A simulation of the Tunguska asteroid explosion, inspired by the possible dynamics of the shock wave and plume. Illustration by Don Davis.

If millions of trees fell in a forest, it would definitely make a sound. But in the case of the Tunguska asteroid explosion, that sound was drowned out by a sonic boom.

This week marks the 100 year anniversary of the Tunguska explosion, the largest asteroid event on Earth in recent history. “Because of the magnitude of what would happen if one of these objects would hit the earth, it’s important to pay attention to the issue,” said Cong. Dana Rohrabacher, R-Huntington Beach.

 ”[The Tunguska asteroid] struck with the force of 1000 Hiroshima bombs. If it exploded over L.A., it would kill millions of people.”

To mark the anniversary, the Planetary Society of Pasadena launched Target Earth, a year-long project that focuses on extra-stellar objects whizzing dangerously close to our home planet. The event featured renowned scientists and speakers, including Rohrabacher, who served as chairman on the House Space and Aeronautics Subcommittee for eight years.

The Tunguska event was an asteroid 164 feet in diameter that exploded in the air over the Tunguska River in central Siberia on June 30, 1908. The shock wave and fireball that resulted from the blast obliterated trees and wreaked general havoc on the ground in a circle about 30 miles wide. The area was largely unpopulated.

“One hundred years ago, we took a big whammy,” said Susan Lendroth, the Planetary Society’s Events and Communications Manager. And it could happen again.

If a Tunguska sized asteroid were to explode over Orange County, “it would cover an area going halfway out to San Bernardino, it would go about to LA, in a 20-30 kilometer [12 to 18 mile] radius going out from the center,” says Alan Harris, Senior Research Scientist from the Space Science Institute. It would flatten buildings and cause mass destruction, similar to the atomic bomb that devastated Hiroshima in 1945.

“To a first approximation, it would be very much like a nuclear blast,” Harris said.

The Tunguska asteroid didn’t actually crash into the Earth; it exploded in the atmosphere above it. A large rock body passing through our atmosphere has a tremendous amount of pressure pushing on it, Harris says. And the bigger the body is, the less force pushes on it, because the force is it distributed over the whole thing. “So a larger rock would be more likely to reach the earth. A smaller rock would explode in the atmosphere,” Harris said.

“Tunguska made it down to about 8 kilometers [about 26,000 feet], where airplanes fly, when it blew up. The damage done was mainly from the airburst, would be from the fireball rather than the crater,” Harris said. And that damage can still be seen today.

	Flattened trees at the site of the Tunguska asteroid explosion. Photo taken in 1984, seventy-six years after the event.

There are approximately 200,000 to 400,000 objects shooting through space that are Tunguska-sized or larger, Harris says. The purpose of Target Earth is to find these near earth objects and track their orbits, to see if they overlap with ours. Many so far have been found.

“The largest asteroids are over 10 kilometers [about 6.2 miles] in diameter. We’ve found all of those. There are three,” Harris said.

But smaller objects could still cause global catastrophe if they smacked into Earth. About 80% of these, that are about 3,260 feet in diameter or larger, have been found. What’s left is yet uncharted, and work is still ongoing to track their trajectories. “If we can find out if something is going to hit in 20 years,” Lendroth says, “We can plan and create a mission, find way to get rid of it.”

But the money for this task is drying up. The best telescope for refining orbits of near-earth objects, the Arecibo radio telescope in Puerto Rico, is in danger of being shut down for lack of funding.

“It’s not something that only happened when the solar system was young,” Lendroth said. “It’s not something that only happened 65 million years ago when the dinosaurs got wiped out. It’s something that happened with a much smaller object that flattened a forest 100 years ago. There are still people alive today that were alive when it hit.

“This is something we can do something about–we can find them, we can track them, we can try to prevent it.”

Fireball streaks across Southern California skies

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