Atmospheric Shock Waves

The effect that an asteroid or meteor has on the Earth's atmosphere depends almost completely on the size of the object. Weak meteors that are up to about 30-90 feet (10-30 m) in diameter usually break up into fragments and completely burn up in the atmosphere before they hit the Earth's surface. The height in the atmosphere that these meteors break up depends on the strength of the meteor body, with most comets and carbonaceous chondrites of this size breaking up above 19 miles (30 km). Stronger meteors, such as irons, in this size range may make it to the surface.

Meteors and comets that enter the Earth's atmosphere typically are traveling with a velocity of about six miles/second (10 km/sec), or 21,600 miles per hour (37,754 km/hr). Small meteoroids enter the upper atmosphere every day; more rarely, large ones enter and compress and heat the air in front of them as they race toward the surface. This heat causes most of these bodies to burn up or explode before they reach the surface, with blasts the size of the Hiroshima or Nagasaki atomic bombs happening daily somewhere in the upper atmosphere, by meteors that are about 30 feet (10 m) in diameter. Larger meteors explode closer to the surface and can generate huge was heard. I was thrown to the ground about three sajenes [about 23 feet, or 7 m] away from the porch and for a moment I lost consciousness. . . . The crash was followed by noise like stones falling from the sky, or guns firing. The earth trembled, and when I lay on the ground I covered my head because I was afraid that stones might hit it.

Local Inuit people reported the blast, as in the following testimony from Chuchan of the Shanyagir Tribe, recorded by ethnographer I. M. Suslov in 1926:

We had a hut by the river with my brother Chekaren. We were sleeping. Suddenly we both woke up at the same time. Somebody shoved us. We heard whistling and felt strong wind. Chekaren said, "Can you hear all those birds flying overhead?" We were both in the hut, couldn't see what was going on outside. Suddenly, I got shoved again, this time so hard I fell into the fire. I got scared. Chekaren got scared too. We started crying out for father, mother, brother, but no one answered. There was noise beyond the hut, we could hear trees falling down. Chek-aren and I got out of our sleeping bags and wanted to run out, but then the thunder struck. This was the first thunder. The Earth began to move and rock, wind hit our hut and knocked it over. My body was pushed down by sticks, but my head was in the clear. Then I saw a wonder: trees were falling, the branches were on fire, it became mighty bright, how can I say this, as if there was a second sun, my eyes were hurting, I even closed them. It was like what the Russians call lightning. And immediately there was a loud thunderclap. This was the second thunder. The morning was sunny, there were no clouds, our Sun was shining brightly as usual, and suddenly there came a second one!

Chekaren and I had some difficulty getting out from under the remains of our hut. Then we saw that above, but in a different place, there was another flash, and loud thunder came. This was the third thunder strike. Wind came again, knocked us off our feet, struck against the fallen trees.

We looked at the fallen trees, watched the tree tops get snapped off, watched the fires. Suddenly Chekaren yelled "Look up" and pointed with his hand. I looked there and saw another flash, and it made another thunder. But the noise was less than before. This was the fourth strike, like normal thunder.

Now I remember well there was also one more thunder strike, but it was small, and somewhere far away, where the Sun goes to sleep.

All of these observers report a generally similar sequence of events from different perspectives. The bolide formed a giant, columnlike fireball that moved from southeast to northwest across the Siberian sky, then exploded in several pieces high above the ground surface, with audible to deafening sounds, and scorching to noticeable heat. The explosions were followed by the air blast that moved down in the center of the area, then outward toward the edges of the blast zone. These eyewitness accounts provide scientists with some of the best observation of airbursts of this magnitude and serve as a valuable lesson in the behavior of meteorites or asteroids that explode before hitting the surface.

FURTHER READING

Elkens-Tanton, Linda T. Asteroids, Meteorites, and Comets. New York: Facts On File, 2006.

Kusky, T. M. Asteroids and Meteorites: Catastrophic Collisions with Earth. New York: Facts On File, 2009.

air blasts like the explosion that leveled thousands of square miles (km) of trees in Siberia in 1908.

The flux of meteoroids of different sizes is calculated in part by comparing crater density on the Moon with the expected result in the higher gravity field of the Earth. Larger bodies that make it though the atmosphere hit with a greater frequency for the small objects, and less often for the larger bodies. Meteors in the 30-foot (10-m) diameter range release about as much energy as the nuclear bomb that was dropped on Hiroshima (0.01 megaton [9,070 tonnes]) of TNT equivalent) when they enter the atmosphere and burn before hitting the surface. Events of this size happen about one time per year on Earth, whereas larger events in the 1 megaton (1 megaton = 1,000,000 tons) range occur about once a century, associated with the burning up of 100 foot (30 m) diameter bodies as they plunge through the atmosphere as shown by the table "Effects of Impacts as a Function of Energy and Crater Size."

A moderate-sized impact event, such as a collision with a meteorite with a 5-10 mile (8-16 km) diameter, moving at a moderate velocity of 7.5 miles/ sec (12 km/sec), would release energy equivalent to 100 megatons, or about 1,000 times the yield of all existing nuclear weapons on Earth. The meteor-oid would begin to glow brightly as it approached Earth, encountering the outer atmosphere. As this body entered the atmosphere, it would create a huge fireball that would crash with the Earth after about 10 seconds. Events of this magnitude happen about

EFFECTS OF IMPACTS AS A FUNCTION OF ENERGY AND CRATER SIZE

Energy of Impact (Megatons)

Diameter of Meteorite or Comet

Crater Diameter Miles (km)

Consequences

< 10

Detonation of stones and comets in upper atmosphere. Irons penetrate to surface

101-102

245 feet (75 m)

1 (1.5)

Irons form craters (Meteor Crater), Stones produce airbursts (Tunguska). Land impacts destroy an area the size of a city (Washington, Paris)

102-103

525 feet (160 m)

1.9 (3)

Irons and stones produce ground-bursts, comets produce airbursts. Land impacts destroy an area the size of a large urban area (New York, Cairo)

103-104

1,150 feet (350 m)

3.7 (6)

Impacts on land produce craters. Ocean impacts produce signi cant tsunami. Land impacts destroy area size of small state (Delaware, Israel)

104-105

0.43 miles (0.7 km)

7.5 (12)

Tsunamis reach oceanic scales, exceeding damage from land impacts. Land impacts destroy area size of a moderate state (Virginia, Taiwan)

105-106

1.06 miles (1.7 km)

18.7 (30)

Land impacts raise enough dust to affect climate, and freeze crops. Ocean impacts generate hemispheric tsunamis. Global destruction of ozone. Land impacts destroy area size of large state (California, France)

106-107

1.9 miles (3 km)

37 (60)

Both land and ocean impacts raise dust, impact ejecta is global, changes global climate. Widespread res. Land impacts destroy area size of a large nation (Mexico, India)

107-108

4.3 miles (7 km)

78 (125)

Global conflagration, prolonged climate effects, probably mass extinction. Direct destruction of continental scale area (United States, Australia)

108-109

10 miles (16 km)

155 (250)

Large mass extinction (K-T in scale)

109-1010

Survival of all life threatened

* Note: table based on David Morrison, Clark Chapman, and Paul Slovic (The Impact Hazard, 1994).

* Note: table based on David Morrison, Clark Chapman, and Paul Slovic (The Impact Hazard, 1994).

once every 1,000 years on Earth, but obviously even in this energy range, impacts are not posing serious threats to the survival of life on Earth.

Objects that break up lower than 12 miles (20 km) above the surface cause much greater destruction. Objects about 165 feet (50 m) in diameter that break up at this height will generate significant air-bursts that pose significant hazards. Larger objects will strike the ground, releasing energy in a manner similar to atomic bomb blasts (but without releasing radioactivity), with the amount of energy proportional to the size of the meteorite or comet. The impact that hit Tunguska in 1908 is estimated to have released about 10-20 megatons (9.1-18 megatonnes), with a radius of complete destruction of 15 miles (25 km), and a much larger affected area. The area of destruction increases to the two-thirds power of the magnitude of the blast.

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