Peter M. Millman

Radar Meteor Counts: Anomalous Increase during 1963

Radar meteor counts at Ottawa, Canada, show an increase of up to 50 percent in the hourly rates of short-duration echoes during the middle months of 1963. A similar increase has been observed in the Southern Hemisphere.

Astronomical Information on Meteorite Orbits

Knowledge of the orbit of any given meteorite can only be acquired through an analysis of the data relating to the associated fireball. For a point near the start of the fireball trajectory the required parameters are: geographic latitude (φ) and longitude (λ), time and date (T), radiant elevation (h) and azimuth (A), velocity relative to the earth (V). One must, in general, rely on observational data from the casual observer, since the time and place of the arrival of meteorites cannot be predicted. Hence, the information available is normally of low accuracy. The above observational parameters are listed roughly in order of increasing percentage error, the velocity almost always being in greatest doubt.

Radio Observations of Meteors

To summarize, we find that the radiotechnique of meteor observation enables us to extendthe systematic recording of meteor rates down to the9th or 10th magnitude; to determine satisfactoryheights and velocities on a scale previously impossible;to calculate the orbits of meteor showers and individual meteors, in particular those that appear onlyin the daytime; and to study wind drift and finestructure in the ionosphere. The radio observationshave quite definitely indicated that down to the 9thmagnitude, corresponding to particles approximately1 mm in diameter, meteors are members of the solarsystem and do not come from interstellar space.

The observational evidence for mass distribution in the meteoritic complex

The integrated mass indexS for solid particles in the solar system is defined by the equationN = (const)m−S, whereN is the number of particles counted down to a lower limit of massm. Independent values ofS found from various observational programs are reviewed. These lie generally in the range from 0.4 to 1.4 for the average background of particles, and include data from lunar craters, satellite impacts, meteors, meteorites and asteroids. The trend ofS with mass is reasonably well established for masses less than one gram, but there are many gaps in our knowledge concerning the objects of greater mass.

The near-infra-red spectrum of meteors☆

Abstract The photographic infra-red spectra of nine meteors are studied. A total of 15 atomic emission lines are identified in the infra-red region, due to NI, OI, and CaII. As yet no infra-red lines have been observed in the spectra of slow meteors. The lines due to CaII become enhanced relative to lines of NI and OI with increasing luminosity of the meteor.

Meteors and interplanetary dust

The contribution of meteor observations to our knowledge of meteoroids and interplanetary dust is reviewed under four headings — flux, mass distribution, physical structure and chemical composition. For lower limits of particle mass ranging from 1 g to 10−5 g the mean cumulative flux into the earths atmosphere varies from 2 × 10−15 to 6 × 10−9 particles m−2 s−1 (2πster)−1, and the mean size distribution of these particles is given by log N = C − 1.3 log M, where N is the cumulative number of particles counted down to a lower mass limit M, and C is a constant. The physical structure of meteoroids in the above range is essentially fragile, with generally low mean bulk densities that tend to increase with decrease in mass. A minor fraction, about 10 or 15 percent, with orbits lying inside that of Jupiter, have densities several times the average densities, approaching those of the carbonaceous chondrites. The mean chemical composition of meteoroids seems to be similar to the bronzite chondrites for the elements heavier than number 10, but with the probable addition of extra quantities of the light volatiles H, C and O.

Topographic nomenclature on planetary bodies

Abstract General guidlines are presented for International Astronomical Union decisions on nomenclature for surface features on the planets and their satellites.

The Visibility of Stars during Twilight

One of the problems in arctic navigation by astro is the twilight period. At this time, if the Moon is below the horizon, suitable objects for sextant observation are not easy to find. The difficulty is aggravated by the fact that on certain flight paths the arctic twilight may last for many hours. It must also be remembered that in these areas the behaviour of the magnetic compass and of radio aids are often unreliable and this increases the relative importance of astro-navigation. With the introduction of the periscopic sextant into air navigation it has become possible to pre-set the instrument for a given star or planet and satisfactory observations may be possible when the heavenly body is still below the level of casual perception for the unaided eye. In this connection it is necessary to know what stars are likely to be seen under twilight conditions if efficient flight-planning is to be carried out.

Origin and Evolution of Interplanetary Objects

A brief summary is given of the current concepts of the icy conglomerate cometary nucleus and of the origin of comets. Evidence that the cores of comets may contain less than average volatile material, wheter in formation or by radiative heating, raises the question of why at least two very faint short-period comets suddenly experienced violent outbursts (~4000 times in brightness). A preliminary study of close double comet nuclei as affected by differential nongravitational forces shows that a collision of a cometary satellite with its primary is a likely outcome. Thus double nuclei may possibly explain these rare but extreme outbrusts. Statistics suggest, however that most comet splitting and comet outbursts represent instrinsic activities in extremely non-homogeneous nuclei.