At 9:00 a.m. 1st February (JST 11:00 p.m. 1st Feb.), US Space Shuttle Colombia broke apart during re-entry. The accident occurred just 7 minutes after an anomaly was detected. All the seven astronauts on board died in the accident. I would like to express my sincere condolence to them. Investigation is underway and I guess factors to have led to the accident will be revealed in the meantime. However, the fundamental cause of the shuttle's breakup was a sharp rise in temperature caused by friction between the shuttle and atmosphere generated when it entered into atmosphere at high speed.
This time, I wrote about objects, which reach on ground disregarding such resistance of atmosphere.
Image - 1. Satellite image of Meteor Crater (ASTER/VNIR on 17 May, 2001)For enlargement (jpeg file 2.4MB)
This is a simulated natural color image of 10km X 10km centering Meteor Crater. Red color shown in the image shows the effect of weathering and oxidation to the surrounding rocks and soils. A large hole made in the middle of a plain is a mark made by a visitor from space. A visitor can come at any time in any places.
Image -2. Bird's eye view imageFor enlargement (jpeg file 1.5MB)
This is a bird's eye view image from 1,000m above ground. The image was made by synthesis of DEM (Digital Elevation Model) data and simulated natural color image. The image is elongated by 150% vertically for the stereoscopic effect of the crater.
Figure - 1. The location of the image on a map
For objects in space to reach earth's surface, they need to overcome the resistance of atmosphere. Most small objects burn away by frictional heating before reaching on earth. This is a phenomenon familiar to us as a shooting star. Shooting stars disappear on their way to earth. In other words, atmosphere is protecting earth as a barrier.
When an object is small and in a speed low enough to drift in the air, it successfully reaches on the ground with small friction. Such minute objects are called cosmic dust. Sometimes, you can find a great deal of cosmic dust contained in collected dust after sweeping the rooftop of an old building. Although it is too small for us to see, visitors from space have been in existence so close to us. There is an estimate that cosmic dust falling on earth amounts to scores of tons per year. Although it is small one by one, cosmic dust arrives on earth huge in mass.
A large solid object survives the resistance of atmosphere and reaches on ground surface. This is so called a meteorite. Meteorite in Japanese ideographically indicates "a fallen stone from the sky above".
A meteorite can cause a huge impact on ground surface. It is believed that there are more than 300 craters on earth, of which 198 are officially identified as craters.
Among craters I have seen actually, Meteor Crater (also called Barringer Meteorite Crater) was the most impressive one.
Meteor Crater appears all at once in the middle of a vast plain in Arizona State, the US. The name comes from meteorite and it is a crater officially identified for the first time in the world's history.
In 1886, a strangely deformed piece of iron was found in Canyon Diablo, to the west of the crater. By the analysis at a university, it consisted of 91% of iron, 7% of nickel, 0.5% of cobalt, as well as a small amount of platinum and iridium. In 1905, D.M. Barringer reported that the crater was made by iron meteorite collision.
D.M. Barringer, who was an engineer and lawyer, received permission from the government and started mining iron from the crater from 1904. His idea was simple. He thought that since there were huge amount of iron meteorite scattered around the crater, there must have been even greater lumps of iron at the bottom of it. He must have thought of making a fortune from selling it. However, he had never hit on a lump of iron no matter how deep he dug into the crater. After all, there were no iron buried in the crater and it was concluded that an iron meteorite was broken in pieces in times of collision and all of them scattered around the crater, which amounted to about 30 tons all in all.
Diameter of Meteor Crater is 1,186m. Let's estimate the diameter of a meteorite to have formed a crater of the size. Energy of a collision increases in proportion to the mass of an object and to the square value of the speed. When a large object collides on earth at high speed, earth suffers an impact similar to an explosion. Since iron is heavy, even a small lump has a big energy. By calculation, it is estimable that a meteorite of 30m diameter at the highest possible speed or 90m diameter at the lowest possible speed formed Meteorite Crater.
Most craters on earth are found on land area. Despite the fact that erosion, weathering and crustal movements cause craters to disappear in a long time, there are no less than 200 craters identified in the world at present. Additionally, from the fact that the total land area is just one third of the earth's surface and craters have hardly been found from the bottom of seas, the 200 craters represent just a small portion of all the meteorite collisions on earth in its history.
It is believed that a large scaled meteorite collision like the one that formed Meteor Crater occurs once in a millennium with an impact comparable to an earthquake of magnitude 6.7. Since Meteor Crater was formed about 50,000 years ago, meteorite collision of the same scale could have occurred 50 times ever since.
Collisions of small celestial bodies and cosmic dust fall on earth are nothing but ordinary events. These are just another aspects of the solar system, to which our mother earth belongs.
1 March 2003, Yoshiyuki Koide
Picture - 1. A panoramic view of Meteor CraterThis is a composite picture made of three pictures taken from northern cliff. Since the crater is not much affected by weathering or erosion, the tremendous scale of the collision is still evident after 50,000 years.
Ficture - 2. Center of the Meteor CraterIn the center of the Meteor Crater, debris of a building and equipment, as well as footprints of men can be observed clearly. These are the remains of boring carried out by D.M. Barringer in search of iron meteorite, which was a dream never to have been come true. Now, this area is off limits to the public.
Right next to a visitor center, command module of The Apollo Program is exhibited. This is a monument commemorating that the Apollo astronauts exercised training in Meteor Crater simulating crater on the moon.
Picture - 3. Apollo SpaceshipPicture - 4. Iron meteorites
It is a large iron meteorite named Canyon Diablo exhibited in a visitor center. All of the iron meteorite pieces were scattered around the crater, and D.M. Barringer never found one from the bottom of the crater.
Copyright of Image - 1 and 2 belongs to JSS and text and Picture 1 - 4 belongs to Prof. Yoshiyuki Koide of Sapporo Gakuin University. Permission of JSS is needed of their use for other purposes.
