Results of radar observatlons of tho Orionid and Eta Aquarid meteor showers carried out In Budrio (Italy) wlthin the IHW program are compared with the simultaneous data from Ondřejov
(Czechoslovakia). The activlty of these showers Is studied in the relation to the motion of large particles ejected from the comet. The activity was found to be independent on tho approach of the parent comet.
We present additional evidence in support of our prevlous work (Crifo, 1987 bc) in which, from theoretical fits to Comet Halley
near -to far- infrared emissions, based on the in-situ flyby probes data, we estimated that the comet was losing half or more of its mass under the form of large (> 1 gram) grains of small (0.3 g cm-^3) density. We confirm that the comet dust-to-gas mass loss rate ratio lies somewhere between the values 0.80 and 18.6, with a best estimate at 3.46. We discuss this result in the context of the general agreement that comets loose less than half of their total mass loss in dust, and this dominantly at very smáli grain sizes. We trace this agreement back to overconfidence placed in a model size distribution which inherently excludes substantial mass loss in
large grains without appropriate experimental justification.
Critical analysis of the results of space experiments, taking into account the contributlon of large particles to the total mass production of comet Halley leads to much higher values of the dust production rate than those derived from the first analysis of space measurements. As a consequence it seems necessary to correct the gas to dust ratio of the mass production by a factor of 10, from 0,1-1,0 to 1.0-10. The corrected values of dust production rate are in much better agreement both with the current concepts of the comeťs history and wlth the evolution of Its meteor stream.
Mechanisms leading to higher particle concentrations in several places along the meteor stream associated with comet Halley are discussed. The positions of the mass concentrations represented by the mean anomaly of the stream orbit, as determined from long series of observations of the Orionids and Eta Aquarida, are correlated with the deviations in the semi-major axis and nodes of the evolving orbit of the comet. It is shown that random deviations in the orbital elements of the comet may be responsible for the nonstable mass concentrations in the stream.
A second part of the pre-perihellon photoelectric measurements of P/Halley obtained at the Skalnaté Pleso Observatory is presented. The observations cover 9 nlghts from November 8, 1985 to January 5, 1986. A set of focal diaphragms of the followlng diameters was used: 29.53", 48.98", 81.08", 137.34" and 220.53". Results of thls paper are magnltudes in focal diaphragms in the fllters Cont. 365.0 nm, Cont. 484.5 nm, CO^, C2, C3 and CN. Coma diameters, photometric parameters and continuum colors are also determined.
From photoeIectric photometry at our 90-cm telescope we derived continuum and emlssion band fluxes within the bandpasses of the.standard IHW filters. These data were converted to give product ion rates for CN, C3, C2 , and soIids. The observations cover the range of pre-periheIion distances from'^2.1 AU to 1.1 AU and include one post-periheI ion measurement at 1.7 AU. The product ion rates of the gaseous components show a strong dependence on heliocentric distance. The result is compared with the behaviour of other comets and theoretical considerations. The dependence is less steep for the solids. This may be due to relatively pronounced backscattering of the grains. During one pre-perihe ion night (r = 1.5 AU) intensity profiles along three sections through the coma of P/Halley were measured. Compared with the Haser model the
profiles show a global anisotropy of the coma and possibly local structure.
Since its recovery in 1982, Comet Halley has been the focus of an unparalleled global scientific effort of exploration. Remote and
in-situ measurements were conducted from the ground, from Earth orbit, from Venus orbit, from interplanetary space, and from the comet itself. Many discoveries, such as the presence of an unexpectedly large and dark nucleus or the abundance of organic presence of organic material, have led to major changes in our ideas about the general nature of comets. In this report, results of various studies are summarized.
Long slit Halley comet spectra taken at ESO Observatory after the perihelion have been analyzed. Relative intensity radial profiles along and perpendicular to the sun direction have been obtained for CN (Δv=0), C2 (Δv=0) and C3 (4050 A) bands. Interpretation of these profiles with the vectorial model allowed us to trace the variability of the gas production rate vs. time before the observations. It has been shown that the comet inereased its gas production rate of a factor 3 on 22.9 March 1986, then it decreased to half the 23.1 March 1986.
Numerous spectrograms of Halley’s Comet have been taken by the author during October 1985 - May 1986 with the Hissar astronomlcal observatory 70-cm reflecting telescope AZT-8 and SPM-1 spectrograph giving a dispersion of 160 A/mm. A two
cascade image intensifier with fiber optics and Kodak 103 aG emulsion have been used. On the basis of four red and near IR spectra taken on 16 December 1985 with 5 to 40 min exposures a set of provisional identifications shown in the Table was given.
About 200 observations from AD66 to 1910 of the tail length of Comet Halley have been used to derive the mean tail length of the comet as visible to the naked eye under very good observing conditions, The curve, covering an interval of -45-^ (t-T)^ 80 days, is skewed and peaks at -SS million km for (t-T) secular decrease of the tail length.