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From: jfc@athena.mit.edu (John F Carr)
Newsgroups: talk.politics.misc,sci.misc
Subject: Re: Darren Leigh at the Edge of Space! :Part I
Message-ID: <4063@bloom-beacon.MIT.EDU>
Date: 26 Mar 88 08:37:29 GMT
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Sender: daemon@bloom-beacon.MIT.EDU
Reply-To: jfc@athena.mit.edu (John F Carr)
Organization: Massachusetts Institute of Technology
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In article <4009@whuts.UUCP> orb@whuts.UUCP (45263-SEVENER,T.J.) writes:
:Mr. Leigh, I beg to bring your attention to the following article
:in the Britannica Micropedia, p.554, on a concept I suppose you haven't
:heard of, called "escape velocity":

:"escape velocity, in astronomy and space exploration, the velocity
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
But Tim, you said when I noted that my majors are aerospace engineering
and planetary science that you did not care and that fact was irrelevant.
If you are at all concerned with the consistency of your articles you
should either retract that statement or the two "edge of space" articles.

:that once attained is sufficient for a body to escape from a
:gravitational centre of attraction without undergoing any
:further acceleration.  Escape velocity decreases with altitude,
:and is equal to the square root of 2 (1.414) times the velocity
:necessary to maintain a circular orbit at the same altitude.
:At the surface of the Earth, if atmospheric resistance could be
:disregarded, escape velocity would be about 11.2 km (6.95) miles
:per second.  The velocity of escape from the less massive Moon
:is about 2.4 km (1.5 miles) per second at its surface.  A planet
:(or satellite) cannot long retain an atmosphere if the planet's
:escape velocity is low enough to be near the average velocity 
:of the gas molecules making up the atmosphere."

Almost true, in fact it is the upper end of the velocity distribution
from which particles are lost.  Such loss is relevant only for small
bodies with no internal source of replenishment for the lost gas.
See for example, the Feb. issue of Scientific American.  

:One might also add that there comes an altitude at which the
:escape velocity approximately equals the average velocity of
:the gas molecules making up the atmosphere.  This is the
:"abrupt end" of the atmosphere.

There are seceral faults with Tim's reasoning: it makes the contradictory
assumptions that the gas is static (so that gas can lost above a 
 certain altitude can not be replenished; gas produced below the surface 
 constantly replenishes that which is lost to space; there is a net flow 
 outward) and free (so that gas can escape); it neglects the fact that the 
mean free path at high altitude is too large for any sharp boundary to be 
maintained; and it assumes that all particles move at the same velocity 
(1: particles have a wide range of velocities, 2: equiparition of energy 
 means that lighter particles move much faster [thus hydrogen is lost 
 quickly]).





   John Carr           "No one wants to make a terrible choice
   jfc@athena.mit.edu   On the price of being free"           -- Neil Peart