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Comets, Meteors, Meteorites and Moons

A comet's core is made by ice and dust. It has an orbit around a star and it is usually very long. It can take several thousand years for the comet to return. One of the most famous is the Haley's comet. Its orbit around the sun takes 76 years. It appeared in 1986 and will appear again in 2062. Some comets may have an "open" orbit. They will pass the sun once, and then disappear in space forever. A comet isn't particularly big and the whole comet usually hasn't a diameter more than a few thousand kilometres. But when it approaches the sun, the gases it possesses will heat up, vaporize and create a big envelope around the comet. This envelope is called coma. The suns energy radiation, called solar wind, repeals the material from the comet. This creates a tail. The tail is made by dust and gas and it is always pointing away from the sun. When the comet eventually is far from the sun, it will freeze again and the tail disappears. Once again the comet is just a dot object.

Meteors are small heavenly bodies made up by stones or metal and they orbit the sun. Their mass is between a few grams and up to several tons. But usually you can't see a meteor until it enters the earth's atmosphere, but then it is on fire. We see it as a glowing stripe on the sky. This phenomenon is called a meteor, or mistakenly called: a shooting star. Bigger meteors don't burn up when they enter the atmosphere. They hit the surface and are called meteorites. When a big meteorite collides with the surface, a crater appears. One of the most famous craters is in Arizona. The crater has a depth of 200 meters and a diameter of about 1,200 meters. This crater was made about 50,000 years ago and there has been a great amount of iron found there.

Our moon is a heavenly body that has an orbit around earth. The moon rotates around its axis, and because of that, we only see one side of it. Because the moon has no atmosphere, it has no defense against meteors. Therefore the moon has a lot of craters. The diameter of the moon is about ¼ of the earths and the distance is about 380,000 kilometres. The highest mountain is more than 8,000 meters high and the largest crater has a diameter of 200 kilometres. The temperature on the moon is between -160°C and +110°C. The moons gravity is only 1/6 of that of the earth. If you can lift an object that weighs 50 kilogram on earth, you can lift 300 kilograms on the moon. The moon has different phases, like half moon and full moon. When the moon is in front of earth, we can't see it. Then when it slowly moves behind the planet, it will show a full moon. Afterward it will move in front of earth and continue that cycle. It takes a month for the moon to finish this cycle.

There are more than 92 satellites in our solar system, and more may be found. Satellites can be like our moon, just a big rock, or have special environment. Jupiter's moons Io and Europa are good examples of this. Io's surface contains a lot of volcanoes, and some of them are even active. Europa has a thin layer of ice on its surface. It is possible that beneath Europa's surface there is liquid water.

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