MadSci Network: Astronomy

Re: If a comet exits the solar system backwards, why doesn't it's tail reverse

Date: Tue Sep 18 20:58:36 2001
Posted By: Erika Gibb, Grad student, Physics & Astronomy/Origins of Life, RPI
Area of science: Astronomy
ID: 1000821757.As

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.


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