1908: The Tunguska Event

Above: Trees knocked down by the Tunguska explosion.

Credit: the Leonid Kulik Expedition.

June 30, 2008: The year is 1908, and it's just after seven in the morning. A man is sitting on the front porch of a trading post at Vanavara in Siberia. Little does he know, in a few moments, he will be hurled from his chair and the heat will be so intense he will feel as though his shirt is on fire.

That's how the Tunguska event felt 40 miles from ground zero.

Today, June 30, 2008, is the 100th anniversary of that ferocious impact near the Podkamennaya Tunguska River in remote Siberia--and after 100 years, scientists are still talking about it.

"If you want to start a conversation with anyone in the asteroid business all you have to say is Tunguska," says Don Yeomans, manager of the Near-Earth Object Office at NASA's Jet Propulsion Laboratory. "It is the only entry of a large meteoroid we have in the modern era with first-hand accounts."

While the impact occurred in '08, the first scientific expedition to the area would have to wait for 19 years. In 1921, Leonid Kulik, the chief curator for the meteorite collection of the St. Petersburg museum led an expedition to Tunguska. But the harsh conditions of the Siberian outback thwarted his team's attempt to reach the area of the blast. In 1927, a new expedition, again lead by Kulik, reached its goal.

"At first, the locals were reluctant to tell Kulik about the event," said Yeomans. "They believed the blast was a visitation by the god Ogdy, who had cursed the area by smashing trees and killing animals."

While testimonials may have at first been difficult to obtain, there was plenty of evidence lying around. Eight hundred square miles of remote forest had been ripped asunder. Eighty million trees were on their sides, lying in a radial pattern.

"Those trees acted as markers, pointing directly away from the blast's epicenter," said Yeomans. "Later, when the team arrived at ground zero, they found the trees there standing upright – but their limbs and bark had been stripped away. They looked like a forest of telephone poles."

Such debranching requires fast moving shock waves that break off a tree's branches before the branches can transfer the impact momentum to the tree's stem. Thirty seven years after the Tunguska blast, branchless trees would be found at the site of another massive explosion – Hiroshima, Japan.

Kulik's expeditions (he traveled to Tunguska on three separate occasions) did finally get some of the locals to talk. One was the man based at the Vanara trading post who witnessed the heat blast as he was launched from his chair. His account:

Suddenly in the north sky… the sky was split in two, and high above the forest the whole northern part of the sky appeared covered with fire… At that moment there was a bang in the sky and a mighty crash… The crash was followed by a noise like stones falling from the sky, or of guns firing. The earth trembled.

The massive explosion packed a wallop. The resulting seismic shockwave registered with sensitive barometers as far away as England. Dense clouds formed over the region at high altitudes which reflected sunlight from beyond the horizon. Night skies glowed, and reports came in that people who lived as far away as Asia could read newspapers outdoors as late as midnight. Locally, hundreds of reindeer, the livelihood of local herders, were killed, but there was no direct evidence that any person perished in the blast.

Comet Lovejoy -- as seen from space

Closeup Comet

By ALICIA CHANG, AP Science Writer – Thu Nov 4, 9:00 pm ET

PASADENA, Calif. – A NASA spacecraft sped past a small comet Thursday, beaming pictures back to Earth that gave scientists a rare close-up view of its center. Mission controllers burst into applause upon seeing images from the flyby that revealed a peanut-shaped comet belching jets of poisonous gases.

"It's hyperactive, small and feisty," said mission scientist Don Yeomans of the NASA Jet Propulsion Laboratory.

The close encounter occurred 13 million miles from Earth when the Deep Impact craft, hurtling through space, flew within 435 miles of comet Hartley 2. It's only the fifth time that a comet's core has been viewed up close.

Scientists are interested in comets because they're icy leftovers from the formation of the solar system about 4.5 billion years ago. Studying them could provide clues to how Earth and the planets formed and evolved.

"The scientific work is just beginning now," principal investigator Michael A'Hearn, of the University of Maryland, said at a post-mission news conference. "The engineers did a fantastic job of getting us data. Now we have to make sense of it."

Thursday's flyby is actually an encore mission for Deep Impact. It set off cosmic fireworks on July 4, 2005, when it fired a copper probe that crashed into comet Tempel 1. The high-speed collision spewed a cloud of debris into space, giving scientists their first peek of the interior.

After the $333 million comet-buster, NASA recycled Deep Impact for a new mission to visit another comet. It was supposed to target comet Boethin in 2008, but it was nowhere to be found. Scientists theorized the comet may have broken up into small pieces.

Deep Impact was then redirected to Hartley 2. Roughly 1 1/2 miles long, Hartley 2 is the smallest comet to be photographed up close. On its way there, the craft spent several months scanning a cluster of nearby stars with known planets circling them.

While its latest task lacks the Hollywood drama of the Tempel 1 crash, researchers still consider it an important mission. Unlike in 2005, viewers could not see Thursday's comet encounter in real time since the craft's antenna was not pointed at Earth as it flew past Hartley 2.

"There are a lot of open questions about comets and their life cycle," said project manager Tim Larson of JPL, which manages the $42 million encore mission. "We have so little data that every time we have an opportunity to go near a comet, it's a chance to expand our knowledge."

Since September, Deep Impact has been stalking Hartley 2 like a paparazzo, taking images every 5 minutes and gathering data. It's the first craft to visit two comets.

Deep Impact will observe Hartley 2 until Thanksgiving and then wait for further instructions from NASA. Thespace agency has not decided whether to reuse Deep Impact again. The craft does not have enough fuel on board to do another flyby.

The latest images add to scientists' cometary photo album, said astronomer David Jewitt of the University of California, Los Angeles, who had no role in the project.

"We're visual animals and nothing seems wholly real to us until we have a nice picture of it," Jewitt said.

Hartley 2 passed within 11 million miles of Earth on Oct. 20 — the closest it has been to our planet since its discovery in 1986.

British-born astronomer Malcolm Hartley, who discovered the comet, said he never imagined a spacecraft would get so close to his namesake find.

"When I saw the comet, it was millions and millions of kilometers away," he said. "I'm extremely excited and feel very privileged. After all, I only discovered it."

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Online:

Comet mission: http://epoxi.umd.edu/