|MadSci Network: Astronomy|
Hello Garrett, A comet is made up of many tiny dust particles covered with ice that stuck together when the Solar System was forming. The comets reside in either the Kuiper Belt, just outside the orbit of Neptune, or the Oort Cloud, which is a spherical distribution of comets as far as 100,000 AU away (an AU is an Astronomical Unit which is equal to the distance between the Earth and the Sun). At these distances, it is very cold and comets do not release any gases. As a comet approaches the Sun, the ices warm until they reach the temperature at which they evaporate (Note: the ices do not melt because the pressure is too low. They go directly from the solid ice to a gas). When this happens, not only the gas, but the dust is also removed from the comet because the particles are only loosely held together. This is where the Sun comes in. The Sun has what is known as the solar wind -- a low density stream of charged particles, mostly protons and electrons, that travel outward at 450 km/s (or about 2,600,000 miles per hour). These particles hit the gas and dust released by a comet and push them outward. Gas molecules are very light so the solar wind pushes them straight away from the Sun to form the ion tail. The dust particles are massive compared to the gas but the solar wind also pushes them away to form the dust tail, which is curved but still points away from the Sun. Even when the comet is moving away from the Sun, the solar wind is still pushing the dust outward so the tail is still pointing away from the Sun. Here is a mental exercise that I hope will help you to visualize what is going on. If you stand in a wind tunnel facing into the wind, your hair (for this exercise, imagine you have long hair) will be blown behind you. As you walk toward the fan, your hair will continue to be blown behind you. If you turn around, your hair is now blowing in front of you, but still away from the fan. Even if you walk away, your hair will be blowing in front of you because the force of the wind is strong enough to push your hair forward. Your hair does not change direction even though you do. The solar wind plays the same role in space as the wind in the tunnel and hair represents the comet tail. Erika For more solar system facts, visit http://www.seds.org/nineplanets/nineplanets/
Try the links in the MadSci Library for more information on Astronomy.