Three networks for the photography of bright fireballs are now in operation; in the central United States, central Europe and western Canada. A detailed comparison is made of the parameters which describe the three networks. Although only two meteorites for which photographic orbital data is available have been recovered, the networks are contributing valuable data on fireball orbits, influx rates and problems of meteor physics.

Meteor counts from the observations of the Harvard-Smithsonian Radio Meteor Project at four magnitudes in the range of about +9 to +15 in the visual scale were used to determine the exponent of the mass-distribution law under certain assumptions. Since for these data no range measures were available, and since the pattern of the transmitting/receiving antennas was very broad, the same range and radiant distribution were assumed for all counts within the same half-hour during which individual counts at different magnitudes were obtained. These echo counts, covering in five successive days 24 hr with overlapping periods, yielded for the exponent a value of 1.95±0.0.02, close to other determinations. Individual values, however, presented a considerable scatter, probably reflecting changes in radiant and mass distribution over the year. From the same data, it appeared that the mass exponent changed during the day, yielding generally a higher value for the evening hours.

Recombination of the Electrons in the ionized column has often been considered (see, for example, Öpik, 1955), but the uncertainties in the atomic and molecular species present and in the rate constants have made it difficult to predict. Furthermore, recombination has not been recognized in the observations, so that it has been judged to be negligible (McKinley, 1961). More recently, however, a relatively high rate constant has been found for dissociative recombination of certain molecular ions. We have now observed recombination at comparable rates in our radar meteor recordings.

Observations from Multiple Sites of a radar network and by television of 29 individual meteors from February 1969 through June 1970 are reported. The primary purpose of the program was to compare ionization with luminosity. Only 12 of the meteors did not appear to fragment over all the observed portion of their trajectories. From these 12, the following relation for the radar magnitude MR to the panchromatic absolute magnitude Mp, in terms of velocity of the meteor V, was found:

Since the advent of low-light-level television (LLLTV) systems, it has been recognized that such devices offer the ability to observe meteors as faint as 10th magnitude which allows the extension of optical meteor data to masses as small as 10-4 gram. The Space Sciences Laboratory at Marshall Space Flight Center has been actively engaged in such observations using image orthicons and intensified SEC vidicons.

The results of these observations are presented along with an interpretation in terms of mass-flux. This interpretation includes the development of a relationship between peak luminosity of a meteor and mass, velocity, and zenith angle that was derived from single body meteor theory and compares favorably with results obtained from the Artificial Meteor Program. Also included in the mass-flux interpretation is an analysis of the observation response of a LLLTV system to fixed and moving point sources.

Atmospheric entry of meteoritic bodies is conveniently and accurately simulated in the laboratory by techniques to be described which employ the charging and electrostatic acceleration of macroscopic solid particles in the 0.02 to 1 μ diameter range. Velocities from below 10 to above 50 km/s are achieved for particle materials which are elemental meteoroid constituents (e.g.: Fe, Si, Mg) or mineral compounds with characteristics similar to those of meteoritic stone (e.g.: FeTiO3). The velocity, mass, and kinetic energy of each particle are measured nondestructively, after which the particle enters a target gas region. Because of the small particle size, free molecule flow is obtained at target pressures ≦ 10 torr. At typical operating pressures (0.1 to 0.5 torr), complete particle ablation occurs over distances of 25 to 50 cm; the spatial extent of the atmospheric interaction phenomena (luminous trails, ionized wakes, etc.) is correspondingly small, simplifying many experiments. Procedures have been developed for measuring the spectrum of light from luminous trails and the values of fundamental quantities defined in meteor theory: heat transfer coefficient λ, drag coefficient Γ, ionization probability β, and photographic luminous efficiency τpg. Results of these measurements are presented, with emphasis on recent, improved evaluations of τpg and β over wide velocity ranges; it is shown that the laboratory values of τpg for iron are in excellent agreement with those for 9 to 11 km/s artificial meteors produced by rocket injection of iron bodies into the atmosphere. Also discussed in some detail is the relevance of these measurements to the interpretation of meteor observations and the methods of inferring from them numerical values of τpg and β for natural meteors.

A crossed beam apparatus has been used to measure the emission and ionization cross sections for the prominent spectral features of Na, Ca, Mg, and Fe in collisions with N2 and O2 over the velocity range of 30 to 120 km/s. From the emission and ionization cross sections, the absolute luminous efficiencies in air were determined over the range of meteor velocities. The maximum luminous efficiencies for the brightest features were: greater than 1 percent for the Na D-lines, 0.2 percent for the CaI(2) singlet, 0.06 percent for the MgI(2) and MgI(3) triplets, and O.4 percent for Fe over the visible spectral range. These luminous efficiencies are valid for free molecular flow conditions for velocities above about 30 km/s and are directly applicable to spectroscopic observations of faint meteors. The luminous efficiency of a typical stone will be presented and compared with the efficiencies determined by other investigators. In contrast to previous work, the luminous efficiency found for stone in the present investigation decreases with velocity above about 50 km/s.

Spectral data with dispersions from 11 to 94 Å/mm on 4 fireballs of actual brightness of —4 to —12 magnitude and with velocities of about 30 km/s at 70 to 80 km heights are used for studies of meteor radiation problems. Previously published analyses need revision for two main reasons: (a) the absolute values of oscillator strengths of Fe I lines from laboratory data were recently recognized to be 1 order of magnitude lower, (b) the luminous efficiency factor τ of Fe I is now much better known from several different experiments. The radiation of fireballs is found to be strongly affected by self-absorption. But if the emission curve of growth is used for correction oj the self-absorption of Fe I lines, a great discrepancy between spectral data and efficiency data for total Fe I light is found. If one assumes that the self-absorption is superposed on another effect, a decrease of the dimensions of the radiating volume with increasing lower potential E1, the spectral data on Fe I lines will be in agreement with the luminous efficiency of total Fe I meteor radiation. Formulas for emission curve of growth and Boltzmann distribution including this effect are derived. This effect is important for fireballs brighter than about —1 or — 2 magnitude, while self-absorption seems to be important even for fainter meteors. The optically thin radiation of all Fe I lines might be expected for meteors fainter than +5 magnitude. Excitation temperature of 5500° K and relaxation time of 0.02 s were found as typical values for the Fe I radiation of fireballs studied. The light of fireballs is emitted during a relatively long relaxation time, which is many orders of magnitude longer than the time necessary for spontaneous radiation of excited Fe I atoms. The dimensions of the radiating volume of Fe I gas for lines with E1 = 0 were found to be 0.3X9 m at 0 absolute magnitude and 2×60 m at —10 absolute magnitude. It ivas not possible to determine any realistic abundances of other elements due to small numbers of lines for an analysis independent of Fe I, while the Fe I curve of growth cannot be used for other elements, because the radiation originates mainly from the effective surface of the radiating volume. A general formula for meteor radiation is also derived and compared with the conventional luminosity equation.

Four meteor spectra from the NASA LRC Faint Meteor Spectra Patrol are analyzed for chemical composition and radiative processes. The chemical compositions of the Taurid, Geminid, and Perseid meteors were found to be similar to that of a typical stony meteorite. The chemical composition of the sporadic meteor was found to be similar to that of a nickel-iron meteorite. The radiation from optical meteors (+ 1 to —10 absolute photographic magnitude) was found to be similar to that of a low-temperature gas, except that strong, anomalous ionic radiation is superposed on the neutral radiation in bright (< — 3 mag), fast meteors.

A brief description is given of the instrumentation, the facilities, and the patrol technique used by the NASA Langley Research Center Faint Meteor Spectra Patrol. A classification of 500 meteor spectra obtained in the first 2-1/2 years of the patrol is given. The general characteristics of “typical” spectra are discussed and preliminary conclusions drawn. Examples of unusual spectra are briefly described.

Thirty-one spectra photographed during the Perseid showers of 1969 and 1970 are found to exhibit greater ionization and stronger, more frequent appearance of the forbidden oxygen line at 5577 Å than Perseid spectra obtained with the same instrument in 1961. Data from 13 Perseid showers indicate a relationship between the frequency of occurrence of the oxygen line and solar activity. In 1969-70, near sunspot maximum, the strength of this so-called auroral green line is greatest near shower maximum, as though the nature of the meteoroids were a function of their distance from the core of the stream, or, alternatively, the strength of the green line were a function of the altitude of the radiant.

The spectra of 25 meteors, recorded with an image-orthicon technique in December 1969, are studied in relation to similar records made in August of the same year. Of 19 Geminid meteors in the absolute visual magnitude range 0 to + 2, only one showed any evidence of the forbidden line of oxygen at 5577 Å, while all Perseid meteors recorded in August exhibited the oxygen line, a result of the large difference in geocentric velocity between the two showers. Atoms identified in faint Geminid meteors include neutral iron, magnesium, calcium and sodium. The molecular bands of nitrogen are also observed.

An extraordinary spectrum of a meteor at a velocity of about 18.5±1.0 km s-1 (approximate uncertainty) was observed from the Springhill Meteor Observatory with an image-orthicon camera at 1970 August 10 d2h 48m 518 UT. The radiant of the meteor was at an altitude of about 49°. It was first seen shoioing a yellow-red continuous spectrum alone at a height of 137±8 km (estimated uncertainty) which we ascribe to the first positive group of nitrogen bands. At 1.608 after its initial appearance the meteor had descended to 116±6 km above sea-level when it brightened rapidly from its previous threshold brighness into a uniform continuum. After a further 0.738 at a height of 106±6 km the D-line of neutral sodium appeared and 0.148 later (height 105±5 km) all the other lines of the spectrum also appeared. The continuum remained dominant to the end 0.408 later (height 87±5 km) or 3.878 after initial appearance.

Water of hydration and entrained carbon flakes of characteristic dimension about 0.2 micron or less are proposed as constituents of the meteoroid to explain these phenomena.

The Fire I test, April 14, 1964, was designed to study the heating of an Apollo-type reentry vehicle. The total reentry complex weighed approximately 230 kg and entered the upper atmosphere at a velocity of 11.5 km/s. The spectrum of the reentry complex has been studied in the wavelength range 8700 to 8800 Å, where 102 multiplets of 21 atoms and the band systems of 5 diatomic molecules have been identified. Comparisons with meteor spectra are made.

Even if we use the classical physical theory of meteors, there may still be some possibility for agreement between observations and theory. The effect of disturbed air behind the main body on the motion and ablation of fragments has not yet been considered. This effect may have important consequences, observed partly as the difference between the photometrically and the dynamically determined masses of the meteor body. By use of extreme mathematical conditions, this difference can be made to reach orders of magnitude during the latter part of the trajectory. However, the physical interpretation is considered only roughly in this paper, and the computed model needs further improvement. But the main purpose here—finding an important effect for the explanation of the discrepancy between the dynamic and the photometric masses, especially for large bodies—has been achieved.

A study is made of the distribution of reciprocal semi-major axis in photographic meteor orbits. A detailed classification of the orbits is made according to quality. The distribution of 1/a in precise orbits is multimodal with two broad maxima approximately centered on 0.05 and 0.40 (AU)-1. Minima in the distribution appear near 0.20 and 0.66 (AU)-1 corresponding to Jupiter’s and Mars’ position in the 1/a, diagram. Considerable fine structure appears in the 1/a distribution. Resonance gaps corresponding to commensurabilities with Jupiter are detected. The gaps are similar to the well studied Kirkwood gaps in the asteroid beli.

This Working List which starts on the next page has been compiled from the following sources:

1. (1) A selection by myself (Cook, 1973) from a list by Lindblad (1971a), which he found from a computer search among 2401 orbits of meteors photographed by the Harvard Super-Schmidt cameras in New Mexico (McCrosky and Posen, 1961)

2. (2) Five additional radiants found by McCrosky and Posen (1959) by a visual search among the radiants and velocities of the same 2401 meteors

3. (3) A further visual search among these radiants and velocities by Cook, Lindblad, Marsden, McCrosky, and Posen (1973)

4. (4) A computer search by Lindblad (1971b) among 1827 precisely reduced photographed meteors from all available sources

5. (5) Visual radiants reported by Hoffmeister (1948)

6. (6) A report on the Phoenicid shower of December 5, 1956, by Ridley (1962)

7. (7) A list of visual radiants by Mcintosh (1935)

8. (8) A report on the June Lyrids by Hindley (1969)

9. (9) Two papers on radar radiants in the southern sky by Weiss (1960a, b)

10. (10) A paper on radar radiants in the southern hemisphere by Nilsson (1964)

11. (11) Several compilations of visual, photographic, and radar radiants by Whipple and Hawkins (1959), McKinley (1961), Millman and McKinley (1963), and Jaechia (1963)

The four most prominent returns of the Leonid shoiver in the past decade fall into two broad classes. The 1966 and 1969 showers were of short duration, had a high proportion of small particles, and occurred with the longest apparent delay after the perihelion passage of the parent comet Temple-Tuttle. By contrast, the 1961 and 1965 returns were of long duration, and had more large particles. The 1961 return preceded the comet.

There are three major influences on particle orbits: ejection velocity, radiation pressure, and close encounters with planets. The observations are explainable in a qualitative way on the basis of the first two. But some speculation concerning the results of planetary perturbations must be invoked.

The present paper critically discusses the method of detection, the magnitude and the rate of occurrence of sudden disturbances in the motions of some short-period comets. The disturbances have recently been suggested as potential indicators of collisions between the comets and interplanetary boulders—minor objects whose existence was predicted by M. Harwit in 1967. The character of explosive phenomena, caused by an impact of such a boulder on a comet’s nuclear surface, depends significantly on the surface texture of the target body. To advance our understanding of the impact mechanism, a method is suggested which would supply a good deal of the missing information about the structure and optical properties of nuclear surfaces from precise photometric observations of cometary nuclei at large solar distances.

Reliable measurements of cosmic dust abundances have been obtained by ionization detectors during particle impact and by collectors controlled either by inflight shadowing or by penetration-hole identification. A description of the techniques used is given.

Crater-number densities observed on the lunar surface and on lunar samples represent an important source of information on cosmic dust fluxes. The related results from the Apollo 11 and 12 missions are reviewed. The overall knowledge gained from these measurements leads to the following flux model: The cumulative flux Φ vs mass m follows the extrapolation from larger meteoroid-size range (Watson’s Law) and can be described by

The Pioneer 8 dust experiment and lunar samples indicate a depletion of the flux at approximately 10-8g. However, cosmic dust particles exist in interplanetary space at least down to 0.3 μ. diameter. They are interpreted as nonmetallic particles in the solar system.

The atmosphere shows an enhancement in particles of about one order of magnitude compared to the flux in interplanetary space at 1 AU. No depletion or cutoff could be detected. These particles are interpreted as lunar debris or as disintegrated products from fireballs.

The numbers of large lunar craters (>140 m diameter) in Mare Tranquillitatis and in Oceanus Procellarum are compared with the meteoroid flux. These comparisons lead to a time-variable flux of Φ.e-Bt, with B = 2.6 and t = time in 109 yr. Thus, the meteoroidflux at the formation of the lunar maria was approximately 4 orders of magnitude higher than today.