Target Earth

Most meteors burn up in the Earth's atmosphere but some larger ones do in fact make it through. Sometimes damages from meteorites do occur. So far, more than 130 cases have been confirmed where meteorites caused death, injury or damages. This isn't so surprising when some meteors are traveling at over 100,000 miles per hour!

In the 1600's a monk was reportedly struck and killed by a meteorite. In 1490, Shasni Province, China, tens of thousands of people were reported killed by a rain of stones the size of eggs. This story though, has not been confirmed. In 1912, some 14,000 meteorites rained down on Holbrook, Arizona.

During the Draconid meteor shower in October of 1992 a meteorite bounced off the hood of a woman's car in upstate New York.

More recently, in 1954, a meteorite slammed through the roof of a house in Alabama. The meteorite continued through the second story floor, bounced off a chair, and landed on a woman resting on a couch! The woman was injured but fortunately, not seriously.

Sometimes larger bodies target Earth and have much more spectacular results. These are very likely asteroids or comets but Earth is scarred in many places with the damage they have done. Such an event happened near the Tunguska River in Siberia.

People riding the Trans-Siberian railroad spotted a huge fireball as bright as the Sun on June 30th, 1908. The object apparently exploded above the ground near the river, flattening forests for over one thousand square miles! The fireball was seen for over a thousand miles and sonic booms were heard for over 600 miles away. There were numerous injuries, some as far away as thirty miles.

Meteorites, and their parent asteroid bodies or comets, may have a major impact on the outcome of life on Earth. About 65 million years ago there was a sudden mass extinction of ocean plants and large land animals, most notably the dinosaurs. In only a few thousand to a million years (a short time in geology) the fossil record shows an abrupt end to the large dinosaurs. This was actually advantageous for us mammals, but it was a nasty end for the ones who perished.

Why are we so sure it was a celestial impact which ended the dinosaurs reign? The evidence is in the rocks themselves, in a layer of iridium which appears almost all over the world and is dated to about 65 million years ago. Iridium is quite rare on Earth, but common in comets and asteroids. It is likely then, that there was an impact at that time.

What would it have been like? We estimate the object to have been about ten km in diameter. It would have been traveling at about eleven km/s when it hit, making our hydrogen bombs look like firecrackers. Temperatures at the impact point would have reached 20,000 K. The impact would have sent a large amount of gas and dust particles up into the atmosphere. Most sunlight would have been blocked from reaching Earth for over a month. This would have killed many plants, and whatever animals which depended on them.

Scientists think that the object which caused the mass extinction of the dinosaurs, left behind a huge crater in the Yucatan, Mexico. It is called the Chicxulub crater and has a diameter of about 180 km. The crater itself has dated to an age of 65 million years.

There are many other craters around Earth, one of the best known, and certainly the best preserved is Barringer Crater, Arizona. This crater is so well preserved because it is a more recent impact, only about 25,000 years old. The energy released with such an impact is on the scale of a 3 megaton hydrogen bomb, digging a crater 1.2 km in diameter.

Recent evidence suggests that mass extinctions take place about every 26 million years. Over one hundred craters on Earth have been located and identified.

Then, in January of 1991 we had a celestial close call when one Earth orbit crossing asteroid whizzed by Earth with only 170,000 km of Earth! That is less than half the distance between the Earth and Moon. With events such as this in mind, NASA began tracking Near Earth Objects. Though they have identified many larger Earth crossing asteroids, there are many more large, and an unknown quantity of boulder sized objects still unidentitied.

Links to other good sites about the Leonids

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