Astronaut Scientific Debriefings, 1962-1963

Il4 MAURICE DUDIN
consideration oflong-duration meteor echoe it ha been suggested that the electron
density in the trail is greater than the critical density for the radio wavelengths
employed in probing meteors, and has led to a revision of the Herlofson ratios for
the production of heat, light, and ionization. GREENHOW and HAWKINS (1952)
thereby found that a mete.or of visual magnitude +6 would produce approxi
mately 1012 electrons per centimetre of path. This is one hundred times greater
than HERLOFSON's estimates, and leads to a discrepancy of roughly five stellar
magnitudes between his theoretical estimate and the experimental determination
of electron-line density in meteor trail . GREENHOW and HAWKINS conclude that
meteors produce more· ionization than was originally estimated. Instead of the
kinetic energy of the meteor being divided between heat, light, and ionization in
the ratio 104 : 102 : 1, the ratios are probably 104 : 102 : 10 for bright meteors, and
10 4 : 10 : 10 for faint meteors. These revised estimates imply that the visual
magnitudes corresponding to ap echo of given characteristics is about five magni
tudes fainter than given by HEBLOFSON. This means that the majority of echoes
of short duration must arise from meteors which are below the limits of naked-eye
visibility, and conversely, that all visible meteors must produce radio echoes of
long duration-a well-known ob ervational fact .
A further consideration in the model of HEBLOFSON is the fact that unmelted
meteorites hav been found on the smface of the earth. WHIPPLE (1950, 1951)
has shown that the micrometeorite, if below a certain size, can dissipate the
energy gained sufficiently rapidly to permit the ·e particles to be stopped by the
atmosphere without melting. Recalling that the Herlofson model required that
ablation of the meteorite was the initial step in the production of light and
ionization, thi point warrants some discussion. GREE HOW and HAWKINS
(1952) have indicated that for radio meteors the amount of light produced is
correspondingly reduced for small meteoric particles but the relative ionization is
not reduced, but rather is generally larger than indicated by HERLOFSON's treatment.
It seems worthwhile to review briefly the physical process involved in the
interaction of a micrometeorite with the atmosphere. Since it is not believed that
ablation results for these very small particle , one might question whether or not
the ionization efficiency would become correspondingly poorer. The physical
problem is one of considering a collision of the micrometeorite with a molecule or
atom of air, with the relative energy of collision in the range from 10 to 800 eV.
The mean free path of the air is much ·greater than the diameter of tho micro
meteorite, and thus the problem may be treated by kinetic theory rather than
fluid dynamics. •
There are everal proces es which might occur: (1) An elastic collision of a
molecule of air with the micrometeorite would yield a molecule with a velocity
capable of ionizing. (2) An inelastic collision of a molecule of air with the Inicro
meteorites such that the air molecule enter the surface of the micrometeorite, and
thereby heats the Inicrometeorite and al o forms a "monolayer" on the surface;
such molecules if not chemically bonded to the surface might very ,easily evaporate
off the surface with the velocity high enough for ionization. (3) Secondary ioniza
tion from collisions with a surface. (4) Attachment to oxygen by the collision and
the eventual addition of a free electron to the atmosphere; and (5) sputtering of