The awesome explosion and devastation at Tunguska a century ago in Siberia -the largest impact event in recent history- may have been caused by an asteroid only a fraction as large as previously published estimates. The Sandia National Laboratories supercomputer simulations run counter to prior theories of a mini black hole or comet as the cause.
The energy of the blast was estimated to be between 10 and 20 megatons of TNT — 1,000 times more powerful than the bomb dropped on Hiroshima, Japan. The explosion felled an estimated 80 million trees over 2,150 square kilometers, and measured 5.0 on the Richter scale.“The asteroid that caused the extensive damage was much smaller than we had thought,” says Sandia principal investigator Mark Boslough of the impact that occurred June 30, 1908. “That such a small object can do this kind of destruction suggests that smaller asteroids are something to consider. Their smaller size indicates such collisions are not as improbable as we had believed.”
Because smaller asteroids approach Earth statistically more frequently than larger ones, he says, “We should be making more efforts at detecting the smaller ones than we have till now.”
The new 3-D simulation — which more closely matches the widely known facts of destruction than earlier models — shows that the center of mass of an asteroid exploding above the ground is transported downward at speeds faster than sound. It takes the form of a high-temperature fireball of expanding gas, which causes stronger blast waves and thermal radiation pulses at the surface than would be predicted by an explosion limited to the height at which the blast was initiated.
“Our understanding was oversimplified,” says Boslough, “We no longer have to make the same simplifying assumptions, because present-day supercomputers allow us to do things with high resolution in 3-D. Everything gets clearer as you look at things with more refined tools.”
The new interpretation also accounts for the fact that winds were amplified above ridgelines where trees tended to be blown down, and that the forest at the time of the explosion, according to foresters, was not healthy. Thus previous scientific estimates had overstated the devastation caused by the asteroid, since topographic and ecologic factors contributing to the result had not been taken into account.
“There’s actually less devastation than previously thought,” says Boslough, “but it was caused by a far smaller asteroid. Unfortunately, it’s not a complete wash in terms of the potential hazard, because there are more smaller asteroids than larger ones.”
Boslough and colleagues achieved fame more than a decade ago by accurately predicting that that the fireball caused by the intersection of the comet Shoemaker-Levy 9 with Jupiter would be observable from Earth.
Simulations show that the material of an incoming asteroid is compressed by the increasing resistance of Earth’s atmosphere. As it penetrates deeper, the more and more resistant atmospheric wall causes it to explode as an airburst that precipitates the downward flow of heated gas.
Because of the additional energy transported toward the surface by the fireball, what scientists had thought to be an explosion between 10 and 20 megatons was more likely only three to five megatons. The physical size of the asteroid, says Boslough, depends upon its speed and whether it is porous or nonporous, icy or waterless, and other material characteristics. “Any strategy for defense or deflection should take into consideration this revised understanding of the mechanism of explosion,” says Boslough.
Extraterrestrial fragments from the impact event have yet to be recovered. A team of Italian scientists believes that a hunk of asteroid may be in a nearby lake, buried deep within a submerged impact crater.
Although Sandia's report doesn't specify the likely size of the Tunguska asteroid, it demonstrates the threat of relatively small near-earth objects. And hard as it is to spot the massive rocks hurtling in Earth's general direction, such as the 1150-ft. asteroid Apophis, smaller asteroids are far more numerous, and even more difficult to track.
Source : dailygalaxy.com
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