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 order to survey the mission possibilities for the short period comets two catalogues are developed. In the first the physical and pertinent orbital characteristics are given for 65 short period comets. Since missions for short period comets are for the most part expected to utilize arrivals near perihelion at a time when the comet is comparatively active, the second catalogue is one containing the predicted perihelia for each of the 65 comets between 1974 and 2010. Included is enough geometry to indicate feasibility of Earth-based observation and sighting within 100 days of perihelion.

This paper summarizes three proposed ballistic spacecraft missions to intercept P/Encke during the 1980 apparition, establishes a baseline physical activity model for P/Encke, and assesses the performance of the neutral mass spectrometer and of the Imaging experiment on each intercept mission, Many of the most fundamental questions about comets are unlikely ever to be answered by ground-based or near-earth observations alone, and a direct intercept of a comet by an appropriately instrumented spacecraft is required in order to directly study the comet and its intercept mission.

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.

Ballistic intercept missions to comets have been strongly endorsed as the best way to initiate a program of cometary exploration (Roberts, 1971). This mission mode is the simplest and least expensive, and can provide a large science return. Currently, a near-perihelion intercept of Encke’s comet in 1980 is receiving serious consideration for the initial cometary mission. Assuming that the 1980 Encke mission will be carried out as planned, a question that should be considered is: what is the next logical step in an evolutionary sequence of cometary missions? Two possibilities are:

• Investigate a particular comet in detail. That is, perform a rendezvous mission.

• Study the physical characteristics of several types of comets. This goal could be achieved by carrying out a series of intercept missions to comets that have exhibited diverse behavior.

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.

This paper considers science aspects of a 1980 spacecraft reconnaissance of Comet Encke. The mission discussed is a ballistic flyby (more exactly, a fly-through) of P/Encke, using either a spin-stabilized spacecraft, without despin of instruments, or a 3-axis-stabilized spacecraft. Celestial mechanics and imaging aspects of such a mission have been considered in more detail by Bender (1) and by Jaffe et al (2), respectively. Engineering designs (3, 4) and more detailed accounts of science aspects are given in other documents. A different approach to an Encke ballistic flyby has been suggested by Farquahar et al (5). Yeomans (6) has considered ephemeris uncertainties associated with such missions.

The present observational problem of understanding the nature and origin of comets is analogous to that we would face in attempting to understand a planet and its atmosphere if we possessed only data on its ionosphere and exosphere. Virtually everything we can observe remotely is a part of the rapidly escaping gas and dust atmosphere of the comet.

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)