Spacecraft encounters with comets Giacobini-Zinner and Halley revealed a great variety of collective plasma phenomena accompanying the interaction of the solar wind with comets. In this review, we discuss the theory and in situ measurements of the Alfvén wave turbulence and the solar wind loading by cometary ions, and the structure of the cometary bow shock.

This review will discuss various plasma waves and instabilities that have been observed near comets. Comments on nonlinear wave evolution and wave cascading, as well as the role of nonlinear waves in wave-particle interactions, will be made.

The solar wind interaction with comets is characterized by the mass-loading of the solar wind with heavy cometary ions that are produced by the ionization of neutrals in the extensive cometary coma. This mass-loading slows down the solar wind and ultimately leads to the formation of a magnetic barrier and a magnetotail. Solar wind protons disappear in the vicinity of the cometopause due to charge-exchange collisions with neutrals. The plasma and fields inside the cometopause of comets Halley and Giacobini-Zinner were observed by instruments on board several spacecraft. Several plasma populations were detected in the inner coma, including cold (less than 1 eV) and energetic (several keV or more) ions, and cold and hot electrons. The Giotto magnetometer observed a diamagnetic cavity surrounding the nucleus, which is a consequence of an outward ion-neutral drag associated with the flow of cometary neutrals past plasma frozen onto field lines in the magnetic barrier. In addition to large-scale structures, many small-scale structures in the plasma and fields have been observed in comets, including tail rays and kinks, and plasma pile-ups and depletions in the barrier. Theoretically, the existence of a very narrow layer of enhanced plasma density just inside the diamagnetic cavity boundary has been predicted. The behavior of plasma in the inner coma (within the cometopause) of an active comet is determined both by plasma processes, such as magnetohydrodynamics, and by collisional processes such as ion-neutral friction, resistivity, electron and ion thermal cooling, ion-neutral chemical reactions, and electron-ion recombination.

Some 24 points are made concerning the author’s understanding of the cometary nucleus as developed in this colloquium. Comparisons with an earlier report are noted, and most of the earlier advances in concept are substantiated. The mean density of the Halley nucleus may have been underestimated. The dust/gas ratio apparently needs to be increased by as much as two times, perhaps to unity or higher. The nature of the rotation remains puzzling. CHON grains now appear to be more important sources of gas than thought earlier. A very low temperature at, and subsequent to, the formation of the nucleus continues to receive support, as does a solar-system composition. The 12C/i:3C ratio may, however, be lower than expected.

The first widely distributed printed comet images appear in the Nuremberg Chronicle, whose Latin edition appeared in 1493, followed closely by a German edition. In the first section, we begin our consideration with the comet image that has frequently been cited as a representation of the A.D. 684 apparition of Comet P/Halley. To better understand this image, we present a thorough survey of the 13 comet images that appear in the Chronicle, all reproduced from four woodblocks, representing 14 apparitions between A.D. 471 and A.D. 1472. In the second part, we present an analysis of the unpublished preparatory drawings for the comet images in the handwritten Exemplars (manuscript layout dummies) for both the Latin and German editions in the Stadtbibliothek, Nuremberg. Finally, in the third part, we demonstrate how the Chronicle presaged the proliferation of broadsides--woodcut prints that functioned like tabloids of the sixteenth and seventeenth centuries. We examine broadsides recording historical comets over such Bavarian cities as Nuremberg and Augsburg. In spite of their superstitious, hysterical journalism, fed by turbulent political and religious upheavals, these broadsides reveal a nascent scientific attitude.