6degrees Astroblog
A Blast From the Past
By Irwin Horowitz, 6-30-08
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The morning of June 30th, 1908 dawned bright and clear in the heart of Siberia near the Stony Tunguska River. The nomadic herdsmen were tending their flocks of reindeer which were busy grazing and foraging. Traders at the post at Vanavara were just starting their daily bartering.
Unknown to them there was a small piece of an asteroid in interplanetary space which was on its final trajectory in their direction. No more than a few dozen meters in diameter, it nevertheless still packed an enormous wallop due to its high relative velocity with the planet in its orbital path.
Shortly after 7 a.m. local time, this chunk of space debris began to encounter the thin upper reaches of our atmosphere. A shock wave started to form at the interface between the leading edge of the rock and the rarefied air. Temperatures and pressures in this region rose to extreme levels as the meteor bore down into the thicker stratosphere. Within moments, the pressure became so great that it completely disrupted this cosmic trespasser resulting in a massive explosion.
A witness located in Kirensk, 400 kilometers distant, reported seeing
“A ball of fire...coming down obliquely. A few minutes later [we heard] separate deafening crash like peals of thunder ... followed by eight loud bangs like gunshots.” After the explosion, they observed “a cloud of smoke rising from the ground.”
The traders at Vanavara, 60 kilometers south of “ground zero” recalled the event thusly:
“I was sitting on the porch of the house at the trading station, looking north. Suddenly in the north...the sky was split in two, and high above the forest the whole northern part of the sky appeared covered with fire. I felt a great heat, as if my shirt had caught fire… At that moment there was a bang in the sky, and a mighty crash… I was thrown twenty feet from the porch and lost consciousness for a moment.... The crash was followed by a noise like stones falling from the sky, or guns firing. The earth trembled.... At the moment when the sky opened, a hot wind, as if from a cannon, blew past the huts from the north. It damaged the onion plants. Later, we found that many panes in the windows had been blown out and the iron hasp in the barn door had been broken.”
Another witness at the post said:
“I saw the sky in the north open to the ground and fire poured out. The fire was brighter than the sun. We were terrified, but the sky closed again and immediately afterward, bangs like gunshots were heard. We thought stones were falling… I ran with my head down and covered, because I was afraid stones may fall on it.”
The shock generated by this explosion was powerful enough to cause seismic tremors recorded at stations all across Europe and Asia. The atmospheric blast packed a sufficient punch to register on barometers in London, England nearly 6000 kilometers distant. Dust that was kicked up in the blast caused a bright glow visible at night for several weeks around the world.
The unfortunate reindeer located within 20 kilometers of ground zero did not survive. Neither did the magnificent forest in the area. Decades after the blast, the first official scientific expeditions that were sent to the area by the Soviet Academy discovered that millions of trees had been blown over such that they were all pointing away from the center of the explosion. However, they did not discover any impact crater or meteorite fragments.
What was it?
How do we know that the interloper was a small piece of an asteroid rather than something else? First, we don’t know with absolute certainty that this was the source of the explosion. However, it is by far the most likely explanation. Evidence which supports this conjecture includes the discovery of tiny particles trapped in resin from the time of the event which contain mineral compositions that are similar to those found in stony asteroids rather than comets. In addition, a comet of the given size would lack the structural integrity to have survived deep into the atmosphere but instead would have broken up at a much higher altitude.
The main problem with the asteroid origin theory is the lack of an impact crater on the surface. However, some scientists have proposed an explanation whereby the body would break apart when atmospheric forces exceeded the cohesive forces holding it together, resulting in the observed explosion.
Subsequent analyses of the blast region suggest that it occurred about 5 miles (8 kilometers) above the surface and released about 15 megatons TNT equivalent energy. This is roughly 1000 times more powerful than the atomic bomb that destroyed Hiroshima at the end of World War II.
Statistical arguments suggest that events like Tunguska occur once every few centuries. Less powerful cosmic impacts occur on a more frequent basis. Indeed, we average about one Hiroshima-sized explosion every year! The reason we rarely note such events is that they generally occur high up in the atmosphere or over large bodies of water or other uninhabited regions of the planet.
Of course, more powerful impacts occur on longer time scales. Some of them have been so massive as to be the source of mass extinction events long ago. The best known example was an asteroid about 10 km in diameter that struck the Earth about 65 million years ago in an area just off the coast of the Yucatan peninsula in Mexico. This impact led directly to the demise of the dinosaurs and indirectly to the rise of mammals as the dominant life form on the planet.
Another impact event that occurred about 50,000 years ago left a massive scar in the Arizona desert. Barringer Meteor Crater was formed when an object composed of nickel and iron about ½ the size of a football field, slammed into the ground at a velocity of about 12 km/sec (28,600 mph). It left behind a hole over one mile wide and nearly 600 feet deep.
More recently, we had a front row seat to an impact that dwarfed the event that killed the dinosaurs. Fortunately, we were about 500 million miles away from the explosions that resulted when comet Shoemaker-Levy 9 struck Jupiter in 22 separate impacts over the course of six days in the summer of 1994.
Can we avoid or mitigate the effects of the next large impact?
Currently there are several programs around the world designed to detect objects at least 1 kilometer in diameter that could pose a potential threat to our planet. By determining their orbital elements from multiple observations, scientists can predict whether each body could make a close approach to our world. Such objects are tagged for extensive monitoring so as to refine their orbits and determine the probabilities of an impact at some point in the future.
Once we’ve identified possible impactors, we then need to develop a plan on how to prevent such an occurrence. Two possible solutions are to disrupt the object while still in space (for instance by using nuclear devices) or to alter its orbital parameters to avoid a collision. It is not presently known whether either solution would be successful if we had to implement them in the near future.
Note that of the over 200 objects currently listed as potential hazards, only one merits a ranking of one (out of ten) on the Torino Impact Hazard Scale. All of the remaining object are either too small (<50 meters) to merit much risk or are extremely unlikely to cross paths with our planet.
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