Bob Doyle, Columnist
On Feb. 15 about 9:20 a.m., there was a brilliant fireball seen prominently over the city of Chelyabinsk, a city near the Ural Mountains in Russia. (The Ural mountains separate Europe from Asia.) Chelyabinsk is 930 miles to the East of Moscow.
NASA estimates that a small asteroid of mass 10,000 tons travelling at 12.4 miles per second or 44,750 miles per hour entered the upper atmosphere.
The asteroid was about 15 meters (about 50 feet) across. After 30 seconds of travel through the atmosphere, the asteroid broke up 12 to 15 miles above the Earth’s surface.
To the people of Chelyabinsk, the fireball outshone the sun (then visible in the clear sky). It was a splendid sight as many viewed the fireball through their windows.
Then the shock wave reached Chelyabinsk, blowing the windows inward. About a thousand people were injured by flying glass.
From the asteroid’s estimated mass, its velocity upon entering the atmosphere and formula for kinetic energy, the energy given off was comparable to 500 kilotons or 500,000 tons of TNT. The 1945 atomic bombs dropped on Japan each had an energy of about 20 kilotons, so the Chelyabinsk fireball was equivalent to 25 atomic bomb blasts.
Damage from the impact was noted as far away as Lithuania, about 1,500 miles from Chelyabinsk.
This impact could have been much worse as most of the energy was released in the stratosphere, not at the ground. It was fortunate that the asteroid entered the Earth’s atmosphere at a grazing angle.
There is enough information about the asteroid’s path to determine that the far side of its orbit was in the asteroid belt, between the orbits of Mars and Jupiter.
The last big impact to the Earth was in 1947 in Eastern Siberia. On Feb. 12 at 10:30 a.m., a mini-asteroid travelling at 8.7 miles/second or 31,300 mph descended towards Earth at a 41 degree angle. The object broke apart at 3.5 miles altitude.
Before entering our atmosphere, this object was estimated to have a mass of 100 tons and was comparable to an SUV in dimensions. The energy of impact was equivalent to 2,500 tons of TNT. About 50,000 pounds of meteoric debris was recovered.
A smoke trail 20 miles long persisted for over an hour. The sound of the impact carried 300 km or 190 miles.
The biggest Earth impact of the 20th century occurred about 7:30 a.m. on June 30, 1908 near the Stony Tunguska river in Central Siberia. An asteroid was on collision course with the Earth but it broke apart about five miles high.
The energy of this impact was estimated to be about 10 million tons of TNT (This is the explosive yield of a hydrogen or fusion bomb, never used in warfare.)
The blast leveled 800 square miles of forests, killing many reindeer but with no known human casualties. A Tungu tribesman many miles away was knocked off his feet by the shock wave.
The site was not scientifically investigated until the mid 1920s due to the political turmoil in Russia. There has been no asteroidal material recovered from the blast area. The scientific consensus is that the object was a low density asteroid, lacking in metals.
Why have these impacts all taken place in Russia? Russia’s area is 6.2 million square miles, over 10 percent of earth’s land area, but only 3 percent of the Earth’s surface area. The impacts should be random, but Russia recently has had more than its share.
An impact in the ocean could trigger a tsunami that would affect far more people than the above impacts. A big tsunami in the Pacific could devastate coastal cities in Oceania, the America’s, Asia and Australia. (At least there would likely be warnings.)
What can be done to learn when significant objects may hit Earth? NASA’s survey of Near Earth Objects estimates that it has found 95 percent of all asteroids larger than 1,000 meters across that may hit us.
The asteroid that flew by the Earth on Feb. 15 at a distance of 17,000 miles from us was about 45 meters or about 150 feet wide.
With better telescopes (some in space), a more intensive survey may detect a majority of objects 100 meters across (size of a football field). But objects like the Chelyabinsk object (45 meters wide) could slip through the sieve.
It is estimated that every day, a basketball size meteor (with a mass comparable to a human) hits the Earth. And every month or two, a meteor the size of a small car will hit Earth. Every day, about 80 tons of meteoric material falls on the Earth, mostly as dust.
SKY SIGHTS AHEAD: The Moon will be full tomorrow evening, shining to the south of Leo, the Lion. The planet Jupiter dominates the western sky, appearing very high in the early evening and dropping low in the west after midnight.
The planet Saturn can be seen low in the east around midnight. Orion is striking in the south with his three-star belt that points left and down to Sirius, the night’s brightest star. His three-star belt points up and to the right to Jupiter.
Bob Doyle invites any readers comments and questions. E-mail him at firstname.lastname@example.org . He is available as a speaker on his column topics.