Back in 1997, the night sky was made all the more beautiful when this stunning comet made its presence known. The comet itself is magnificent, but what people most notice is the gorgeous, colorful tail that sweeps away behind it. However, the comet's tail is the source of a common misconception that we will be exploring on today's FactRoulette!
Here on Earth, we're used to tails or wakes following behind an object as it moves forward. Note the direction of the trail of dust behind a vehicle as it travels down a dusty country road, or the wake left behind a ship or boat as it travels across the surface of a body of water.
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| The dirt kicked up by this bike follows behind the rider as he travels forward. |
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| The wake follows behind the U.S.S. Enterprise as it sails the high seas. |
However, comets are a little different. We all drew comets as children with the tail following behind it, just as we would expect. However, the truth is a little more complicated. Think of the previously-mentioned car driving on a dirt road. If it's a calm day, the dust follows directly behind the car in the direction of travel. However, if it's a very windy day, and the wind is blowing perpendicular to the direction of travel, the dust will appear to be coming off the car sideways. The same happens to comets.
"But," you ask, perplexed, "there's no air in space. How can there be wind?"
Not the wind we're used to here on Earth, of course. That would be impossible. No, the tail of a comet is formed by something else entirely: solar wind.
It is the radiation and heat from the local star that creates the tail, by melting gases and ice on the comet. Therefore, the tail always points away from the star, not necessarily in the direction the comet is coming from.
However, there is a small wrinkle in all of this. If you look closely at the picture of Hale-Bopp above you will notice that there is not merely one tail, but two. The large, white tail is indeed dust. This tail does in fact curve somewhat as the comet travels through space. It points in the direction away from the star as the solar wind "blows" the debris into space, but is "left behind" as the comet travels along its orbit.
The second, smaller tail is made up of gasses "blown" off the comet by the intense radiation of the star. This tail generally points directly away from the local star, as seen in the diagram below.
So, contrary to what one might expect, the tails you see coming off of a comet do not extend behind the comet, but rather away from the local star (our sun) as the comet is bombarded by the intense radiation and solar wind it puts out!
Photos:
"Comet Hale-Bopp 1995O1" by E. Kolmhofer, H. Raab; Johannes-Kepler-Observatory, Linz, Austria (http://www.sternwarte.at) - Own work. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Comet_Hale-Bopp_1995O1.jpg
"Motorbike rider mono". Licensed under GNU Free Documentation License 1.2 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Motorbike_rider_mono.jpg
"Enterprise Cruising" by DoD photo by Petty Officer 1st Class Todd Cichonowicz, U.S. Navy. (RELEASED) - Source. Licensed under Public domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Enterprise_Cruising.JPG
"Cometorbit". Licensed under Public domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Cometorbit.png
