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Re: (meteorobs) P/2000 G1 & Vgeo



I am not sure that the thermal capacity of the meteor has much to do with this.
I think the meteor is behaving just like the ablation nose cones that were
designed in the 60s to deliver atom bombs.  Despite the high temperature at the
surface and the large amount of energy that was generated the interior right
below the surface remained cool.  The heat was carried off by the ablated
material.

Paul O. Johnson wrote:

> ----- 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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