METEORS AS PROBES OF THE UPPER ATMOSPHERE 
By FRED L. WHIPPLE 
Harvard College Observatory 
INTRODUCTION AND ASTRONOMICAL 
BACKGROUND 
Tn this short discussion of the use of meteors in upper- 
atmospheric research no attempt is made to achieve 
historical completeness. For an interesting historical 
account of meteors and the basic theory of the earth’s 
attraction, the reader is referred to Olivier’s book on 
the subject [61]. Watson’s Between the Planets [85] 
covers in semipopular style the related subjects of 
comets, meteors, meteorites, and minor planets. Pio- 
neering work on meteoric and atmospheric theory such 
as that by Schiaparelli [76] in 1871 is discussed by 
Kopff [37] in his section on meteors in the Handbuch 
der Astrophystk. Hoffmeister [30] has summarized much 
of the other material in his book on the subject. A 
recent account, concentrating mostly on the early work 
by Lindemann and Dobson, is given by Mitra [59] 
in his valuable volume Th Upper Atmosphere. 
The reality of stones falling from space was not gen- 
erally recognized in the scientific world until the first 
decade of the nineteenth century. Previously, the 
French Academy had remained particularly obdurate 
in denying the phenomenon, even though Halley had 
been convinced of the general idea in 1686 and the con- 
cept was very commonly accepted in ancient times. 
Once the fact is accepted that meteors arise from an 
interaction between the atmosphere and bodies fall- 
ing from space, certain limits on the phenomenon can 
be set immediately. The lower limit of the velocity 
near an altitude of 100 km is about 11.1 km sec, the 
velocity of fall from rest at infinity. The earth itself 
is moving in a nearly circular orbit about the sun with 
a mean velocity of 29.7 km sec~, while the velocity of 
escape from the sun at the earth’s distance is about 
42.1 km sec. Hence, the maximum velocity of en- 
counter is nearly 73 km sec, representing the head-on 
case and allowing for the earth’s attraction [61]. 
The average velocity rises statistically from 6:00 P.M. 
to 6:00 4.m. as the observer is turned from the following 
hemisphere of the earth to the leading hemisphere. 
Meteors of maximum velocity cannot be observed before 
midnight. 
There is still no proof from photographie or radio 
meteor studies that any meteoric bodies originate out- 
side the solar system. The Harvard two-station photo- 
graphic studies of some sixty brighter meteors give 
vectorial velocities with a precision of about one per 
cent. The radio-doppler studies of some 3000 meteors 
by McKinley [52] provide scalar velocities of somewhat 
less accuracy, but to a brightness limit well below the 
capacity of the naked eye. Lovell! has observed scalar 
velocities of more than eighty radio meteors coming 
1. Private communication. 
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from directions including specifically the apex of the 
earth’s motion. None of these studies yield a statisti- 
cally decisive excess of velocities above the solar-system 
limit within the visual magnitude range from —4 to 
possibly +7 or +8. 
To clarify terminology, the term meteor will be used 
here to indicate the meteoric phenomenon and the 
term meteoroid to indicate the material body producing 
a meteor. Meteors sufficiently bright to cast shadows 
are called fireballs, while detonating fireballs are bolides. 
In case a meteoric body is sufficiently large for part of 
it to reach the earth’s surface as a solid, the resultant 
body is a meteorite. These distinctions are necessary 
because of the rapidly accumulating evidence that the 
meteoroids producmg the common meteors observed 
visually, photographically, and by radio techniques 
have a different origin than the meteorites. Hence, there 
is no justification for assuming that the chemical and 
physical structures of the meteoroids commonly seen as 
meteors are exemplified by the meteorites. 
A large percentage of meteors are observed in showers, 
repeated with variable intensity at the same dates 
from year to year. Since the meteoroids in a shower 
strike the earth in parallel paths relative to the observer, 
the resultant meteors appear to radiate from a radiant 
on the sky. The various meteoric streams or showers 
are usually named for the constellations in which the 
radiants are located. Schiaparelli first showed that the 
Perseid shower, observable for more than half the month 
of August, is associated with Comet 1862 III. Since 
then a number of the major meteor showers have been 
associated with comets [85] and there is every reason 
to believe that essentially all shower meteors arise from 
comets. 
The remaining meteors not associated with known 
showers are called sporadic meteors. The Harvard pho- 
tographic meteor studies show that the major percent- 
age of the bright sporadic meteors move in very elon- 
gated and randomly oriented cometlike orbits about 
the sun before striking the earth. The remainder move 
in orbits of low eccentricity, small major axis, and low 
inclination to the fundamental plane of the planets. 
These orbits are similar to those of the few minor planets 
or asteroids that approach the sun within the earth’s 
distance, but also similar to the orbits of short-period 
comets. Wylie [95] has shown that several fireballs 
followed similar orbits. It is not possible to state defi- 
nitely whether these sporadic meteors are of cometary, 
of asteroidal, or of other origin. 
The recent studies of meteorites by Brown and 
Patterson [9] and Bauer [6], on the other hand, indicate 
strongly that meteorites represent debris from a broken 
planet or planets. The writer doubts seriously that 
comets can have such an origin. Hence, the commonly 
