g-rh@cca.CCA.COM (Richard Harter) (03/26/88)
In article <4009@whuts.UUCP> orb@whuts.UUCP (45263-SEVENER,T.J.) writes: ... in the course of some interminable argument >> 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. In support of this contention, he produces the following, >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." This quotation is, of course, correct. Tim then goes on to say: 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. This is technically correct, but irrelevant -- the atmosphere has vanished well before this point. The average velocity of the gas molecules is drastically lower than escape velocity. At ground level the gas molecule velocity is ~700 mi/hr whereas escape velocity is ~22,000 mi/hr. Gas molecule velocity is higher in the outer atmosphere (higher temperature) and the escape velocity is marginally lower. These factors do not make up for a difference of 30 to 1. I will commend to Mr. Sevener the article in the Enc. Brittanica on the atmosphere. There he will find that the atmosphere extends out to several thousand kilometers, albeit at very low pressures. I will also suggest to him that he might try to calculate at what distance from the Earth the escape velocity is 1000 mi/hr. He may find the answer illuminating. -- In the fields of Hell where the grass grows high Are the graves of dreams allowed to die. Richard Harter, SMDS Inc.