboratory, has tried to untangle other astronomical observations from ancient tales told from the Middle East to Hawaii. He referred to an impact event in the Rio Cuarto region of northern Argentina, in which an asteroid 100 to 300 yards (or meters) in diameter apparently broke up as it zoomed through the atmosphere, leaving a trail of craters.

Image: South America
This map shows the shallow path of a near-Earth object that blazed through the atmosphere and hit northern Argentina thousands of years ago. Archaeologist Bruce Masse says the black spots indicate areas where cultures told tales of a catastrophic "Great Fire" in ancient times.
This event, which has been dated to around 2000 B.C., may square with tales told of a catastrophic “Great Fire” by cultures in a large region around the impact site, Masse said.

Masse went on to say that at least some of the tales of a Great Flood, found in hundreds of cultures around the world, may relate to an oceanic impact that set off a tsunami.

“The preliminary picture emerging from this analysis is that on or shortly after May 10 in 2807 B.C., the earth was struck by a cosmic impact in the deep waters between Africa and Antarctica. ... These data indicate that the flood itself was the very real product of a combination of large tsunami and atmospheric rainout.”

Masse cautioned, however, that his analysis was not yet complete. Other researchers have sought to link the biblical account of a Great Flood to a catastrophic inundation of the Black Sea, touched off in the wake of the last Ice Age around 5000 B.C.

The sky isn't falling
NASA’s Morrison said the linkages between folklore and celestial phenomena could easily be taken too far. He cited the example of Russian psychologist Immanuel Velikovsky, who tied all sorts of ancient accounts to cosmic collisions and near-collisions. “One has to be careful not to fall into Velikovsky-like situations,” Morrison said.

He said the computer simulations were useful for putting the risk of cosmic impact into perspective, but shouldn’t necessarily be taken at face value.

“There’s nothing really new in these simulations,” Morrison told MSNBC. “All they do is generate output that’s based on what’s put into them ... so what you get generally reflects the assumptions that you put in.”

Even the projected death toll had to be put into perspective, Morrison said. When compared with total deaths during the same time frame, impact-related fatalities generally worked out to far fewer than 1 in 10,000, which he said was consistent with the view that asteroids and comets were a significant but not a major cause of death.

Morrison, who has participated in NASA’s 1992 Spaceguard Survey and other efforts to assess the risk from near-Earth objects, said he was “quite skeptical” about tying cosmic impacts to historical events.

But Peiser said such events should be considered among the environmental factors that could affect history. For instance, small-scale impacts — along with increased levels of cosmic dust in the atmosphere — could have contributed to climate changes, he said.

“You have to look for something before you find it,” he said.

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