Why comets have tails

This isn't the only comet of importance. Hopefully, in the fall of we will have a super awesome comet to look at -- ISON. It might be the best comet since I don't know when. So let's look at some interesting things about these comet tails. Be warned, I am not an astrophysicist. Instead, I am going to use some fundamental principles to try to explain why comets do what comets do. Oh, sure I could just look this stuff up.

However, speculation is quite entertaining at least for me. Not every comet is the same, but it wouldn't be terrible to say that a comet is a dirty-icy object in the solar system. When they come near the sun, they melt I'm not sure "melt" is the most appropriate term here and produce gas and dust.

The gas and dust form both a coma and a tail or two tails. If the comet is large enough and close enough to Earth, you can see the comet from the sunlight that reflects off this gas and dust. There are two tails because there are two ways the comet can interact with the sun. Everyone thinks about light coming from the sun. However, there is also the solar wind. The solar wind is really just charged particles like electrons and protons that escape from the sun due to their high velocities.

These charged particles then interact with the ionized gas produced from the comet. The other tail is due to an interaction with the dust produced by the comet and the light from the sun. Really, it is this interaction that I want to talk about. Important idea number 1: Matter is made of positive and negative charges. If you have anything with structure like dust particles then it has to have atoms in it. Basically, dust is made of a combination of electrons, protons and neutrons.

That's it. Important idea number 2: Light is an electromagnetic wave. What does this even mean? It can mean lots of things. For this discussion, the important thing is that if you have a region of space moving at the speed of light an electric and magnetic field can move in accordance with a set of rules we call Maxwell's equations.

Here is a typical representation of a sinusoidal EM wave from the awesome textbook Matter and Interactions. The electric field and magnetic field in this light must both be perpendicular to each other and to the direction the wave moves.

That's important. Important idea number 3: If you have a charged particle in an electric field, it will experience a force. For a positive charge, this force will be in the same direction as the electric field. For negative charges, the force is in the opposite direction as the electric field.

In the above diagram, I am using the yellow arrows to represent a region with a constant electric field. The red ball is a positive charge and the blue is a negative charge.

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