After a 200-foot-wide meteorite sped toward the ground near the Tunguska River in 1908, it unleashed an explosion in the remote Russian region 500 times more powerful than the bomb dropped on Hiroshima.
The blast, called the Tunguska Event, was detected in London by monitoring equipment, and it leveled millions of trees over an 830-square-mile area.
Had the meteorite hit a populated area such as London, the result would have been a catastrophe.
It’s that rare but plausible scenario of a large meteorite striking an increasingly crowded Earth that has a network of scientists, including David Dearborn of Lawrence Livermore Laboratory, devising strategies to monitor and then thwart these celestial menaces.
Dearborn, a research physicist, spoke at three free seminars Saturday in Livermore, describing his strategy for using nuclear blasts to prevent devastating meteorite strikes on Earth.
Throughout the ages, asteroids circling the sun have intersected Earth’s orbit and entered the atmosphere. Most of them are so small they quickly burn up in a blaze, giving us a brief glimpse of their final moments, which we call shooting stars. But about every 500 to 1,500 years, earborn said, larger ones the size of the Tunguska meteorite reach Earth. And every year, smaller ones carrying a lesser but still potent punch get through, although most hit the ocean or remote terrestrial regions.
“It’s not uncommon,” he said.
And sometimes they do hit populated areas, Dearborn added. “Cars have actually been hit by meteorites.”
When they hit, these cosmic bodies leave “impact craters” or can create tsunamis if they land in the ocean. There are about 170 known impact craters, including the 120-mile-diameter Chicxulub crater on the Yucatan Peninsula in Mexico, and many thousands that either eroded away or remain undetected on land or the ocean floor. The huge meteorite creating the Chicxulub crater struck 65 million years ago, and scientists believe it brought about the extinction of the dinosaurs, along with three-quarters of life on Earth, by radically altering the atmosphere by kicking up clouds of dust and sulfur gases. An event that size is expected every 50 million to 100 million years, the NASA stated.
Hundreds of thousands of asteroids form a belt that circles the sun from Mars to Jupiter, and they range in size from less than a mile in diameter to nearly 500 miles across. In the 1990s, scientists started paying more attention to the dangers posed by this asteroid belt, Dearborn said. Now about 4,500 of them are deemed “near-Earth objects” that are likely to cross Earth’s path at some point, or get dangerously close.
Dearborn proposes sending spacecraft bearing nuclear explosives to an asteroid identified as a threat by a NASA program called “Space Guard.” The program is tasked with tracking near-Earth asteroids larger than 1 kilometer. Congress asked NASA to create the program in 2005.
The nuclear blasts could change the speed of the asteroid enough to prevent it from slamming into Earth, much like slowing down or speeding up a car to stop it from striking another object.
Nuclear material has the advantage of packing far more energy into it per ton than any comparable explosive material â?” a key consideration on weight-conscious space missions, Dearborn said. Detractors of this approach worry about the spread of radioactive material in space with such a system, but Dearborn said it would be detonated so far out that minuscule amounts – far less than that found naturally on Earth – would reach the atmosphere. Others are leery of former weapons being used in the neutral realm of outer space. NASA, however, in a 2007 report described the use of nuclear blasts as the best approach for handling threatening asteroids.
Other strategies under consideration include using a gravity tractor, which could slightly alter an asteroid’s trajectory by exerting on it the gravitational pull of nearby spacecraft, although Dearborn said that approach is marred by the enormous fuel requirements of such spacecraft.
Dearborn, however, pointed out that with the Space Guard program, there’s ample time to prepare for a threatening asteroid or comet, since they can be detected decades in advance. “We’ll have time to think about all sorts of things,” Dearborn said. What’s most critical, he said, is to maintain the monitoring system.
“With the warning, you have options.”
Suzanne Bohan
Contra Costa Times
