Fred L. Whipple

Physics and chemistry of comets

The space-age view of cometary physics and chemistry is dealt with in this book. It presents in detail current knowledge of comets, based on the scientific data from the spacecraft missions of 1986 to Halleys Comet. Nucleus, neutral coma, plasma, dust and the origin and evolution of comets are treated in chapters written by leading experts in the field. The book is intended as a reference source for advanced students and researchers in space physics, planetology, geophysics and cosmochemistry.

Comet P/Halley’s nucleus and its activity

The Halley Multicolour Camera on board ESA’s Giotto spacecraft observed the nucleus of comet P/Halley and its environment and returned more than 2000 images. The observations are summarized, their calibration is described, the status of the analysis and the results are discussed. Topographic features on the nuclear surface and areas of activity are identified. The optical thickness of the dust produced in jet-like features is estimated. The impact and constraints of the observations on cometary nucleus models are discussed.

Cometary brightness variation and nucleus structure

The Bobrovnikoff and Beyer photometric data for more than 100 comets have been analyzed for intrinsic brightness variations,before andafter perihelion, according to ther−n law, wherer is solar distance. The Oort and Schmidt classification of comet ‘age’ has been extended and applied with Marsdens new determinations of inverse semi-major axis, 1/a, original. All classes of comets withP>25 yr show statistically the same value ofn after perihelion. New comets approach perihelion with smaller values ofn and older comets with increasingly larger values (Table II). For comets ofP<25 yr,n is larger and erratic.A physical interpretation involves the quick loss of a frosting of super-volatile materials from new comets; then, for all comets, the development of an insulating crust after perihelion. The crust also includes ‘globs’ of meteoroidal and icy material. The crust tends to be purged near perihelion but generally to grow in a spotty fashion with cometary age. The orientation of the axes of rotating comets is shown to be an important unknown factor in cometary brightness variations. A speculation is made concerning the axis of rotation for C/Kohoutek, 1973 XII.

The Meteoritic Risk to Space Vehicles

The Meteoritic Risk to Space Vehicles. Consideration is given to the distribution of meteoritic material and its rate of fall on the earth as functions of mass and velocity. With a simple theory, the probabilities are calculated that surfaces in space in the neighborhood of the earth may be punctured by meteoric action. A table of data and probabilities is given. It is calculated that a near-earth satellite of radius 20 inches and skin thickness 0.5 mm Al will be punctured on the average of once in five days.

Sources of interplanetary dust

Attention is centered on cosmic dust measures made by sensors on Pioneers 8 and 9 in Earth-like orbits. The conclusion follows Zook and Berg that the particles are largely “β-meteoroids,” interplanetary impact debris expelled by solar radiation pressure. An analysis of periodic comet orbits and comets observed during the missions failed to yield correlations, except possibly for debris from Comet Encke. A treatment of β-meteoroids from this stream is presented.

The cometary nucleus: Current concepts

This paper deals with current ideas concerning the nature of cometary nuclei as modified by observations of recent years, particularly by those of Halley’s comet during the 1986 apparition. The major trend has been to increase the postulated dimensions of the nuclei and reduce their albedos. The Vega and Giotto space missions have established invaluable check points with regard to comet nuclei. Striking is the low geometric albedo of 0.044 deduced from the directly observed dimensions of the nucleus and the brightness at large solar distances. The dimensions, the observed nongravitational motions and the jet activity combine to suggest an extremely low density for the nucleus of Halley’s cornet — in the range of 0.1 to 0.5 g cm−3.

Comet P/Holmes, 1892III: A case of duplicity?

Abstract The observations of comet P/Holmes 1892III, exhibiting two 8- to 10-mag bursts, have been carefully analyzed. The phenomena are consistent with the grazing encounter of a small satellite with the nucleus on November 4.6, 1892, and the final encounter on January 16.3, 1893. The grazing encounter produced, besides the first great burst, an active area on the nucleus, which was rotating retrograde with a period of 16.3 hr and inclination of nearly 180°. After the final encounter, the spin period was essentially unchanged, but two areas became active, separated some 164° in longitude on the nucleus. After the first burst the total magnitude fell less than 2 mag from November 7 to 30 (barely naked eye) while the nuclear region remained diffuse or complex, rarely of ever showing a stellar appearance. The fading was much more rapid after the second burst (barely naked eye at maximum) while the nucleus frequently appeared stellar after the first day. It seems reasonable to conclude that the grazing encounter distributed a volume of large chunks in the neighborhood of the nucleus, maintaining activity for weeks. The final encounter activated a new area on the nucleus, the shock and fall back disturbing the area already exposed by the grazing encounter. Several details of this scenario are fitted rather well.

Dust distribution in the inner coma of Comet Halley - comparison with models

Abstract The images of the inner coma of comet Halley that were taken with the Giotto camera have been evaluated to obtain information on the near-nucleus dust distribution and its relationship to the nuclear source regions. Over two-thirds of the dust in the inner coma can be modeled remarkably well with only three source regions, each emitting dust into broad jets with opening half-angles of 30 to 50°. The dust in the jets is not sharply bounded but disperses with a Gaussian cross section, consistent with a model in which both gas and dust emissions are confined to finite source regions. Departures from the simple R−1 relationship for the radial distribution of dust near the nucleus are reproduced by a model that considers the source region as a composite of adjacent subareas, each emitting dust into cones with opening half-angles of about 10°. Comparison of the azimuthal and radial dust distributions with model predictions gives information on the coma dust, the dust-gas interaction, and the properties of the emitting regions.

The meteoritic environment of the Moon

Diverse evidence concerning the probable particulate influx on the Moon is assembled along with estimates of the rather large uncertainties. The masses under consideration range from 10–14 to 10+18 g, the total estimated influx amounting to the order of 4 × 10–16 g cm–2 s–1. The occurrence of large lunar craters assumed to have accumulated over 4 x 109 y appears consistent with the expected production rate by Apollo-type asteroids. Cometary nuclei may or may not have contributed significantly. There is possibly a dearth of smaller craters of diameter less than 50 m as compared to the probable production by meteoritic influx. The overturn of lunar material by cratering impacts may have largely blanketed craters with diameters smaller than 3 to 30 m.

Interstellar Molecules [and Discussion]

Some 70 different molecular species have so far been detected variously in diffuse interstellar clouds, dense interstellar clouds and circumstellar shells. Only simple (diatomic and triatomic) species exist in diffuse clouds because of the penetration of destructive ultraviolet radiations, whereas more complex (polyatomic) molecules survive in dense clouds as a result of the shielding against this ultraviolet radiation provided by dust grains. A current list of interstellar molecules is given together with a few other molecular species that have so far been detected only in circumstellar shells. Also listed are those interstellar species that contain rare isotopes of several elements. The gas phase ion chemistry is outlined via which the observed molecules are synthesized, and the process by which enrichment of the rare isotopes occurs in some interstellar molecules is described. Reference is also made briefly to some very recent work in interstellar ion chemistry. A list of the atomic and molecular species that have been detected in cometary atmospheres is given and attention is drawn to the similarities and differences between interstellar and cometary molecules. The physical and chemical processes by which these observed cometary species may be generated from material that sublimes from the cometary nucleus are discussed.