In these days of expanded research poten- tialities, the study of meteors has matured as a research area and many of the major problems of yesterday are now solved; for example, the controversial problem of hyperbolic meteors.
Both the precise photographic measures of brighter meteors and the radar measures of the fainter meteors have failed to prove the exist- ence of hyperbolic meteors. If such meteors exist, they represent a minority below the 1- percent level. Furthermore, the photographic and visual meteors are clearly of cometary origin except for a small fraction not exceeding 10 percent that may be of asteroidal origin.
Most photographic and radio meteors, how- ever, move in direct orbits of relatively low inclination to the ecliptic; their aphelia tend to lie beyond the asteroid belt and usually beyond Jupiter's orbit, but many of the fainter radio meteors exhibit quite small aphelion distances.
The cometary meteoric mass is proved to con- sist of extremely fragile, crumbly material that is probably of very low density. The spectra by P. M. Millman indicate a composition some- thing like that of stony meteorites, but it is un- likely that any sizable pieces have reached the ground. Micrometeorites, on the other hand, may well represent the cometary solids.
Although photographic and radio techniques have been exploited to an amazing degree in the past several years, serious gaps remain to be bridged before we can reach a satisfactory understanding of meteoric phenomena and of the origin of these tiny bodies from interplane- tary space. I should like to mention briefly certain of these difficult observational problems and devote most of my attention to the almost untouched areas in laboratory and theoretical work. As long as these areas are unexplored, our knowledge of meteors, their nature, and
1 Harvard College Observatory, Cambridge, Mass., and Director, Astrophyslca) Observatory, Smithsonian Institution.
their origin will remain conspicuously in- complete.
In the area of two-station direct photography of meteors for velocities, trajectories, decelera- tion and light curves, the J. B. Baker Super- Schmidt cameras in the Harvard Meteor Pro- gram have largely completed the basic observa- tional research in terms of their present-day capabilities. Analysis is being carried along rapidly and effectively by L. G. Jacchia, R. E.
McCrosky, and A. F. Cook. I must point out, however, that no serious effort has yet been devoted to the use of the television image tube in optical meteor methods. There is every reason to believe that the sensitivity can be in- creased more than an order of magnitude, since the transient character of the meteoric radi- ation is highly suited for the present-day photo- electric techniques. Utilization of the image tube, on the other hand, requires a major effort since the telescope and the receiver must be integrated into a new and specialized form.
Such a technique shows promise of reaching to the ninth magnitude visually for very slow meteors, and to the sixth apparent magnitude for the fastest meteors. Applied as in the Har- vard Program, the image tube may accomplish a corresponding improvement (some four mag- nitudes) in the registration of persistent meteor trains shortly after the meteors have passed.
Thus, measurement of upper winds as a function of altitude in the range from 80 to 112 kilome- ters could be systematic and highly precise.
The train type of observation is important not only for the understanding of upper atmos- pheric turbulence but also for solving the basic problem of meteoric masses. The "coasting motion" in meteor trains presents to date the only direct method of measuring meteoric masses, via their momentum transfer to the ambient air. Either photographic or image- tube methods should be applied with larger base
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84 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS V O L . 1
lines than those of the present stations, about 50 rather than 20 miles, to clarify the question as to the average masses associated with mete- ors of a given brightness and, from a more fundamental point of view, the question of the densities of cometary solids. A single example leads to a density of only 0.05 gm/cm3!
I t has not yet been possible to utilize the full power of the Baker Super-Schmidt meteor cameras for the photography of faint meteor spectra. Relatively coarse transmission rep- lica-gratings with a high concentration of light in one first order have not yet become available for use on fast cameras of large aperture.
Coupled with image tubes, such coarse gratings might lead to remarkable progress in the under- standing of the spectra of fainter meteors and of train and wake phenomena for brighter meteors and might make it possible to de- termine the composition of cometary solids.
D. W. R. McKinley at Ottawa, Canada, and A. C. B. Lovell with his colleagues at Jodrell Bank, England, have made remarkable strides in measuring the velocities of faint meteors by radio techniques. J. G. Davis has now devel- oped a three-station method for measuring radiants, and, therefore, orbits as well. Only a major effort utilizing this technique can pro- vide the many data critical to our understand- ing of the orbits and origins of meteors, as well as their interaction with the atmosphere. In particular, it is essential that such a system be used with a number of receiving stations in order that observations may be made along the full length of meteor trails to determine ioniza- tion as a function of distance and height, dif- fusion effects at great altitudes, deceleration in the atmosphere, precise correction to outer atmospheric velocities and orbits, heights, winds, and other important meteoric observables.
Such a program is being undertaken at Harvard under the technical direction of G. S. Hawkins.
This technique should be carried to its prac- tical limit.
Fortunately, many radio groups are studying the properties of forward-scatter by meteors because of its importance in communication.
In the near future, such information needs to be collated, summarized, and interpreted with a view to increasing our understanding of the
nature, decay, and diffusion of the electron clouds produced by meteoroids.
There is every reason to hope that the planned Earth Satellite Program of the United States and the International Geophysical Year of the National Academy of Sciences will provide direct above-the-atmosphere measures of small meteoroids in space, so far undetectable by any of the meteoritic methods. I t is extremely important that such observations be made in order to clarify the relationships among comets, meteors, and the zodiacal light and the effect of corpuscular radiation on small particles in interplanetary space.
Although sound observational techniques for the study of meteors are being utilized vigor- ously, the opposite can be said of the theoretical and laboratory research which should now be underway to consolidate these observational gains. The basic meteoric phenomenon still remains unexplained in terms of even an approximate physical theory. This deplorable situation exists not only because the problem is difficult, with many facets, but because no serious effort has been directed towards the laboratory determinations of the fundamental data required in such a theory. Desperately needed are measures of atomic and molecular cross sections for dissociation, excitation, ioniza- tion, attachment, and recombination for me- teoritic and atmospheric elements in the energy range up to 103ev. These quantities are need- ed for such elements as silicon, iron, nickel, magnesium, sodium, calcium, manganese, and others in air, or, better, in pure atmospheres of nitrogen or oxygen. Such laboratory meas- ures coupled with the remarkable progress of recent years in shock-tube research and ultra high-velocity phenomena should make it pos- sible to utilize the observations of meteors to establish the beginning, at least, of a satisfactory basic theory for meteors. The needs in the area of astroballistics are elaborated in a paper of this volume by R. N. Thomas.
Artificial meteors, either in evacuated ranges or at moderate altitudes in the atmosphere, could provide extremely valuable information regarding the meteoric phenomena under con- ditions in which the nature and mass of the meteoric body would be known. Such re- search, started under the direction of J- S.
NEW HORIZONS IN ASTRONOMY 85 Rinehart at the Naval Ordnance Test Station,
Inyokern, Calif., should be pursued vigorously to provide the confirmation of theories based upon the laboratory measures.
In quite a different area of meteoritic studies, laboratory measures would also be of extreme value. Corpuscular radiation from the sun almost certainty plays an important role in the disintegration of meteoroids in space. We require laboratory measurements of the sputter- ing phenomena produced by protons in the energy range 10*-10*ev on common meteoritic surfaces—stony minerals as well as nickel-iron.
A knowledge of the rate of disintegration under these circumstances would greatly increase our understanding of the cometary meteoroid.
Once such a body has been ejected from a comet, it is subjected to corpuscular radiation which causes it to spiral inward towards the sun and also to disintegrate. There seems little doubt that cometary particles are the major source of scattering and diffraction of sunlight in the solar Fraunhofer corona, in the zodiacal light, and in the Gegenschein. We cannot integrate our information concerning comets, meteors, the corona, the zodiacal light, and the Gegenschein until we know the effect of corpuscular radiation on small meteoritic particles.
Similar laboratory measures of atmospheric components impinging on meteoritic material with energies in the range 10-103ev would provide a sound foundation for an understand- ing of the interaction of extremely small meteoroids with the upper atmosphere. If the sputtering effects produced under these condi- tions are sufficiently small, then "micromete- orites" can indeed pass through the upper atmosphere without destruction; by black-body radiation at temperatures below the melting point they can radiate away the heat derived from atmospheric encounter. Such sputtering information would be of extreme value in interpreting the observations of meteoritic dust in the atmosphere and in deep sea oozes. It would also help clarify the question of whether micrometeorites provide condensation nuclei to initiate heavy rainfall at extremely great altitudes.
In the laboratory-ballistic area of measure- ment, particularly needed are the values of such
3S1O14 — 30 7
physical quantities as the heat transfer coef- ficient and the luminous efficiency. These quantities are, respectively, the percentage of the available translational energy of the me- dium that is transferred to the surface of the meteoric body and the percentage energy of the ablated material that is transferred into observable radiation. A corresponding quan- tity is the ionization efficiency, measuring the number of electrons freed in the meteoric process by each atom of the meteoroid. These quantities are, naturally, functions of the velocity and very likely are functions of the air density and composition of the meteoroid.
The physics of the meteoric process involves the integration of physical data, physical phenomena, and meteoric phenomena into a coherent whole. The development of such a theory has obviously been the goal of much research. In one area, there is hope that such a theory can be developed without so much sub- sidiary information, viz, the theory of the per- sistent meteor train- The evidence points strongly to the conclusion that meteoric energy is transferred rapidly to some retentive agency in the atmosphere, such as active nitrogen, and is transformed slowly into radiation after the meteoric phenomenon is ended. Such a theory will be closely integrated with the problems of the physics of the upper atmosphere but does not seem to be hopeless in terms of our knowl- edge at the moment.
A number of theoretical problems fall in the realm of celestial mechanics. The problem of the "Jupiter barrier" requires much more attention. The Jupiter barrier arises from the perturbational disturbances by a large planet which tend to prevent a particle spiraling in towards the sun from reducing its aphelion distance below the perihelion distance of the planet. Perturbations by close approaches to the planet necessarily throw the particle into an orbit that will return near the point of encounter, and thus an orbit with aphelion beyond the planet's orbit. Many of the perturbational problems of interactions between the planets and small particles of the solar system take on a stochastic character and, in fact, are soluble by modern stochastic theory.
An interesting problem combining stochastic theory and celestial mechanics concerns E.
SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS V O L . 1
Opik's suggestion that perturbations of aster- oidal material by Mars are responsible for the existence today of asteroidal material crossing the earth's orbit. Here we require a more comprehensive theory of the perturbative effects of a smaller planet upon microparticles in the solar system.
This enumeration of problems in the field of meteors is far from complete; in particular it omits a number of important questions in the category of correlation-interpretation- theory. These questions involve the relation- ship between the orbits of comets and meteors, the distribution of particle orbits to produce the observed zodiacal light, the nature of the Ge- genschein, the development of meteor streams, identification criteria for meteor streams of great diffusion or low population, and a number
of problems relating to the orbits of comets, meteors, meteorites, and asteroids.
The author hopes that this report, incomplete though it is, will at least tend to eliminate complacency with regard to the present com- pleteness of meteoric research.
References
KAISBH, T. R., BD.
1955. Meteors. Pergamon Press, Ltd., London.
LOTBLL, A. C. B.
1954. Meteor astronomy. Oxford at the Clar- endon Press, New York.
POBTEB, J. G.
1952. Comets and meteor streams. John Wiley
& Sons, New York.
RlNEHABT, J. 8.
1954. Behavior of metals under impulsive loads.
American Society for Metals, Cleveland, Ohio.