We have recently reported detection of hydrogen cyanide and the first quantitative observations of the velocities of neutral gas jets in the inner part of the coma while the comet was at small heliocentric distances (Huebner, et al., 1974). Now we report the detection of two line transitions from unidentified cometary molecules, provide further evidence of the variability of neutral gas jets, and give a summary of our search program for microwave transitions in molecules of cometary origin.

The observations presented here were made with a 3-mm line receiver mounted on the 11-m NRAO radio dish at Kitt Peak. Observations were carried out before perihelion (15 to 20 December 1973) and after perihelion (3 to 7 January 1974). During these periods the comet was between 0.3 and 0.5 AU heliocentric distance.

The present authors (Clark et al, 1971) previously attempted to detect the 1.35 cm microwave line of water in Comet Bennett (1969i) using the 26 m Maryland Point radio telescope of the Naval Research Laboratory. An upper limit for the H2O column density of 1017 molec/cm2 was obtained assuming the rotational levels were in thermal equilibrium, A more recent search in Comet Kohoutek using the 37 m radio telescope of the Haystack Observatory with a low noise traveling wave maser preamplifier was also unsuccessful. In this comet the identification of H2O+ in the visible spectrum gave strong support to the idea of water as a major parent molecule. The radio detection of HCN (Heubner et al, 1974) and CH3CH (Uhlich and Conklin, 1974) in Comet Kohoutek suggested that the attempt to detect H2O should be repeated in a suitable comet.

According to current theories, the radicals optically identified in cometary spectra stem from more complex parent compounds which have no transitions at visible wavelengths. In recent years radio transitions of likely molecules have repeatedly been searched for but were never found (e.g., Huebner and Snyder, 1970; Clark et al., 1971). Only in Comet Kohoutek 1973f have microwave lines of three molecules been detected, OH (Biraud et al., 1974; Turner, 1974), CH3CN (Ulich and Conklin, 1974), and HCN (Snyder et al., 1974). Here we report on our search for transitions in the 22-25 GHz band of the molecules H2O, NH3, and CH3OH in this Comet and give upper limits on line temperatures and column densities.

Comet Tago-Sato-Kosaka (1969 IX, hereafter TSK) was the first medium bright comet passing by in the era of ultraviolet satellites. The Orbiting Astronomical Observatory (OAO-2), observed in January 1970 the strong Lyman alpha signal at 1216 A which led to the first detection of cometary hydrogen. The peak signal in the photometer field-of-view (FOV) of 10′ diameter was about 70 kR (Code et. al , 1970) The resonance scattering emission of hydrogen was optically thick in the central part of the coma. A rocket experiment of Jenkins and Wingert (1972) using an objective grating spectrograph confirmed the OAO-2 observations Later in Spring 1970 comet Bennett (1970 II) was observed in Lα by OAO-2 and by two photometers onboard the Orbiting Heophysical Observatory (0G0-5) (Bertaux and Blamont, 1970; Keller and Thomas, 1973).

The Lyman alpha emission from Comet Bennett (1970II) was measured near perihelion (March 1970) by the University of Colorado ultraviolet photometer experiment on OGO-5 The spectrometer field of view of about 3° crossed the cometary hydrogen coma four times. The hydrogen coma was observed to extend more than 30 x 106 km in the antisolar direction.

A model for the hydrogen density was developed which took the actual cometary motion and the gradients of the forces of gravitation and radiation pressure into account Exact trajectories of atoms in the orbital plane representing the column densities perpendicular to the plane were calculated. The variation of the hydrogen lifetime along the trajectory as well as the solar Lα profile were considered.

Comet Bennett (1970 II) was observed with the ultraviolet photometers on OAO-2 from April 13.39 to May 13.88, 1970, while its heliocentric distance increased from R = 0.77 to 1.26 a.u. An analysis of the photometer data for the emission features of OH λ 3090 and H λ1216 indicates the production rates of OH and H were 2.0 x 1029 molecule sec-1 and 5.4 x 1029 atom sec-1, respectively, at R = 1 a.u During this period the production rates of H and OH varied as R-2.3 . This is consistent with the assumption that water vaporization controls the production rate of gas in comets at small heliocentric distances.

Comet Bennett (1970II) was observed with an ultraviolet spectrometer on the OAO-2 spacecraft during April 1970 The instrument consisted of a plane grating, a parabolic mirror, a slit, and a photomultiplier tube. The exit slit provided a 2′ x 8′ rectangular field-of-view and a 23 A bandpass. At this resolution emission features such as OH λ3090 and CN λ3883 were essentially monochromatic. Since the instrument had no entrance slit, movement of the grating produced a shift in the field of view, with 1 arc min of spatial offset corresponding to a 10 A wavelength shift.

The first infrared observations of a comet were made by Becklin and Westphal (1966) who studied Comet Ikeya Seki (1965f) Their data at wavelengths of 1.6 to 10 microns revealed that this comet was a bright infrared object because of thermal radiation by dust grains in the coma. This is just another example of the importance of dust in infrared astronomy. The success of infrared is largely due to the extreme visibility of dust which reveals itself by reradiation in the infrared and by scattering or extinction in the visible. Optical opacities are χ = 104cm2/gm for one micron dust particle, for plasma free electrons, χ = 10-4cm2/gm for neutral molecules. By the time Bennett came along (1969i) infrared techniques were well developed, and the presence of an emission feature at 10 and 20 microns had been discovered in the infrared energy distributions of late type luminous supergiant stars (Woolf and Ney, 1969, for a summary see Ney (1972)).

Because of Comet Kohoutek's anticipated large gas production, which seemed to offer a unique chance to reveal parent molecules, two Fabry-Perot Tilting Filter Photometers were designed with the purpose to detect and study the behaviour of CH4 and its photolysis product H2 The importance of these two molecules is well known and their detection would have given valuable indications about the structure of the nucleus, its thermal history and conditions of formation.

Similar to CH4, H2 has no dipole moment and cannot be detected by radioastronomy. The most obvious way for measuring H2 in extended cometary comae is certainly on the basis of fluorescence from the Lyman bands around 1000Å, there are, however, vibrational quadrupole transitions within the overtone bands of the ground electronic state which give rise to emissions in the near infrared, accessible by means of ground based telescopes. Three of the stronger lines are: λ = 0.8748 μ; 0.8560 μ and 0.8497 μ. Methane is more readily detectable in the infrared, since it has strong fundamental (1-0) infrared vibration rotation bands at 3.3 μ (ν3).In order to measure both the CH4 concentration and its rotational temperature, a. very high resolution (~3.7A) high throughput instrument was designed which could isolate several individual vibration-rotation lines in the v3 band, namely the P2, P3 and P9 lines. The instrument consisting of a Fabry-Periot Tilting Filter Photometer with InSb detector interfaced with the 30 cm f/30 Dahl-Kirkham Telescope is described in detail elsewhere.( l). The observations were made in January from the NASA Convair 990 (Galileo II) at an altitude of 13 km, where atmospheric methane absorption can be minimized but not avoided. Doppler shift of cometary and atmospheric lines with respect to one another by at least a few A caused by the orbiting velocity of the comet would be sufficient to allow for high transmission measurements. Though long integration time measurements with Lock-In- Amplifier technique have been carried out, no signals from the CH4-rotational lines of the comet coma could be detected. Using the planet Venus as a calibration source for the photon flux and as a result of delicate laboratory measnrements an upper limit of

could be derived. This value is several orders of magnitude less than the original predictions for Kohoutek during close approach. Therefore, one could conclude that volatile components like CH4 boiled off the comet well before perihelion, at large (~4 AU) distances from the sun and were responsible for the high brightness of the comet at that time. Such a fractionation is only possible if the nucleus was composed of relatively loose, porous ice, rather than compact ice. This hypothesis was strongly supported by the second experiment for search of H2 in the near infrared at the 182 cm telescope of Asiago. Also in this case a Fabry-Perot tilting filter photometer was designed to match with the f/9 optics of the telescope. The instrument (2) consists in a high resolution (~0.7A) tilting filter system with photon counting technique which allows phase-sensitive background subtraction. On the basis of the best data achieved between January 10 and 15 the occurrence of H2-lines with an intensity larger than 2% of the continuum could be excluded, viz. the flux averaged over the field of view was less than 4.105 photons/cm2 sec sr A. Since the pre- and post-perihelion measurements were not affected by molecular fluorescence, they represent only the light scattering flux from dust particles. The data display that the comet's dust coma was definitely brighter during approach than during recession from the sun. However, the quantity of more fundamental interest is the difference in dust production rates, and a derivation of the mass-production rate of dust could be derived. The study shows that both the dust and gas production rate differ greatly in the pre-perihelion period as compared to the post-perihelion period, as conjectured previously for "virgin" comets. (Dust production rate/gas production rate: pre-perihelion 0.1, post-perihelion 1). The pronounced asymmetry in the production rates strongly suggests that fractionation and dust entrainment effects have to be considered in brightness predictions of young comets, the nucleus of which will generally consist of a multi-component mixture of parent molecules.

In this review, we concentrate on an aspect of the physics of ion tails stimulated by observations of Comet Kohoutek (1973f), namely, the nature of moving structures in the tail. These motions could be bulk motions themselves or waves moving at the information speed of the medium; the resolution of this question from simple photographic data alone may not be possible.

We do not discuss the principal problem of ion tails, namely, the physical mechanism responsible for the creation of CO+ ions. Indeed, results from studies of gas phase chemistry presented at this Colloquium indicate that the solution may be quite complex. Also, we do not discuss either the details of the solar-wind comet interaction or dust tails.

By combining observations of Comet Kohoutek (1973 f) made in the Southwest US, Alaska and Hawaii, a cometary tail movie has been made. Parts of the movie were shown at the conference and some frames of the movie are reproduced in Fig. 1, In this paper we give some details of the observations and describe what we see on the movie.

Observers who contributed to the movie are listed in Table 1 with their institutions, instruments, and site locations. In all but one case, fast Schmidt cameras were used; high-speed optics are needed for this work because the interval between successive pictures should not exceed 15 minutes if plasma tail motions are to be properly visualized. The 36 cm f/2 Schmidt camera of the Joint Observatory for Cometary Research (JOCR) is clearly well-suited because of its flat field of about 8 x 10°.

Six JOCR photographs of Comet Kohoutek taken on January 19, 1974 (UT) show concentrations of plasma about ten degrees from the head of the comet which are very similar to the “barred spirals” reported by Bostick (IAU Symposium No. 6: Electromagnetic Phenomena in Cosmical Physics, B. Lehnert, (ed.) Cambridge Univ. Press, 1958) from laboratory observations of colliding plasmoids. The photographs were taken about one day after a solar magnetic sector boundary crossed the comet's head. Also, the comet's plasma tail splits near the head into two distinct segments. It appears that the two segments reconverge and form the plasma concentrations by collisions. This represents an increase in scale over Bostick's work of about 1014 Possible historical observations of similar phenomena are discussed.

By far the greatest number of observations of the brightness of comets, and the only ones that cover a long enough time span for investigation of secular effects, have been made by visual methods, mostly with small instruments. Such observations record the contribution of a large part of the coma, and possibly some light from the tail, at wavelengths to which the eye is sensitive. Three distinct observational techniques have been defined:

1. 1) Comparison of similar-appearing extrafocal images of comet and comparison stars for equal apparent brightness. (Most observations have been made by this method.) The technique has been described by Bobrovnikoff (1941a, 1941b).

2. 2) Comparison of the in-focus image of the comet with extrafocal images of comparison stars for equal apparent brightness. This method has been described by Sidgwick (1955).

3. 3) A method used extensively by Max Beyer (1950), in which grossly out-of-focus images of the comet and comparison stars are examined for similarity of extinction against the sky.

Visual estimates of comet total magnitude have been made for well over one hundred years. In this paper no attempt has been made to review all previous work on comet magnitudes. Instead we prefer to concentrate on developing a conceptual framework upon which previous work can be evaluated. We have tried to unify the approach as much as possible by filling in gaps that occur between previously published accounts. The present work represents an extension and revision of an earlier attempt at understanding comet magnitudes (Meisel, 1970).

This review states and defends seven conclusions on the origin of comets and the evolution of their orbits:

1. 1. There is a N-½ law of survival of comets against ejection on hyperbolic orbits, where N is the number of perihelion passages.

2. 2. The short-period comets are not created by single close encounters of near-parabolic comets with Jupiter.

3. 3. Observable long-period comets do not evolve into observable short-period comets.

4. 4. Unobservable long-period comets with perihelia near Jupiter can evolve into observable short-period comets.

5. 5. Long-period comets cannot have been formed or created within the planetary region of the solar system. (This conclusion is somewhat qualified because of possible effects of stellar perturbations. )

6. 6. It is possible that some of ths short-period comets could have been formed inside the orbit of Neptune, but it is certain that others have the same distant source as the long-period comets.

7. 7. The circularly-restricted 3-body problem, and its associated Jacobi integral, are not valid approximations to use in studying origin and evolution of comets.

The study of the nongravitational effects on comets began slightly more than a century and a half ago. As is well known, Encke (1819) demonstrated that comet 1819 I had a revolution period of not more than a few years and that the same comet had also been observed in 1786, 1795 and 1805. The observations clearly required that the revolution period be about 3.3 years, and Encke went on to remark that, after approximate allowance had been made for the perturbations by the planets, the average revolution period seemed to be 1207.9 days between 1795 and 1805, but only 1207.3 days between 1805 and 1819. As a result of a more refined computation of the planetary perturbations, the following year he (Encke 1820) was able to confirm these figures and find in addition that the average period between 1786 and 1795 was as much as 1208.1 days.

The problem of the cosmogony of the solar system has long occupied a central position in astronomical research. The origin of comets plays a substantial role in the solution to this problem. It is therefore necessary to study in detail the “original” and “future” orbits of the comets with nearly parabolic orbits and to examine in particular the great orbical changes that take place when comets pass within the spheres of action of the giant planets.

In the U S.S.R. — especially at the Institute for Theoretical Astronomy (I.T.A.) in Leningrad — considerable attention is paid to investigations of this type. In this respect it is useful to refer to IAU Symposium No. 45, organized in Leningrad in 1970, and in particular to the introductory report by Chebotarev (1972)