About 2 million years ago the Earth and the other planets circling our star, the Sun, passed through the plane of our galaxy, the Milky Way.
This crossing takes place about every 26 million years, and when this has happened in the past, some of this debris has hit the Earth.
The Geological Survey of Canada
created this map of large craters on Earth.
You can click on the map to see photographs of the craters .
These pictures were taken by the Calar Alto Observatory in Spain.
While it looks small in the pictures, Jupiter is a very big planet
and this explosion was as wide as the Earth!
You can click on Jupiter to see more pictures of the explosions .
The National Space Society is the largest group in the United States working to make sure that NASA gets the money it needs. You can visit the National Space Society by clicking here , or click on the dinosaur to learn more about their magazine AD ASTRA . The National Space Society wants to make sure that a comet or asteroid does not hit the Earth, and you can read what they think by clicking here .
While the largest pieces of debris strike the Earth only once or twice after our solar system crosses the plane of our galaxy, the Milky Way, the smaller pieces of debris strike the Earth much more often. One of these pieces struck in Tunguska in Siberia in Russia in 1908 and another one struck in the jungle of Brazil in 1931. Our best guess is that one of these small pieces strikes the Earth every 50 to 100 years on average. The explosions caused when these small pieces hit are as big as those that were caused by large hydrogen bombs, and these were as powerful as the explosion of 10's of megatons of TNT. (By way of comparison, 9 M-80 firecrackers altogether contains only 1 ounce of explosive: there are 16 ounces to a pound, and 2,000 pounds to the ton, and 1,000,000 tons to the megaton. That's a lot of M-80's!)
While these small pieces are only around 30 to 150 feet across, they can cause big explosions because they are moving very fast, about 26 miles per second. Depending on whether they are traveling with the Earth in the direction of its movement around the Sun or against it, they will hit the Earth at a speed between 8 to 44 miles per second. (By way of comparison, there are 60 seconds in a minute, and 60 minutes in an hour. So 44 miles per second is a lot of miles per hour!)
Just above and to the right of the center of this picture you can see
a circle where the trees in the jungle have still not grown back.
The circle is many miles across.
It is likely that these pieces are much too small to be seen from the Earth by radar or by telescope, but no one knows that for sure right now. No one knows, because right now very few people are being paid to try to find them: fewer people, in fact, than the number of people who work at your local McDonald's, Burger King, or Pizza Hut!
Most of the people who are looking for these small pieces of debris expect that no one else will be hired to look for them until one of these small pieces hits a city and kills many people, instead of hitting in the jungle or in a place where very few people live. When this happens, men (and women) will probably return to the Moon, because a telescope or radar built there would be able to see these small pieces.
This NASA picture by Pat Rawlings shows what a radar station on the Moon might look like.
This picture by the Japanese Space Agency NASDA shows what a telescope on the moon might look like. You can click here to visit NASDA or click on the picture to learn more about what they think about a lunar telescope.
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