They think that about , million comets orbit the Sun. Some comets orbit the Sun like planets. Their orbits take them very close to and very far away from the Sun. A comet is made of dirty ice, dust, and gas. When a comet gets close to the Sun, part of the ice starts to melt. The solar winds then push the dust and gas released by the melting ice away from the comet.
This forms the comet's tail. Every time a comet comes close to the Sun, a part of it melts. Over time, it will completely disappear. But when it approaches the sun, the heat evaporates the comet's gases, causing it to emit dust and microparticles electrons and ions.
These materials form a tail whose flow is affected by the sun's radiation pressure. We can observe two types of comet tail that reflect the light spectrum differently.
One is a plasma trail, which draws a straight line like a broomstick. The other is a dust tail, which opens like the bristles on a broom. The plasma tail comprises electrons and ions that are ionized by the sun's ultraviolet radiation. The dust tail consists of micrometer-scale particles. The dust tail is wide and slightly bent because of the pressure of the light from the sun and the orbital action of the comet's nucleus. Another term is "radiation pressure. The momentum change acts as a pressure and you will remember that light also acts dynamically.
Photons have no mass weight , but because they have the characteristics of waves, they also have momentum. The force can be quite significant. The radiation pressure of sunlight on one square meter of the earth is as strong as that from accelerating a one-gram object at seven millimeters per second.
In reality, there is little affect on the earth's surface because of air friction and gravity. But the impact on dust particles in space can be massive. So, the concept has been proposed of fabricating a "photon rocket" that would be propelled using this force.
Comet periods typically range from a few years to millions of years. Where do comets come from? Comets are divided into short-period comets and long-period comets. Short period comets — such as Comet Halley — revolve around our Sun in orbits that take less than years. Their orbital paths are close to the same plane of orbit as Earth and the other planets, and they orbit our Sun in the same direction as the planets. Based on these orbital characteristics, short-period comets are believed to originate in the Kuiper belt , a disk-shaped region extending beyond Neptune.
The Kuiper belt contains small, icy planetary bodies, only a few of which have been imaged. Occasionally the orbit of a Kuiper belt object will be disturbed by the interactions of the giant planets in such a way that it will have a close encounter with Neptune and either be flung out of the solar system or pushed into an orbit within our solar system. Their orbital path is random in terms of direction and plane of orbit. Based on calculations from their observed paths, long-period comets are believed to originate in the Oort cloud.
The Oort cloud is a spherical envelope that may extend 30 trillion kilometers approximately 20 trillion miles beyond our Sun. Oort cloud objects have never been imaged. As a comet approaches our Sun, it begins to heat up and the ice begins to sublimate — to change from a solid to a gas with no liquid stage. Some of the dust is left behind as the ice sublimates. It forms a dark, protective crust on the surface of the nucleus and slows the melting.
In some places the protective layer is thinner, and jets of gas break through. The gas and dust form the cloud of the coma. Our Sun emits a solar wind, a constant flow of gas and particles mostly protons and electrons that streams outward at kilometers about miles per second.
Sunlight and solar wind sweep the dust and gas of the coma into trailing tails. Because sunlight and solar wind always flow outward from our Sun's surface, the tails always point away from our Sun no matter what direction the comet is moving in its orbit.
This means that the tails can be in front of the comet as the comet moves away from our Sun on its return to the outer part of its orbit.
Two distinct tails develop — the plasma tail and the dust tail. The different shapes and angles of the tails are caused by the way different particles are affected by our Sun. The thinner, longer plasma tail forms a straight line extending from the comet.
The particles in this ion tail are electrically charged and are pushed away from our Sun by the solar wind. The shorter dust tail is curved slightly. The larger particles in the dust tail do not have an electric charge and are not affected by the solar wind. Comet tails get longer and more impressive as the comet gets closer to our Sun. As the comet approaches our Sun, it gets hotter and material is released more rapidly, producing a larger tail.
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