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.

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 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.

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 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.

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.

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.

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.

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.

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.