Meteorite Hits

January 19, 2012 by staff 

Meteorite Hits, An impact event is the collision of a large meteorite, asteroid, comet, or other celestial object with the Earth or another planet. Throughout recorded history, hundreds of minor impact events (and exploding bolides) have been reported, with some occurrences causing deaths, injuries, property damage or other significant localised consequences.

Impact events have been a plot and background element in science fiction since knowledge of real impacts became established in the scientific mainstream.

A bolide undergoing atmospheric entry
Small objects frequently collide with the Earth. There is an inverse relationship between the size of the object and the frequency that such objects hit the earth. The lunar cratering record shows that the frequency of impacts decreases as approximately the cube of the resulting crater’s diameter, which is on average proportional to the diameter of the impactor. Asteroids with a 1 km (0.62 mi) diameter strike the Earth every 500,000 years on average. Large collisions – with 5 km (3 mi) objects – happen approximately once every ten million years. The last known impact of an object of 10 km (6 mi) or more in diameter was at the Cretaceous-Tertiary extinction event 65 million years ago.

Asteroids with diameters of 5 to 10 m (16 to 33 ft) enter the Earth’s atmosphere approximately once per year, with as much energy as Little Boy, the atomic bomb dropped on Hiroshima, approximately 15 kilotonnes of TNT. These ordinarily explode in the upper atmosphere, and most or all of the solids are vaporized. Objects with diameters over 50 m (164 ft) strike the Earth approximately once every thousand years, producing explosions comparable to the one known to have detonated above Tunguska in 1908. At least one known asteroid with a diameter of over 1 km (0.62 mi), (29075) 1950 DA, has a possibility of colliding with Earth on March 16, 2880, but the Torino scale only works for impact possibilities within 100 years, and thus cannot apply to this asteroid.

Objects with diameters smaller than 10 m (33 ft) are called meteoroids (or meteorites if they strike the ground). An estimated 500 meteorites reach the surface each year, but only 5 or 6 of these are typically recovered and made known to scientists.

Earth has gone through periods of abrupt and catastrophic change, some due to the impact of large asteroids and comets on the planet. A few of these impacts may have caused massive climate change and the extinction of large numbers of plant and animal species.

The Moon is widely attributed to a huge impact early in Earth’s history. Impact events earlier in the history of Earth have been credited with creative as well as destructive events; it has been proposed that impacting comets delivered the Earth’s water, and some have suggested that the origins of life may have been influenced by impacting objects by bringing organic chemicals or lifeforms to the Earth’s surface, a theory known as exogenesis.

Eugene Merle Shoemaker was first to prove that meteorite impacts have affected the Earth.
These modified views of the Earth’s history did not emerge until relatively recently, chiefly due to a lack of direct observations and the difficulty in recognizing the signs of an Earth impact because of erosion and weathering. Large-scale terrestrial impacts of the sort that produced the Barringer Crater, locally known as Meteor Crater, northeast of Flagstaff, Arizona, are rare. Instead, it was widely thought that cratering was the result of volcanism: the Barringer Crater, for example, was ascribed to a prehistoric volcanic explosion (not an unreasonable hypothesis, given that the volcanic San Francisco Peaks stand only 30 miles (48 km) to the west). Similarly, the craters on the surface of the Moon were ascribed to volcanism.

It was not until 1903-1905 that the Barringer Crater was correctly identified as being an impact crater, and it was not until as recently as 1963 that research by Eugene Merle Shoemaker conclusively proved this hypothesis. The findings of late 20th-century space exploration and the work of scientists such as Shoemaker demonstrated that impact cratering was by far the most widespread geological process at work on the solar system’s solid bodies. Every surveyed solid body in the solar system was found to be cratered, and there was no reason to believe that the Earth had somehow escaped bombardment from space. In the last few decades of the twentieth century, a large number of highly modified impact craters began to be identified. The largest of these include Vredefort Crater, Sudbury Crater, Chicxulub Crater, and Manicouagan Crater. The first observation of a major impact event occurred in 1994: the collision of the comet Shoemaker-Levy 9 with Jupiter; to date, no such events have been observed on Earth.

Based on crater formation rates determined from the Earth’s closest celestial partner, the Moon, astrogeologists have determined that during the last 600 million years, the Earth has been struck by 60 objects of a diameter of 5 km (3 mi) or more. The smallest of these impactors would release the equivalent of ten million megatons of TNT and leave a crater 95 km (60 mi) across. For comparison, the largest nuclear weapon ever detonated, the Tsar Bomba, had a yield of 50 megatons.

Besides direct effect of asteroid impacts on a planet’s surface topography, global climate and life, recent studies have shown that several consecutive impacts can have effect on the dynamo mechanism at a planet’s core responsible for maintaining the magnetic field of the planet, and can eventually shut down the planet’s magnetic field.

While numerous impact craters have been confirmed on land or in the shallow seas over continental shelves, no impact craters in the deep ocean have been widely accepted by the scientific community. Impacts of projectiles as large as 1 km in diameter are generally thought to explode before reaching the sea floor, but it is unknown what would happen if a much larger impactor struck the deep ocean. The lack of a crater, however, does not mean that an ocean impact would not have dangerous implications for humanity. Some scholars have argued that an impact event in an ocean or sea may create a tsunami (a giant wave), which can cause destruction both at sea and on land along the coast, but this is disputed.

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