Brightness changes in P/Halley

Thirty photometric magnitudes, in each filter (UBV), of P/Halley (1982i) have been measured on 1986, January 12. They show short-term variability of about 0.15 magnitude. The present measurements show the active nature of the comet.     

The visual brightness behaviour of P/Halley during 1982–1986

The overall long-term behaviour of the total visual coma brightness of P/Halley during October 1982 – June 1986 is presented. The observations used in this study consist of the raw total magnitude data collected from the International Astronomical Union Circulars. We have compared the observed light curve with the predicted one.     

Variation in comet P/Halley

Photoelectric photometry (UBV) of Comet P/Halley performed on April 29, 1986 has been presented. The observations show short-term variability in light. The amplitude of variability in U filter is much more than in B and V filters.     

Observations of HCN in comet P/Halley

We present observations of the HCN J = 1-0 rotational transition at 3.4 mm wavelength in comet P/Halley. The data were obtained during a total of 56 individual observing sessions between November 1985 and May 1986 and represent the first time that a cometary parent molecule has been so extensively monitored. The HCN production rate is well correlated with the total visual magnitude of the comet, and comparison of the HCN production to the total gas production of the comet indicates that it is a relatively minor constituent with 0.1% the abundance of H2O. Comparison of HCN and CN production suggests that HCN is a major parent molecule of CN, but probably not the sole parent. HCN spectra obtained by binning the data with heliocentric distance show that the line width, and thus the parent outflow velocity, increases with decreasing heliocentric distance, and that there is a tendency for the lines to be blue shifted due to anisotropic outgassing from the nucleus. Finally, there is evidence of day-to-day time variability in the total HCN emission and in the hyperfine ratios. The time variation of the total emission is consistent with the known time variable behavior of the comet, and detailed comparisons to optical data, where possible, confirm this interpretation. However, non-LTE values of the hyperfine ratios are not consistent with theoretical modeling of the excitation of these transitions.     

Multi-fluid model of comet 1P/Halley

A new three-dimensional magnetohydrodynamic model of the coma of a comet has been developed and applied to simulations of a Halley-class coma using the solar-wind conditions of the Giotto flyby of Halley in 1986. The code developed for high-performance parallel processing computers, combines the high spatial resolution of smaller than 1 km grid spacing near the nucleus, with a large computational domain that enables structures nearly 10 million km down the comet tail to be modeled. Ions, neutrals, and electrons are considered as separate interacting fluids. Significant physical processes treated by the model include both photo and electron impact ionization of neutrals, recombination of ions, charge exchange between solar-wind ions and cometary neutrals, and frictional interactions between the three fluids considered in the model. A variety of plasma structures and physical parameters that are the output of this model are compared with relevant Giotto data from the 1986 Halley flyby.     

Charge-coupled device photometry of Comet P/Halley

Comet P/Halley has been observed during its approach to perihelion at heliocentric distancesR = 11.0 AU and R = 8.2 AU. No extended coma is seen and limits can be placed on the fraction of the total light contributed by coma. The brightness of the comet varies on a short time scale. The variations may be due to transient activity or to rotation of the irregular nucleus.     

Spectrophotometry of P/Halley (1982i)

Spectral scans of the head of periodic Comet Halley (1982i) have been presented in the optical region (3200–7000 Å) for six nights. Emission features due to NH, CN, CH, C₃, C₂ and NH₂ molecules have been identified. It is found that the comet exhibits night-to-night variation of emission bands. The abundances and production rates of CN and C₂ species have been derived.     

BVR broadband photometry of comets 1P/Halley and 4P/Faye

We analyzed the visible broadband photometric data of comets 1P/Halley and 4P/Faye, obtained during their perihelion passages of 1986 and 1991, respectively at the Sanglokh Observatory (Tajikistan) and the European Southern Observatory (Chile). Applying the Lomb-Scargle periodogram in the V-band magnitudes and B-V color index, we find that the most probable periodicities are 79 ± 6 and 7.36 ± 0.04 days for 1P, and 6.1 ± 0.3 days for 4P. After comparing results of color and magnitude periodograms, we argue there is a systematic difference in the number of signals identified and the level of confidence of the same periodicity in the periodograms. Our results suggest the quest for periodicities in the color of the coma of active comets should be complementary to ones in magnitudes. We have verified that the distribution of the color B-V of Faye’s coma was invariable during and after the possible occurrence of a post-perihelion outburst. We verify a symmetry in the pre- and post-perihelion H₀ photoelectric absolute magnitude of the comet Halley. The same issues were not observed in the B-V color index. We verify that the absolute magnitude H₀ of the comet Halley differs from each other when calculated from the visual or photoelectric magnitudes, due to the section of the coma used to estimate these magnitudes. We also verified that this difference in the photometric aperture can compromise comparisons of B-V color distributions between active comets.     

On the brightness variations of comet Halley at large heliocentric distances

Sporadic variations of its intrinsic brightness of up to 500%, with time scales as short as a few hours, has been exhibited by Halley's comet at large heliocentric distances (11-8 AU). It is shown that many of these brightness enhancements are closely correlated to the encounter of high-speed solar wind streams by the comet. It is proposed that during such periods the night side of the comet gets charged to numerically large negative electrostatic potentials, with consequent electrostatic levitation and blow-off of fine charged dust grains lying on it. This gives rise to the observed brightness variations.     

Scanner observations of P/Halley (1982i)

We present an outline to calculate the principal and indirect portions of the planetary disturbing function, based on the method of W. M. Smart. We truncate the Taylor expansion at the third power of eccentricity-inclination. The outline of a new method to obtain the negative powers of the mutual distance between two planets is also given, neglecting powers higher than the fourth in the eccentricity-inclination.     

Spectrophotometric study of P/Halley during post perihelion passage

Spectrophotometric observations of P/Halley are presented for four nights in the optical region (3200–7000 ) during post-perihelion period. Emission features due to CN, CH, C₃ and C₂ molecules are identified and the total apparent fluxes in each emission band are measured. It is found that the comet display large variations in the emission bands as well as continuum.     

Negative ion chemistry in the coma of comet 1P/Halley

Negative ions (anions) were identified in the coma of comet 1P/Halley during in situ Electron Electrostatic Analyzer measurements performed by the Giotto spacecraft in 1986. These anions were detected with masses in the range 7–110 amu, but with insufficient mass resolution to permit unambiguous identification. We present details of a new chemical‐hydrodynamic model for the coma of comet Halley that includes—for the first time—atomic and molecular anions, in addition to a comprehensive hydrocarbon chemistry. Anion number densities are calculated as a function of radius in the coma, and compared with the Giotto results. Important anion production mechanisms are found to include radiative electron attachment, polar photodissociation, dissociative electron attachment, and proton transfer. The polyyne anions C₄H^? and C₆H^? are found to be likely candidates to explain the Giotto anion mass spectrum in the range 49–73 amu. The CN^? anion probably makes a significant contribution to the mass spectrum at 26 amu. Larger carbon‐chain anions such as C₈H^? can explain the peak near 100 amu provided there is a source of large carbon‐chain‐bearing molecules from the cometary nucleus.     

On the non-gravitational forces in the 1986 return of P/Halley

In this paper we study the time evolution of the non-gravitational forces (NGF) along with the observational bias, during the 1986 return of comet Halley. First, evidence is presented which shows that a NGF model with constant coefficients along the radial and transverse directions (A1 andA2), and a constant coefficient radial-only observation bias model fails to represent the observations within the conservative optical measurement noise of one second of arc. Second, we present a stochastic approach to the problem, where coefficients for the radial, transverse and normal components of both the NGF and the observation bias, are allowed to vary in time as three statistically independent Gauss-Markov processes. Finally, an orbit is estimated with this model, to fit observations made during the last apparition, employing a stochastic optimal smoother.Results are given in terms of time histories of the coefficient estimates along with their smoother computed uncertainties. A plot of the observation residuals is also included, showing a uniform and unbiased behavior. The analysis of the results confirms some of the assumptions often made when modelling cometary motion, but questions others. In particular, the normal bias coefficient shows an unexpected pre-perihelion peak (–1200±250 km) which proves that a radial-only observation bias model may lead to biased orbital estimates and unrealistic computed uncertainties.     

Evidence for methane and ammonia in the coma of comet P/Halley

Methane and ammonia abundances in the coma of Halley are derived from Giotto IMS data using an Eulerian model of chemical and physical processes inside the contact surface to simulate Giotto HIS ion mass spectral data for mass-to-charge ratios (m/q) from 15 to 19. The ratio m/q = 19/18 as a function of distance from the nucleus is not reproduced by a model for a pure water coma. It is necessary to include the presence of NH3, and uniquely NH3, in coma gases in order to explain the data. A ratio of production rates Q(NH3)/Q(H2O) = 0.01-0.02 results in model values approximating the Giotto data. Methane is identified as the most probable source of the distinct peak at m/q = 15. The observations are fit best with Q(CH4)/Q(H2O) = 0.02. The chemical composition of the comet nucleus implied by these production rate ratios is unlike that of the outer planets. On the other hand, there are also significant differences from observations of gas phase interstellar material.     

Possible collisions of P/Halley and P/Machholz 1 with Jupiter and the terrestrial planets

The perturbed motion of comet Halley and comet Mackholz 1 1986 VIII was investigated within a time interval of about 20 millennia. The minimal distance of 0.043 AU between P/Halley and Venus may occur on April 4, 4868 AD. The distance of 0.036 AU between P/Halley and Jupiter will take place on April 1, 6616 AD. The orbit of P/Machholz 1 crosses the orbits of Mercury and Venus eight times, that of the Earth six or eight times, and the orbit of Mars four times per a period of advance of the argument of perihelion. A distance of about 0.06 AU between P/Machholz 1 and the Earth may take place in August 2576 AD and 5751 AD and in February 4770 AD. The minimal comet-Earth distance of 0.035 AU occurs on September 14, 5971 AD. The closest encounter between P/Machholz 1 and Jupiter at the distance of 0.098 AU may be in May 4499 AD. These results may be considered as a forecast of possible collisions.     

Possible collisions of P/Halley and P/Machholz 1 with Jupiter and the terrestrial planets

The perturbed motion of comet Halley and comet Mackholz 1 1986 VIII was investigated within a time interval of about 20 millennia. The minimal distance of 0.043 AU between P/Halley and Venus may occur on April 4, 4868 AD. The distance of 0.036 AU between P/Halley and Jupiter will take place on April 1, 6616 AD.The orbit of P/Machholz 1 crosses the orbits of Mercury and Venus eight times, that of the Earth six or eight times, and the orbit of Mars four times per a period of advance of the argument of perihelion. A distance of about 0.06 AU between P/Machholz 1 and the Earth may take place in August 2576 AD and 5751 AD and in February 4770 AD. The minimal comet-Earth distance of 0.035 AU occurs on September 14, 5971 AD. The closest encounter between P/Machholz 1 and Jupiter at the distance of 0.098 AU may be in May 4499 AD. These results may be considered as a forecast of possible collisions.     

Thermodynamic model of comet 29P/SW brightness changing

We analyze the thermodynamic model of changes in comet brightness focusing on the exceptional case which is comet 29 P/Schwassmann-Wachmann(hereafter 29 P/SW). This object demonstrates quasiregular flares that occur in a short period of time; most often it lasts about a dozen hours. The analysis of observational data shows that the average number of comet outbursts for 29 P/SW is 7.3 per year. This is the only known comet with such a number of outbursts per year, which demonstrates its uniqueness. In the proposed approach to analyze the outburst of comet 29 P/SW, we took into account the complex structure of particles which are in its coma and assumed that they are composed of water ice, dust and organic matter.The paper explains how this diversity affects the more efficient scattering of incident sunlight during the outburst of comet 29 P/SW.