The
infamous Tunguska explosion, which mysteriously leveled an area of Siberian
forest nearly the size of Tokyo a century ago, might have been caused by an
impacting asteroid far smaller than previously thought.
The fact that a relatively small asteroid could
still cause such a
massive explosion suggests "we should be making more efforts at detecting the
smaller ones than we have till now," said researcher Mark Boslough, a
physicist at Sandia National Laboratory in Albuquerque, N.M.
The
explosion near the Podkamennaya Tunguska River on June 30, 1908, flattened some 500,000
acres (2,000 square kilometers) of Siberian forest. Scientists calculated the
Tunguska explosion could have been roughly as strong as 10 to 20 megatons of
TNT — 1,000 times more powerful than the atom bomb dropped on Hiroshima.
Wild
theories have been bandied about for a century regarding what caused the Tunguska
explosion, including a UFO crash, antimatter, a black hole and famed
inventor Nikola Tesla's "death ray." In the last decade, researchers
have conjectured the event was triggered by an asteroid exploding in Earth's
atmosphere that was roughly 100 feet wide (30 meters) and 560,000 metric tons
in mass — more than 10 times that of the Titanic.
The
space rock is thought to have blown up above the surface, only fragments
possibly striking the ground.
Now
new supercomputer
simulations suggest "the asteroid that caused the extensive
damage was much smaller than we had thought," Boslough said.
Specifically, he and his colleagues say it would have been a factor of three or
four smaller in mass and perhaps 65 feet (20 meters) in diameter.
The
simulations run on Sandia's Red Storm supercomputer — the third fastest in the
world — detail how an
asteroid that explodes as it runs into Earth's atmosphere will generate a
supersonic jet of expanding superheated gas. This fireball would have caused
blast waves that were stronger at the surface than previously thought.
At
the same time, previous estimates seem to have overstated the devastation the
event caused. The forest back then was not healthy, according to foresters,
"and it doesn't take as much energy to blow down a diseased tree than a
healthy tree," Boslough said. In addition, the winds from
the explosion would naturally get amplified above ridgelines, making the
explosion seem more powerful than it actually was. What scientists had thought
to be an explosion between 10 and 20 megatons was more likely only three to
five megatons, he explained.
All
in all, the researchers suggest that smaller asteroids may pose a greater
danger than previously believed. Moreover, "there are a lot more objects
that size," Boslough told SPACE.com.
NASA
Ames Research Center planetary scientist and astrobiologist David Morrison, who
did not participate in this study, said, "If he's right, we can expect
more Tunguska-sized explosions — perhaps every couple of centuries instead of
every millennia or two." He added, "It raises the bar in the long
term — ultimately, we'd like to have a survey system that can detect things
this small."
Boslough
and his colleagues detailed their findings at the American Geophysical Union
meeting in San Francisco on Dec. 11. A paper on the phenomenon has been
accepted for publication in the International Journal of Impact Engineering.
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