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Re: (meteorobs) P/2000 G1 & Vgeo
----- Original Message -----
From: "Lew Gramer" <dedalus@latrade.com>
To: "Meteor Observing Mailing List" <meteorobs@jovian.com>
Sent: Thursday, June 01, 2000 9:56 AM
Subject: Re: (meteorobs) P/2000 G1 & Vgeo
> Vgeo is a *vector* thingy, and as far as I know the lower limit to
> speed is more to do with terminal velocity than Earth's gravity, ie what
> the minimum speed is that still allows enough friction for burn up to
occur
> in the upper atmosphere. Anybody know better?
There is no such a minimum speed. The friction due to air drag converts the
meteor's kinetic energy into thermal energy; the more kinetic energy lost,
the more thermal energy gained. The temperature increase caused by all that
thermal energy depends on the heat capacity of the meteor, and that depends
on its mass and its specific heat capacity (i.e., on the material it's made
of). For a given amount of thermal energy, the smaller the heat capacity,
the greater the temperature rise.
For example, for every 100 mph it slows, a 100-kg meteor would undergo an
increase in temperature of 4.0 degrees centigrade if it were iron, 2.4
degrees if it were granite, and 0.8 degrees if it were ice. In other words,
it takes five times the thermal (heat) energy to heat ice 1 degree that it
takes to heat iron 1 degree.
For the meteor to burn up in the atmosphere requires that every kilogram of
it be heated to its boiling point temperature (much hotter than its melting
point). That requires a humongous slowing down due to air drag. Of course,
the drag force and resulting deceleration increase as the meteor falls
through the atmosphere because of the increase in air pressure, so the delta
vee increases as it falls. For practical purposes, you can assume that a
meteor which vaporizes (burns up entirely) loses all the velocity it had
before it entered the atmosphere. Of course, if it doesn't lose all its
initial velocity it won't all vaporize and what's left of it will impact the
ground.
Any falling object reaches terminal speed when the drag (upward) force on it
equals the gravitational (downward) force on it. The drag force depends in
its speed, its shape, its cross sectional area, and the air density.
Therefore, a big cross sectional area (like a falling body under a deployed
parachute) has a much slower terminal speed than a small area (a falling
body with no parachute or a roughly spherical meteor). The drag force
increases as the meteor gets lower (increasing air pressure), so its
terminal velocity decreases.
Paul O Johnson
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