The ground 2π3/2 state of OH consists of 2 A-doubled levels which are separated by about 1666 MHz. The upper (parity = +1) and lower (parity = −) levels each have eight hyperfine subleveis which consist of a three-fold degenerate F=1 and five-fold degenerate F=2 energy state, and transitions between these levels give rise to the OH-18-cm radiowave spectrum. Of the four possible transitions the F=2 → 2 and F= 1 → 1 transitions are most intense and are the source of the 1667 MHz and 1665 MHz signals observed from comet Kohoutek (Biraud, et al (1973), Turner (1973)). The peak antenna temperature ΔTb/Tb for these lines are approximately proportional to the ratio i = (N+-N-)/(N++N-) where N± are the total concentrations of 2 π3/2, J = 3/2 molecules in the indicated parity state. In the optically thin, collisionless atmosphere of a comet these populations are determined predominantly by the fluorescent scattering of solar u.v. radiation by 12 absorption lines of the OH(A2 Σ+ ← X2 π) transition. The steady state distribution is only a function of the relative solar flux at these 12 absorption wavelengths. The molecules are pumped into a large set of 2π states, which then rapidly cascade by infrared transitions back to either the + or − levels of the ground state. Because of the Doppler shift of the absorption spectrum relative to the solar Franhaufer spectrum, the ratio i is a sensitive function of the heliocentric velocity Vh of the comet, and the radio signals can be seen either in absorption or stimulated emission relative to the galactic background temperature Tb, depending on whether the levels are anti-inverted, i < o, or inverted, i > o.

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 A3Π – X3Σ− transition of NH is a common feature of cometary spectra. Since the NH molecule is likely to be formed by photodissociation of molecules such as ammonia or hydrazine, identifying the final states of the photolysis would shed light on the identity of the parent. Stief and DeCarlo noted that the photolysis of ammonia at 123.6 nm results in emission at 324.0 nm in the c1Π – a1Δ system. They suggested that in the absence of collisions in the coma, the NH(a1Δ) should accumulate if ammonia is the source of NH radicals. The absence of the singlet system in this view would suggest another parent molecule than ammonia as the source of the NH radical. However, the pumping rate of the UV transitions in a comet is also very small. This note will show that the transition rate for the a1Δ – X3Σ− transition is sufficiently fast to deplete any a1Δ concentration formed in the original photolysis process.

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.

The main lines of OH at 18 cm wavelength were observed in Comet Kohoutek (1973f) from December 1973 through February 1974 with the Nancay radio telescope. They were detected in absorption in early December and reappeared in emission around mid-January. In a preliminary approach these results are interpreted in terms of U. V. pumping by the sum when the Fraunhofer spectrum is taken into account.

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).

It has long been recognized that collisional cooling and recombination processes are likely to be important in the inner cometary coma, in a 104 km radius region for the larger comets (Biermann & Trefftz 1964) Cherednichenko (1970) laid stress on dissociative recombination processes, as possibly playing a role in the production of observed ions and radicals. Oppenheimer, in his spirited contribution to this conference, emphasized that a variety of ion-molecule interactions occur relatively rapidly and probably take part in the production of known cometary radicals.

Type I or ionic comet tails on the average make an angle of a few degrees at the nucleus with the prolonged radius vector in the direction opposite to the comet’s orbital motion. This fact was explained by Biermann (1951) as the aberration angle caused by the comet’s motion in the outflowing solar wind plasma, and, as is well known, led to the discovery of the solar wind itself.

It is almost generally accepted that the essentially structureless and often significantly curved tails of comets are composed of sunlight-scattering solid particles of various sizes, ejected from the comet’s nucleus by evaporating gases. Much less agreement has so far been achieved as to the character, composition, and size distribution of the particles.

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.

The origin of the Zodiacal cloud has been attributed to an influx of cometary debris which maintains a stable meteoritic complex (Whipple, 1955). Objections to a cometary origin of the Zodiacal cloud were presented by Harwit (1963) without denting the cometary theory (Whipple, 1967). Since then, the Finson-Probstein theory of dust production has been applied successfully to dusty comets. As a consequence size distributions of dust particles have been deduced for Arend Roland, 1957 III, (Finson and Probstein, 1968), Seki-Lines, 1962 III, (Jambor, 1973), Bennett, 1970 II, (Sekanina and Miller, 1973) and Kohoutek, 1973 f. (Jambor unpublished). Only careful consideration of the size distribution of the dust from periodic comets can resolve the problem of the origin of the Zodiacal cloud.. The following reexamines the production and history of the dust released from periodic comets using the Finson-Probstein theory and compares it to the size distribution of dust deduced from the above mentioned comets.

The HEOS 2 satellite was launched into a highly eccentric orbit around the earth on January 31, 1972 and re-entered the earth’s atmosphere after a successful mission on August 2, 1974. Due to the orbit (apogee: 240 000 km, perigee: 300 – 5000 km) the satellite spent most of the time in the interplanetary region where the influence of the earth’s gravitation field is negligible with regard to its effect on interplanetary dust particles.

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.

This paper presents physical properties of interplanetary dust determined by in-situ techniques. It is probable that, like millimeter-sized meteoroids (Jacchia, et al. 1967), most interplanetary dust is cometary matter. Although a cometary origin for interplanetary dust is widely accepted (Whipple 1967) (Millman, 1972) there is currently no unambiguous proof of this hypothesis. The results presented here must be interpreted accordingly. It must also be remembered that even if interplanetary particles are cometary, they might possibly be altered in the interplanetary medium by collisions and by thermal effects during close perihelion passages, so the dust particles may not be representative of unaltered cometary material.

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)).

Several recent studies have been undertaken to optimize mission strategies and to select appropriate instrumentation for in situ studies of short period comets (Farquhar et al, 1974; Bender, 1974; Newburn, 1973; Meissinger, 1972; Roberts, 1971). Although some studies have contrasted the physical characteristics of several proposed target comets, few have comprehensively studied the orbital history and ephemeris uncertainties of target comets. In general, the navigational accuracy of cometary flyby probes is almost entirely dependent upon the target comet’s position uncertainty at the time of intercept. Although cometary error analyses are necessary for realistic mission planning, such analyses cannot be conducted in the standard fashion. Comets are affected by nongravitational forces (Marsden et al, 1973), they occasionally exhibit slight discontinuities in their orbital motions, and at least one comet (Biela) has completely disintegrated (Marsden and Sekanina, 1971). Each comet is an individual. Comets have steadfastly resisted recent attempts at classification. Hence, it seems clear that, for each comet of interest in mission planning, a separate in-depth error analysis study must be undertaken to realisticly determine the target comet’s ephemeris uncertainty at the time of intercept. Such studies should consider a number of criteria in order to assure accurate ephemerides for prospective cometary targets. Using the 1980 apparition of comet Encke as an example, these criteria are outlined below.