orb@whuts.UUCP (SEVENER) (03/25/88)
In article <2177@mit-amt.MEDIA.MIT.EDU> dlleigh@media-lab.MEDIA.MIT.EDU (Darren L. Leigh) writes: >In article <3884@whuts.UUCP> orb@whuts.UUCP (45263-SEVENER,T.J.) writes: >> >>My, what a crop of scientific genuises we are training >>up at MIT!! > I suggested there might be an abrupt shift from the atmosphere > to space at the edge of the atmosphere. A poster has suggested > this is not correct. Personally I don't know. > Logically, it seems to me that there will be some kind of > abrupt shift or discontinuity at the point of escape from the > Earth's gravity. On the other hand, air is an amorphous gas > which is not rigidly bound and constantly in motion. So this > will blur the boundary. > >Isn't this rich? Obviously Tim hasn't heard Newton's law of universal >gravitation (F = GMm/(r^2)) or at least he doesn't understand it. >With Tim's understanding of physics at this level, I feel it best that >we ignore his postings on the subject for the time being. I repeat again: My, what a crop of scientific geniuses we are training up at MIT!! 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 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." 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. More from a classic paper on the subject in Part II... tim sevener whuts!orb
jfc@athena.mit.edu (John F Carr) (03/26/88)
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 Peartjfc@athena.mit.edu (John F Carr) (03/27/88)
After I wrote a reply last night to Tim Sevener's article, I checked the atmospheric tables in the Handbook of Chemistry and Physics. I found that at high altitudes the scale height of the atmosphere increases. That is, the real state of the upper atmosphere is exactly opposite what Tim claimed: not an abrupt end, but an ever slowing decrease in density. John Carr "No one wants to make a terrible choice jfc@athena.mit.edu On the price of being free" -- Neil Peart