Comets are the only large solar system bodies where nongravitational forces directly affect their dynamic motions. Their approach to within a few AU of the Sun initiates the vaporization of nucleus ices, and the resulting rocket-like effects either add to or subtract from the comet’s orbital energy; the sign of the energy change depends upon the comet’s rotation direction and its spin pole orientation. The cometary outgassing phenomena have generally been modeled by assuming a rapidly rotating nucleus of water ice that outgasses symmetrically with respect to perihelion. Although this nongravitational acceleration model has been quite successful in providing accurate orbits and ephemerides, several comets exhibit water production rates and visual light curves that are noticeably asymmetric with respect to perihelion. New asymmetric models are being developed that attempt to represent more closely the cometary outgassing phenomena. For the same comet, derived nongravitational parameters can differ widely, depending upon which model is used to fit the astrometric data. The uncertainties in the data and in the nongravitational acceleration model prevent realistic extrapolations of these objects’ motion beyond a few hundred years, particularly if close planetary encounters are involved. Accurate orbits, ephemerides and efforts to model the nongravitational effects ultimately depend upon the quality of the astrometric data. Using a combination of long-focus telescopes, charge coupled device (CCD) detectors, microdensitometer reductions and modern star catalogs, cometary astrometric data can be generated that are accurate to the sub arcsecond level. While occultation, spacecraft, and radar observations can provide powerful astrometric data when available, it is still the ground-based optical observations that must provide the vast majority of data for cometary astrometry in the foreseeable future.

I review the primary methods used to determine the spin state of cometary nuclei and the pitfalls and successes experienced in their use. There are in excess of 60 reported determinations of rotational periodicities, but only a few (~4) appear to be reliable, and even these do not necessarily fully describe the true rotational state. An adequate rotational ephemeris is not, at present, available for a single cometary nucleus.

Because recent studies indicate that active cometary nuclei could be in excited spin states, I also review the theory of rigid body rotation from the point of view of remote (astronomical) observers, covering what is known of the effects of nutation on lightcurves, the influence of torques induced by jet activity, and the effects of internal energy and mass dissipation, and nuclear splitting.

The available knowledge on rotation for 8 comets, including P/Halley, is reviewed. Outstanding questions that need early resolution are: (1) Can a consensus be achieved on the rotational state of P/Halley? (2) Is it possible to accurately determine the amplitude of the transverse non-gravitational force associated with rotation? (3) Are the orientations of fan-like comas a valid indicator of the orientation of the spin vector – can definitive observational checks be made in a few cases?

Improved observational and interpretational techniques are needed to advance this field. Improved time-series and zero-date analyses are needed to connect existing and future data sets and to search for multiple periodicities in cometary lightcurves; improved sampling and extension of time-series observations with moderate- and large-aperture telescopes at good sites is needed; near-simultaneous photometric and radiometric observations made when cometary activity is low are particularly significant; well-sampled time-series imaging of near-nuclear phenomena (together with adequate software to analyze them) is essential to diagnose rotational states if nutation is present.

In this paper, those results of the VEGA imaging experiments are summarized that probably have general validity for any comet. Shape, size, surface structure, jet activity, and rotation pattern are considered in this respect. It is pointed out that imaging data provide indispensable information for an understanding of cometary activity.

The study of comets using modern optical photometric techniques is reviewed. Particular emphasis is given to the physical constraints imposed on the nature of comets by photometric data. The photometric study of bare nuclei and of active comets is discussed.

The infrared spectral region (1–1000 μm) is important for studies of both molecules and solid grains in comets. Infrared astronomy is in the midst of a technological revolution, with the development of sensitive 2–dimensional arrays leading to infrared cameras and spectrometers with vastly improved sensitivity and resolution. The Halley campaign gave us tantalizing first glimpses of the comet science possible with this new technology, evidenced, for example, by the many new spectral features detected in the infrared. The techniques of photometry, imaging, and spectroscopy are reviewed in this chapter and their status at the time of the Halley observations is described.

Cometary nuclei are often considered to be the most primitive bodies of the solar system. Thus it is particularly important to find out what structural changes may have been brought about as a result of their thermal evolution. Is there reason to believe that the bulk composition of the nucleus of a present-day short-period comet may differ from that of the original planetesimal in the solar nebula? Apart from the development of a non-volatile surface layer (‘dust mantle’), what further depth-dependent differentiation can we expect in such a nucleus? These are the ultimate questions addressed in this paper, and attention is focused on the two most active stages of thermal evolution: the early planetesimal stage with internal heating basically due to radioactive decay, and the recent or present cometary phase with strong external heating due to insolation of the surface.

The first in situ observations of a cometary nucleus by imaging techniques on board the VEGA and Giotto spacecraft in 1986 opened the possibility of directly studying the properties and structures of a cometary surface. This new information, combined with results from other experiments made by these spacecraft and with results from laboratory experiments and Earth-bound radar observations, gave a better basis for coming from speculation to more precise knowledge in describing properties of cometary surfaces and related activity. It is the aim of this paper to discuss the different views developed to understand properties of cometary surfaces and to propose a consistent model for understanding topological surface properties and related cometary activity. We accomplish this by extending former hypotheses and approaches and making them more precise in the light of the new and more complex data.

Most cometary parent molecules do not strongly fluoresce at ultraviolet and visible wavelengths, and some do not possess permanent electric dipole moments, preventing their study in the radio region as well. However, many of these molecules have strong ro-vibrational transitions in the near infrared (λ ∼ 2 – 5 μm). Since the solar flux at these wavelengths is quite strong, parent molecules in cometary comae can be probed directly via fluorescence in these infrared transitions. The feasibility of this approach was convincingly demonstrated by the detection of H2O in comet Halley (1986 III) from the Kuiper Airborne Observatory and by the detection of H2O, CO2, and H2CO using an infrared spectrometer (IKS) on VEGA. Tentative detections of near infrared lines of CH4 were also reported during ground-based and airborne observations of comets Halley and Wilson (1987 VII). High resolution spectroscopy of the infrared water transitions has yielded a wealth of new information on cometary physics: the absolute line intensities and spatial brightness profiles are used to determine water production rates and lifetimes, the relative line intensities probe the kinetic temperature profile in the coma, the line widths and line positions shed light on coma outflow dynamics, and the temporal variability in the lines provides information on the structure of the nucleus. These observations also allow the determination of the water ortho-to-para ratio, which may provide fundamental insight into the origin and/or evolutionary history of cometary nuclei. Similar observations of other molecules (those mentioned above plus others) will provide important complementary data and will also allow us to compile a volatile inventory for cometary nuclei, but such observations are extremely difficult due to the low abundances of these molecules (≤10% relative to water) and the limitations of present infrared facilities. Recent advances in infrared instrumentation promise to extend sensitivities for parent molecule searches to relative abundances well below 1%, especially if cooled, Earth-orbiting facilities are available.

The Comet Halley observations showed that carbon is a major component of the comet nucleus, with mass spectroscopic data giving near-cosmic C/O ratios. Gaseous and solid compounds were also observed with infrared spectroscopy, which gave detections of CO and CO2, probable detections or upper limits of H2CO and CH4, and a tentative detection of OCS. The CH4/CO ratio of less than unity in Comet Halley points to a CO-rich solar nebula; however, the ratio is higher than in interstellar clouds. A broad, complicated emission feature near 3.4 μm is evidence for carbonaceous compounds containing C-H groups in gas or solid phases. Analysis of radiation mechanisms and abundance constraints suggests that thermal emission or transient heating by single photons can account for the 3.4-μm emission feature. The band resembles (but is not identical to) bands of carbonaceous chondrite organic material, synthetic materials, and interstellar carbonaceous bands. Direct connections among these materials are possible, but have not been established.

Major progress has recently been achieved in the understanding of the coma morphology of dust comets. The conceptual models for the various types of feature observed in the coma (jets, spirals, halos, fans, etc.), their computer simulation, and the hydrodynamic models for jet formation are critically reviewed, and evidence for anisotropic, strongly collimated flows of ejecta emanating from discrete active regions (vents) on the rotating cometary nuclei is presented. The techniques employed to generate synthetic comet images that simulate the observed features are described and their relevance to the primary objectives of coma-morphology studies is exemplified. Modelling of temporal variations in the water emission from discrete active regions suggests that production curves asymmetric with respect to perihelion should be commonplace. Critical comparisons are offered with the activity profiles of Encke’s comet and with light curves of disappearing comets and comets that undergo outbursts. Discussed next is the potential evolutionary significance of nuclear shape, the fractional area of the surface that is active, the vents’ erosion rate, and available information on their activation, aging, dormancy, migration, rejuvenation, and extinction. Recent developments in the understanding of the processes that cause the nongravitational perturbations of cometary motions are reviewed and the observed discontinuities are identified with birth of new sources and/or deactivation of old vents.

During the past decade, vacuum ultraviolet spectra of over 30 comets have been obtained with the International Ultraviolet Explorer (IUE) satellite observatory. With few exceptions, the spectra of these comets appear to be similar, with OH and H produced by the photodissociation of water being the dominant species and emissions of C, O, S, CS and CO+2 usually present. Although signs of variability of many kinds in comet spectra appear, the evidence from the ultraviolet observations suggests that all comets have the same basic chemical composition and that observed differences are due to evolution and ageing processes. The principal exception is S2, which was detected by IUE in comet IRAS-Araki-Alcock (1983 VII), but not in any other comet to date. During the 1985–86 apparition of comet Halley, ultraviolet spectra were also obtained by other spacecraft and by sounding rocket instruments, including a long-slit imaging spectrograph. Further advances await future ultraviolet observations of comets by the Hubble Space Telescope and other planned ultraviolet astronomy missions.

A cometary outburst is an unexpected flare-up in the brightness of a comet, with the brightness normally increasing by 2 to 3 magnitudes, but on occasion by up to 5. This outburst is superimposed on the general change in brightness that is related to the comet’s heliocentric and geocentric distance, spin phase, spin axis orientation and activity geography. The brightness variation during outbursts usually has an abrupt, eruptive onset followed by a quasi-exponential decrease and lasts for a few tens of days. Several cometary outbursts are accompanied by the expansion of a spherical halo from the brightest point at the centre of the visible cometary coma.

Radio observations of comets complement studies at other wavelengths as well as providing certain kinds of unique information. Studies of continuum emission may probe the thermal emission of the nucleus and large-size dust particles. Spectroscopic observations of gas in the coma allow searches for parent molecular species that have no signatures in the optical spectral range. The 18-cm wavelength spectral lines of the OH radical now are relatively easy to detect, and observations of them permit long-term and short-term monitoring of the cometary gaseous output. Moreover, with the unique spectral resolution of radio techniques, aspects of the kinematics of the coma may be also studied, such as the gas expansion velocity and the anisotropy of gas production from the nucleus. In this review, we present recent results of cometary radio observations, and discuss what may be learned from such studies in the future.

The chemical composition of gases in the coma of Comet Halley obtained from in situ and remote observations is reviewed. Water vapor with an abundance of approximately 80% by number is the dominant parent gas. Carbon monoxide is the second most abundant molecule, with about one-seventh of the water abundance. The other expected parent gases—carbon dioxide, methane, and ammonia—contributed with at most a few percent to the coma gas. Molecular nitrogen is found to be a minor species, with an abundance of less than one percent. Formaldehyde is relatively abundant, with a few percent relative to water. Gas-phase isotope ratios of nitrogen, oxygen, and sulfur are found to agree with solar system values within experimental errors. Halley’s hydrogen isotope ratio is comparable to the values found in other solar system objects poor in hydrogen, but distinctly different from the protosolar nebula and objects that accreted hydrogen in gaseous form. Carbon in the cyanide radical is enriched by 35% in 13C compared to the bulk solar system value that indicates the presence of non-homogenized interstellar carbon in Halley. Volatiles released from grains contribute noticeably to the coma gas. Complex organic molecules, inferred from various observations as constituents of the dust grains, are believed to be the origin of the distributed CO source, CN, and other jet structures in the coma.

We present an overview of cometary observations carried out with the VLA. These include OH imaging observations, studies of other molecules and searches for continuum emission. Primarily, we discuss the OH observations obtained of Comet Halley, a periodic comet, and Comet Wilson, a “new” comet. The emission from Comet Halley is confined to a region a few times 105 km. The emission from Comet Wilson shows sporadic blobs at large distances (∼ 106 km from the center), which vary abruptly in both position and velocity. The generally accepted picture of excitation by UV pumping in a spherical outflow of cometary gas explains the coarse phenomenology, but other effects seem to cause significant modulations of the OH emission. We suggest that for Comet Halley, collisional effects near the edge of the quenching region, which is coincidently the approximate distance to the cometopause, may be important, and that for Comet Wilson, outgassing from cometesimals ejected from the nucleus may be significant. We further show that asymmetric line profiles are more common than previously thought. Lengthening integration times and lowering the velocity resolution results in a smooth, round shape for the comet’s brightness distribution. In addition, spectral profiles become broader and more symmetric after long integration times and after integration over larger spatial regions. In addition to the OH observations we summarize the H2CO detection experiments for comets Halley and Machholz.

The isotopic abundances depend on the universal evolution of elements and on the individual history of particular objects. Since it is believed that unprocessed material of the solar nebula is preserved in comets, the data concerning the abundance of stable isotopes in these primitive bodies are of some importance in the cosmological context. The present status of this problem is reviewed. The reliability of results for nuclear species with cosmological and cosmogonical implications, such as D/H, C 12/13, N 14/15, O 16/18, and Mg 24/25/26, is discussed. Significant variation is found for the isotopic abundance of carbon, depending upon which carbon reservoir is sampled. Deuterium is probably enhanced relative to the interstellar ratio. For other isotopes, the ratios are close to those of the terrestrial data. The tendency of the D/H ratio to be at higher values indicates a low temperature in the environment of the comet’s formation, and, together with similar effects in the outer planets, suggests that there were two different primordial reservoirs of deuterium in the solar system. The 12C/13C ratio inferred from in situ mass spectrometry of the dust, as well as from the ground-based optical spectra of the Swan band, tends to be approximately equal to the average terrestrial ratio (89) or larger. Recent results obtained from the CN band provide a significantly lower value (about 65), which corresponds to the carbon isotopic ratio in the diffuse interstellar clouds. The enhancement of deuterium and the possible differences of the carbon isotopic ratio in different species and refractory material are indicative of chemical fractionation processes in the protosolar nebula.