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NASA NTRS1986

International Ultraviolet Explorer (IUE) observation of Halley's Comet

The main objectives of the IUE Halley's Comet program are: study of the physical and chemical properties of a comet clearly different from those observed previously with IUE; study of the long-term evolution of the gas and dust coma and the ion and dust tails, by combining IUE data with UV and in-situ observations obtained by other spacecraft; and assessment of the gas and dust environment of Halley in terms of its possible impact on the trajectories of the flyby spacecraft. Another program investigates the CS scale length, and the rotational profile of CS in order to test and calibrate dynamic-flow models of comets. These observations will begin in August 1985 when the heliocentric distance of the comet is 2.8 AU and its total visual magnitude + 13. They will continue into the ninth IUE observing period, through mid-1986.

Kondo, Y.↗
NASA NTRS1985

The spacecraft 'fleet' to comet Halley - An introduction

Questions related to the formation and the characteristics of comets are discussed, and it is suggested that an evolutionary link exists between comets, prebiotic organic synthesis, and the origin of life as it is known. Fundamental questions about the solar wind interaction with comets, are considered, giving attention to the ionization mechanisms in the different cometary regions, the presence of field-aligned currents, and the cometary plasma tail. It is pointed out that some of the questions will soon be answered when the NASA-ESA International Cometary Explorer (ICE) goes through the tail of Giacobini-Zinner in September 1985, and when a five spacecraft 'fleet' arrives at comet Halley in March 1986.

Tsurutani, B. T.↗
NASA NTRS1986

Modeling of dust halo formation following comet outbursts Preliminary results

Evolution of gas and dust distributions following a spatially and temporally localized comet outburst was calculated using a hybrid kinetic - hydrodynamic method. It was found that a comet outburst resulted in significantly increased dust terminal velocities. As these higher terminal velocity values result in larger apex distances, therefore the outburst generates distinct dust envelopes in front of the regular dust coma. Such envelopes were observed at several comets (cf. comet Donati).

Gombosi, T. I.↗
NASA NTRS1986

Orbit determination for the Mariner Mark II Comet Rendezvous/Asteroid Flyby mission - The orbiting phase

The Comet Rendezvous/Asteroid Flyby (CRAF) mission is the first of the Mariner Mark II mission set, designed to explore the outer solar system. Major objectives of orbit determination will be determine the positions and masses of the comet and asteroid and the relative position of the spacecraft, which is important to accurate pointing of the scan platform on which the narrow angle camera and scientific instruments are positioned. Position prediction is also important, since continuous commuication with the spacecraft will not be possible. The small gravitational attractions and poorly known ephemerides of the comet and asteroid, and the small, slow spacecraft orbit about the comet, pose significant new problems for orbit determination. Results of simulations studying the effectiveness of key data types, the accuracies of estimates, and prediction capabilities, are presented.

Weeks, C. J.↗
NASA NTRS1986

Optical investigation of Comet Halley

The optical properties of comet p/Halley are being investigated. Initial research has revealed the basic characteristics of the nucleus and provided unique insights into the formation and structure of the coma and the plasma tail. The study is among the most intensive conducted for any comet. The results from comet p/Halley may be useful in the interpretation of results from other comets.

Jewitt, D. C.↗
NASA NTRS1986

Astrometric observations of comets and asteroids and subsequent orbital investigations

During the past year some 500 observations were made on 66 nights and published on the MPCs (Minor Planet Circulars/Minor Planets and Comets). In addition, a handful of measurements of earlier plates were completed and published. 121 of the observations published referred to comets. Of special importance were observations of comets (P/Giacobini-Zinner and P/Halley) in connection with the NASA ICE and ESA Giotto missions, but a special effort was made to get good coverage of almost all of the observable comets. Observations were also made of (2060) Chiron and of the earth-approaching objects (1627) Ivar, (1866) Sisyphys, (1943) Anteros, (3362) 1984 QA, 1985 JA, PA, TB and WA, and 1986 DA and EB. 46 minor planets were given permanent numbers entirely as a result of the observations.

Mccrosky, R. E.↗
NASA NTRS1986

Extended atmospheres of outer planet satellites and comets

An analysis of the extended atmospheres of outer planet satellites and comets is made. Primary emphasis is placed on cometary atmospheres because of the return of Comet P/Halley. As part of a collaborative effort with A.I.F. Stewart, observations of the hydrogen coma of Comet P/Giacobini-Zinner obtained from the Pioneer Venus Orbiter ultraviolet spectrometer (PVOUVS) were successfully analyzed at AER and are reported. In addition, significant pre-modeling and post-modeling activities to support and analyze the PVOUVS observations of Comet P/Halley successfully acquired in late 1985 and early 1986 are also discussed. Progress in model preparation for third-year analysis of the Voyager UVS Lyman-alpha brightness distribution emitted by hydrogen atoms in the Saturn system is also summarized.

Smyth, W. H.↗
NASA NTRS1986

General features of the Comet Halley-solar wind interaction from plasma measurements

The Giotto Complete Positive ion, Electron and Ram Negative Ion measurements near Comet Halley plasma experiment identified regions in which the solar wind interaction with the cometary plasma displayed characteristic features. Beginning 4.6 million km from the comet there is an upstream region with sporadic connection to the comet. An electron foreshock is present up to 250,000 km away from the bow shock. A bow shock is detected at 1.15 million km. Between the bow shock and the cometopause the outer regions can be divided into three parts. A cometopause is found at 135,000 km, and a density decrease is detected at 45,000 km from the comet. The detailed plasma features associated with these regions are described.

Reme, H.↗
NASA NTRS1986

Comet Halley: The Curtis Schmidts-Isla de Pascua observations

Halley's comet plasma tail disturbances and attendant tail phenomena were observed. Nearly simultaneous exposures with two telescopes serve to correlate information obtained with the two instruments. Photographs of 14 pre-Halley comets taken on 54 nights were examined with a view to cross-interpretation of phenomena seen in Halley with the earlier comets, as recorded on a homogenous collection of plates taken with the same instrument. The tail of Halley was highly active. This contrasts sharply with pre-Halley comets where undisturbed tails are the rule. During March and April, disturbances appeared in the tail of Halley at an average of 1 new distrubance every 3.7 days. It is considered that 10 of the 11 observed disturbances had common characteristics which allow them to be characterized as disconnections.

Miller, Freeman D.↗
NASA NTRS1986

The ICQ photometric archive for P/Halley and other comets

The International Comet Quarterly (ICQ) archive is introduced. There is still no electronic (e.g., photoelectric photometers, CCDs) means for readily obtaining total visual magnitudes of comets when the coma diameters are greater than 20 arcsec and visual estimates of the total brightness are still the most abundant type of data for comets during the past 100 yr besides astrometric data. The ICQ published greater than 18,000 visual observations of comets made by 600 observers since 1940. What is available, what is accomplished thus far, and potentials and limitations inherent in the ICQ photometric data archive are discussed.

Green, Daniel W.↗
NASA NTRS1986

Pre-perihelion observations of Comet Halley at 2-13 microns

Between Nov. 8, 1985 and Jan. 13, 1986 comet Halley brightness was obtained as a function of time at 2.3, 3.6, 4.6, and 10.3 microns. Brightness outbursts were noted, an especially significant one beginning Jan. 10, 1986 and persisting for several days during which the IR brightness more than doubled. The heliocentric brightness indices for 2.3, 3.6, 4.6, and 10.3 microns are -5.7, -7.1, -8.5, and -6.2, respectively. Comet Halley's absolute magnitudes at these wavelengths are 7.3, 4.4, 2.8, and -1.8. The 4.6 to 10.3 micron color temperature dependence on heliocentric distance (R) is slightly less steep than the Rsup -1/2 expected theoretically. The albedo of the comet is 8% to 9%. Based on spectral differences between outbursts on Nov. 8, 1985 and Jan. 10 to 13, 1986, it is concluded that the comet displays composition inhomogeneities.

Lynch, David K.↗
NASA NTRS1986

The brightness variations of Comet Halley at large heliocentric distances

The reasons for the intrinsic brightness variations of up to 500 percent on time scales as short as a few hours detected by Sekanina (1984) in Comet Halley between October 1982 and February 1984 are discussed. It is shown that solar wind-modulated electrostatic dust blowoff from the night side of the comet is consistent with the observed brightness variations. The variations coincide with the encounter of high-speed streams with the comet. The stream's propagation time to the comet and the sun's rotation during this transit were used to locate the stream origin on the coronal surface, and the results are shown.

Flammer, K. R.↗
NASA NTRS1986

A disturbance of the ion tail of Comet Halley and the heliospheric structure as observed by Sakigake

In order to study the interaction between the solar wind measured by Sakigake and ion tail disturbances of comet Halley, more than 500 photographs of the comet taken on the ground during this apparition are surveyed. The focus of the present study is the December 31, 1985, event, when various types of disturbances occurred, including an outstanding disconnection event (DE)-like knot. Analysis of the Sakigake/IMF data reveals that comet Halley did not encounter the heliospheric neutral sheet on that day, demanding a new explanation for the DE-like event, different from the Niedner-Brandt model. During this event the comet encountered a high-speed solar wind stream from a coronal hole tongue of the sun. The event can be explained by a dynamic pressure model, according to which the DE-like plasmoid was caused by a sudden increase in the dynamic pressure of the solar wind. A result of the simulation work by Ogino is found to support this interpretation.

Saito, T.↗
NASA NTRS1986

ICE plasma wave measurements in the ion pick-up region of Comet Halley

In late March 1986 the plasma wave instrument on the International Cometary Explorer (ICE) detected sporadic bursts of strong plasma turbulence with average wave characteristics very similar to those detected six months earlier, during the ICE traversal of the Comet Giacobini-Zinner (G-Z) heavy ion pick-up region. In both cases the observations of enhanced wave levels were generally correlated with simultaneous detection of energetic ions. The 1986 activity is interpreted in terms of plasma instabilities associated with solar wind pick-up of ions produced by heavy neutrals from Comet Halley. On March 25, when the distance between Comet Halley and ICE was 28.1 million kilometers, the ICE-to-comet range was about six times greater than the distance that marked the measured outer boundary of the turbulent heavy ion pick-up region of G-Z. Based on comparison with G-Z data and with earlier Halley observations, plausible arguments suggest that in late March, Halley should have produced detectable levels of energetic ions and associated plasma turbulence in a region with a spatial extent of 30-40 million kilometers.

Scarf, F. L.↗
NASA NTRS1986

Comets, X-ray bursts, and gamma-ray bursts

The proposal, revived by Tremaine and Zytkow (1985), that accretion of comets by neutron stars may be the origin of gamma-ray bursts is considered. This mechanism has difficulty accounting for the observed gamma-ray spectrum and optical counterparts of the bursts. The survival of comets near supernovae is investigated. Ablation rates and the thermal structure of an ablating surface layer are calculated. In some circumstances, mechanical disruption will erode a comet more rapidly than evaporation. The accretion of comets by neutron stars may produce a class of X-ray burst sources with novel properties.

Katz, J. I.↗
NASA NTRS1986

Comment on the letter 'On the influx of small comets into the earth's upper atmosphere'

A critical comment is made on the proposal by Frank et al. (1986) that a flux of small comets constantly impacts the earth's atmosphere. The seismic amplitudes generated in the moon by such comets impacting that body are calculated, and it is concluded that the proposed flux of comets is 100,000 times greater than would be supported by either the seismic impact rate of the moon or the amplitude of its steady state reverberation. Frank et al. reply that this discrepancy may be traced to the insensitivity of the lunar seismic stations for the detection of the impacts of tenuous, weakly bound comets relative to those of dense, stony meteoroids.

Davis, P. M.↗
NASA NTRS1986

Cometary grain scattering versus wavelength, or 'What color is comet dust'?

Optical and near-infrared observations of comets are combined in a systematic study of the wavelength dependence of the scattering from cometary grains. The normalized rate of change of the reflectivity of cometary grains with respect to the wavelength of observations decreases as the wavelength increases. The observed wavelength dependence of the reflectivity gradient is consistent with an origin by scattering from micron-sized or larger, slightly absorbing spheres. The optically important comet grains are about an order of magnitude larger than the optically important interstellar grains. This size difference is a probable result of grain growth in the cloud from which the comets condensed. Grain properties differ so widely among the comets that any phase angle or heliocentric distance dependences of the continuum color are hidden.

Jewitt, David↗
NASA NTRS1986

The chemical composition of comets and possible contribution to planet composition and evolution

The theory of cold aggregation of interstellar dust into comets is shown to support the ideas that comets provided not only a substantial part of the earth's water but also of its complex organic surface material. Comparison of the density of interstellar dust with meteor densities leads to the conclusion that comets are very fluffy bodies whose solid matter occupies no more than 2/5 of the total volume and possibly substantially less. This implies an extremely low comet albedo of about 0.05 in the visual even for slightly absorbing individual components.

Greenberg, J. Mayo↗