Its interesting to note that all the bodies in the solar system seem to have been impacted at one time or another. This Earth of ours is no exception to that rule. There are a lot of dangerous objects whizzing around and sometimes hitting the Earth. We have been hit by large objects in the past and there is no reason to believe that we won't be hit by large objects again. The Earth has many craters where comets and asteroids have hit it. The asteroid belt exists between Mars and Jupiter and these objects range up to 568 miles in diameter. Ceres is the largest with the 568 mile diameter. The problem with these asteroids is that they don't always stay in their orbits and have been know to have hit the Earth in the past with devastating consequences. The Asteroid Belt puts the Earth at risk enough but there is more danger waiting for us. There are near Earth objects that have been knocked into orbits that send them near us. Want more? There is something called the Kupier Belt, it is beyond Neptune and contains, by best estimate, over 70,000 objects with diameters over 100 km. This is the home of short period comets such as Halley's Comet, they usually repeat their appearance in under 200 years. Think you heard it all now? Not yet, there is also a place called the Ort Cloud which is thought to be about 50,000 times the distance from the Earth to the Sun. The Ort Cloud contains the long term comets, those that take over 200 years to appear. It would only take an object only a couple of miles across to wipe out most or all life on Earth.

Lets look at some of the former impacts on this planet of ours.

The Manicouagan Crater in northern Canada is one of the oldest impact craters known. Formed during a surely tremendous impact about 200 million years ago, the present day terrain supports a 70-kilometer diameter hydroelectric reservoir in the telltale form of an annular lake. The crater itself has been worn away by the passing of glaciers and other erosional processes. Still, the hard rock at the impact site has preserved much of the complex impact structure and so allows scientists a leading case to help understand large impact features on Earth and other Solar System bodies. Also visible above is the vertical fin of the Space Shuttle Columbia from which the picture was taken in 1983.

What happens when a meteor hits the ground? Usually nothing much, as most meteors are small, and indentations they make are soon eroded away. 49,000 years ago, however, a large meteor created Barringer Meteor Crater in Arizona, pictured above. Barringer is over a kilometer across. In 1920, it was the first feature on Earth to be recognized as an impact crater. Today, over 100 terrestrial impact craters have been identified. Recent computer modeling now indicates how some of the Canyon Diablo impactor melted during the impact that created Barringer.

The shape of this twenty-eight-mile-diameter (forty-five km) crater is fairly obvious. Kara-Kul Lake has formed inside the crater, which is less than 5 million years old.

This 2-billion-year-old crater is the oldest known crater on Earth. One estimate of its original diameter is 187 miles (300 km), which would make it one of the largest craters on the planet. Other estimates place it at only 87 miles (140 km).

This 142-million-year-old crater has a diameter of almost fourteen miles (twenty-two km). Like many craters on Earth, it is misleading. The raised ring that is clearly visible to the left is not the crater rim. It is an erosional remnant. The remains of the actual rim are found farther out from the ring.

Most circular features in Tibesti, Chad are volcanoes, but there is one exception, the Aorounga impact crater in the windstreaks southeast of Emi Koussi volcano.

This space radar image shows the Roter Kamm impact crater in southwest Namibia. The crater rim is seen in the lower center of the image as a radar-bright, circular feature. Geologists believe the crater was formed by a meteorite that collided with Earth approximately 5 million years ago.

This image shows Gosses Bluff, an impact crater sandwiched between the MacDonnell Range to the north and the James Range to the south in Australia's Northern Territory -- it is about 160 kilometers (99 miles) west of Alice Springs. It is one of the most studied of the Australian impact craters.

There are many more craters on this planet of ours, over 100 is the stated amount but remember that this planet is mostly water and there may be hundreds more under the ocean, but I think you get the point. These objects become dangerous at approximately 10 meters across, but it also depends on their makeup. If an object is composed or iron. and 10 meters across, a lot of its pieces will reach the ground. In 1908 the explosion at Tunguska in Siberia was believed to have been caused by an object about 100 meters in diameter. The meteor crater in northern Arizona, which is 4000 feet in diameter and 600 feet deep is believed to have been caused by an object 60 meters in diameter that had the energy equivalent of 15,000,000 tons of TNT. Fifteen million years ago a 1500-meter (5000 feet) asteroid or comet hit, excavating more than a trillion tons of material and scattering it all over Europe. There are now more than 150 asteroids known that come nearer to the Sun than the outermost point of Earth's orbit. These range in diameter from a few meters to about 8 kilometers.