Barnard (1891) appears to have been the first to recognize the significance of systematic observations of comets at large distances from the sun. His successful tracing of two 1889 comets to heliocentric distances over 5 and even 6 a. u. caused him to notice that some of the short-period comets might be within the reach of the Lick Observatory’s 36-inch refractor throughout their entire orbits around the sun. Although it is clear nowadays that the short-period comets would be a good deal fainter at comparable distances than the two nearly parabolic comets referred to by Barnard, his original idea proved basically correct, except for the necessity of using photographic plates. Periodic Comet Encke was probably detected near aphelion during Barnard’s lifetime, in September 1913 (Barnard 1914a; Marsden and Sekanina 1974). Undisputed images of the comet just several days off aphelion were obtained in 1972 (Roemer 1972; McCrosky and Shao 1972).

My approach to the nature and origin of the nucleus is strongly influenced by the quantitative data on amounts of dust and gas being. released near perihelion. Calibrated spectra and photoelectric photometry (Gebel 1970, O’Dell 1971, Stokes 1972) have shown that the morphological division of Oort and Schmidt into dust-rich and dust-poor seems to be correct. More accurately, I can say that we have observed a wide variation in the ratio of scattered light continuum to molecular emission. This may or may not reflect a large variation of the dust to gas ratio in the nucleus. In fact, the apparently dustiest comets may have smallest dust to gas ratio at the nucleus! This is because the process of accelerating the particles out from the nucleus by means of viscous gas flow depends sensitively on the amount of gas leaving. (Finson and Probstein 1968). The comet with weak scattered light is probably one that is unable to lift the particles from the nucleus, leaving a residual surface of particles. Since small particles can be lifted more easily, the remaining surface would selectively become one of large particles, forming an insulating layer of low albedo with internal degas if ication occuring at an even slow rate. This in turn would diminish the particle loss rate, with a rapid convergence to a particle cover and nucleus. This model would explain the variation from continuum strong to continuum poor in the Oort-Schmidt new and old comets, in addition to the intrinsically low luminosity and photometric radius of old comet nuclei.

Almost all information about the physics of the nucleus is based on deductions from observations of the coma and tails. It is well to keep in mind the hierarchy of events on which these deductions are based:

1. The material properties of the constituents of the nucleus and the detailed physical and chemical structure of the nucleus form the basis for the behavior of coma and tails.

2. Interaction of solar radiation with the surface of the nucleus determines the overall temporal development of the coma.

3. The subsequent interaction of solar radiation and solar wind with the coma determine the gross features of the tails.

4. Short term fluctuations primarily in the solar wind (and associated magnetic, field) cause disturbances of comparable duration observable mostly in the tail but also in the coma.

Photometry and polarimetry of the cometary heads still constitute one of the most important sources of information about the physical processes in comets. For instance, most of the present estimates of molecular lifetimes are based on the observed distribution of molecules in the cometary head and the assumption of a particular kinematical behaviour of the matter in the cometary atmospheres.

The study of kinematics and dynamics of cometary heads and tails has been based upon the analysis of the forms and apparent motions of well-defined envelopes, halos, knots in tails and streams. Direct inspection of a large number of photographs (or drawings from the last century) of several bright comets demonstrates that the cometary head is generally a complicated object. The heads consist of nearly circular diffuse patterns with superposition of different features, particularly of curved streams. This is illustrated by the Atlas of the Cometary Forms compiled by Rahe, Donn and Wurm (1970).

What I would like to discuss here is the chemical nature of the nucleus.

This implies the consideration of the vaporization of the nucleus into the coma because the coma is our only source of information on the nucleus. The nature of the parent molecules vaporizing from the nucleus has been for a long time the missing link for our understanding of the molecular processes. A major development has been brought about by the first identification of three neutral parent molecules HCN and CH3 CN identified in Comet Kohoutek, H20 in Comet Bradfield, confirmed by H2O+ in Kohoutek.

Comet Kohoutek (1973f) has been observed with the 50 cm (f/15) reflecting telescope of the European Southern Observatory, La Silla, Chile, on fourteen nights between January 16 and 30, when the heliocentric and geocentric distances of the comet were r=0 66 - 1 00 A. U and Δ =0 81 - 0.96 A. U. , respectively The 40″ and 80″ diaphragms were used for the photometry of the cometary head (EMI 6256 A photomultiplier) in the UBV system and with six interference filters: CN 3884 Å, CO+ 4267 Å, C2 4737 Å, C2 5172 Å, cont. 5300 Å and Na 5893 Å.

The atmospheric conditions were good but the accuracy of our observations was lower than usual due to large extinction (air mass 25 to 43) and twilight. The mean error of one measurement of log F in all but Na 5893 Å filters can be estimated at ±0.02, whereas the accuracy through the Na filter was substantially lower

My remarks will present some views of the behavior of the nucleus and problems with their explanation.

One thing we definitely know about comets is that there has to be a permanent structure which revolves around the sun in the comets orbit. Permanent means carrying over from apparition to apparition during the lifetime of the comet. I adopt some form of the “icy nucleus” model as proposed by Whipple. This structure reasonably fits most of the known cometary features although no completely consistent model accounting for all known phenomena in a satisfactory manner has yet been described. The icy nucleus does not appear to have any major flaws. It has a real advantageous feature in that detailed models can be constructed and their behavior more or less accurately predicted, e.g. (Donn 1963; Delsemme and Miller, 1971; Sekanina 1972, 1973; Huebner, 1975).

Photometric observations of emission line features of the coma of comet Kohoutek (1973f) were made with narrow band interference filters. The emission features observed were CN(3879 A°), C3(4057 A°), and C2(4732, 5165,5634 A°). The radial dependence of the emission was investigated by employing six diaphragms set concentrically about the brightest part of the comet's head.

The photometric observations were made at the Optical Research Facility and Observatory of NASA/Goddard Space Flight Center (GSFC) in Greenbelt, Maryland. The observatory is located 13 miles Northeast of Washington, D.C. (longitude = 5h07m18s22, latitude = +39° 01′ 11.″48, altitude = 49 meters) and is operated by the Laboratory for Optical Astronomy of Goddard (GSFC).

In discussing the nucleus of comets and the role of comets in the evolution of the solar system I summarize the observation only in Table 1, the observed composition. Note that gas-phase chemistry obscures the nature of the active atoms, molecules and radicals in the nucleus.

I wish to stress the fact, so obvious from this symposium, that the comprehensive observational attack on Comet Kohoutek (1973f) has led to an enormous step forward in deciphering the comet enigma.

With few exceptions, measurements of cometary brightness and polarization have been restricted to regions in or near the coma and therefore to a relatively small range of phase angles. Photoelectric techniques are required for detailed wavelength coverage, whereas large-field photographic techniques are better suited for mapping the large regions of sky spanned by a comet tail. Observations with a small field of view provide high spatial resolution but generally restrict multicolor measurements of brightness and polarization to a small region of the comet. Observations with a large field of view (diameter larger than 1 or 2 deg) provide adequate color and spatial coverage but can result in the loss of detail. A compromise is afforded by Fabry photometry, using a modest telescope of small aperture and relatively large field of view.

The cosmogony of the planetary and satellite systems consists of understanding the physio-chemical processes leading to their formation and also trying to decide at what time and over what period their formation took place. The cosmogony of the comets require answers to not only these two questions but also as to where, in relation to the solar system, the observed and inferred distributions of comets were formed.

Narrow-band filler photographs of comet TSK were taken in the light of C2, CN and C3 by Rahe McCracken and Donn, have been analysed in terms of Haser's model of the coma. The isophotes obtained from these photographs were corrected for sky background. The isophotes were circularly symmetric. Radial intensity profiles were obtained along the sunward, antisun and the two perpendicular directions. In each case, these profiles were the same within the experimental errors in intensity (±5%).

Theoretical curves based on Haser's model were computed for different combinations of the parameters β0 and β1 where l/β0 is the scale length for the decay of the unobserved parent molecule and 1/β1 the corresponding quantity for the observed daughter molecule. A comparison of the theoretical and observed intensity profiles, yielded our best estimates for β0 and β1 for the different molecules. These are listed in Table 1. In some cases, we were able to obtain lower limits only to the ratio β0/β1 because the isophotes do not go out far enough, a limitation caused by the sky background. It is to be noted that the same values of the parameters were obtained from observations on different dates indicating no unusual activity in the comet in the intervening period. We believe ours is the first result ever obtained for C3.

In Table 2, we summarize all the results for C2 and CN in different comets. It appears that the scale lengths differ between comets, a result which cannot be attributed to different helio-centric distances alone.

The current evidence concerning the nature of comet nuclei suggests that comets may be sizeable aggregations of interstellar grains. This is a progress report on an effort to find circumstances and processes whereby such aggregations might be formed in the solar system at distances far beyond the proto-planets during the early stages of solar-system development. Under investigation are interactions between the early solar “gale” and the surrounding interstellar gas and dust—the so-called “snowplow effect.” Compression of the gas and resultant motions of the dust coupled with the pressure radiation from the Sun and nearby new stars may, under certain idealized circumstances, produce a high enough concentration of dust for gravitational instability to occur in the dust, thereby producing km-sized coherent bodies. The likelihood or probability of actual comet formation by such processes remain to be determined.

It is now a generally accepted concept that comets are a residue of the early history of the solar system from the time when the planets were forming. Because of the approximately 0.1% loss of material from the nucleus during perihelion passage near 1 A.U., lifetimes of short period comets are limited to 104-105 years. This requires an astronomically recent source of the comets seen at the present epoch. From the statistics of the aphelia of parabolic and long period comets, Oort (1951) proposed the existence of a comet cloud between 50,000 and 100,000 A.U. which serves as a reservoir from which presently observed comets have recently been perturbed. Although there are various difficulties with populating the cloud (Opik, 1973) and its subsequent evolution (I.A.U. Symposium 45, 1972; Everhart, 1974) it is the basis for nearly all current studies on the origin and evolution of comets.

This paper consisted of a running commentary by Ernest Hildner on the movie as it was shown. The data from the movie has not yet been fully analyzed and will be published elsewhere. Discussion immediately followed the movie. (ed.)

The question of the present day usefulness of the great wealth of observational material on comets buried in the archives of the older observatories is an important problem that should be evaluated soon if any action is to be taken before much of it is lost or destroyed. The beginning step has been taken with the publication of the isophotometric atlas and the Atlas of Cometary Forms which is a direct result of Recommendation No. 1 of Commission 15 of the IAU meeting at Prague in 1967. At that time a repository for cometary photographs and spectrograms was also discussed. However, in the light of observational advances and the present capability of quantitative correlation of structural changes with condensations or annomalies of the solar plasma, it is desirable to establish again that this old observational material may be useful.

At present only four polyatomic molecules or ions have been identified in the spectra of comets and their tails. They are C3, CO2 +, NH2, and H2O+. The first two are linear molecules. The C3 radical gives rise to the well-known 4050 group. It was first obtained in the laboratory in an interrupted discharge through CH4; was definitely identified by Douglas as being due to C3, and was later investigated in considerable detail in absorption in the flash photolysis of CH2N2. The complicated vibrational structure of this spectrum was first understood when it was realized that the bending frequency in the ground state is very low (64 cm-1) and that in the excited state the interaction of the vibrational angular momentum with the electronic angular momentum leads to large splittings (Renner-Teller splittings).