Ten Things You Don't Know About Comets

By Phil Plait | April 20, 2010 6:00 am




Comets can have two tails


I’ve been talking about the tail of a comet, but actually they can have more than one! It’s common to see two tails from a comet, like in the picture above of Comet Hale-Bopp. As gas sublimates from the head of the comet, the solar wind — a stream of charged particles streaming off the Sun — can blow it back. The solar wind is very high velocity, much faster than a comet moves, so that tail blows straight off the comet. The gases get ionized by the wind; that is, the electrons in their atoms get stripped off. The ions are then affected strongly by the magnetic field of the solar wind, which sweeps them up and drags them along. The blue color of this ion tail is due to carbon monoxide. which scatters blue light toward us (similar to why the sky is blue). As electrons recombine with the ions they give off light, causing the tail to glow predominantly blue (though other colors are present due to other elements and molecules as well).

Mixed in with the gas blowing off the comet is also dust: ground up silicates, minerals, and other more stable substances. This material is denser and not as subject to the solar wind. It has a yellowish or reddish color because it reflects sunlight. This tail can curve, following the comet’s path itself. In many pictures of comets, you can see the blue ion tail screaming straight out from the comet head, while the yellower dust tail gently curves, sometimes for millions of kilometers.

In reality, comets can have lots of tails. The dust tail can be broken up into several straight features called dust striae, the cause of which is unknown but most likely due to large (well, boulder-sized) pieces of the comet breaking off and forming their own tails. Some comets have sported as many as a half-dozen tails!



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