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NASA NTRS1986

Detection of CN emission in Comet P/Halley

The first detection of the CN 3883 A band in emission in Comet P/Halley (1982) in February 1985 is reported. The detection technique, which is described, used blind offsets from secondary reference stars to the comet's predicted position, and involved plotting the comet's track on an enlargement of an appropriate section of the Palomar Sky Survey E plates. The strong solar continuum in the resulting cometary spectrum demonstrates the dominance of the dust reflection spectrum and the reflection from the nucleus. The 3883 A feature is identified with the v = 0 band sequence of the blue system of CN; no other significant features were identified in the spectrum.

Wyckoff, S.↗

NASA NTRS1986

In-situ observations of cometary pick-up ions greater than 0.2 AU upstream of Comet Halley - ICE observations

Burst-like enhancements of energetic ions were observed by the EPAS instrument on the International Cometary Explorer (ICE) during its closest approach to (28 x 10 to the 6th km upstream of) Comet P/Halley, in late March 1986. The ion intensity was modulated by the varying solar wind speed (the latter reaching maxima of around 600 km/s), as was found to be the case for heavy cometary ions accelerated by pick-up in the solar wind flow, during the ICE encounter with Comet P/Giacobini-Zinner (G-Z). Therefore it is concluded that the observed pick-up ions (most probably greater than or equal to 65 keV oxygen ions) are produced by heavy neutrals from Comet Halley. The observations of energetic ions at such large distances suggest the presence, in the neutral atmosphere surrounding the nucleus, of a component with an ionization scale length of 5-10 million km, resulting from a relatively high expansion speed of a few km/s and/or an ionization time scale of a few times 10 to the 6ths.

Wenzel, K.-P.↗

NASA NTRS1985

Far-infrared photometry of Comet Halley from NASA's Kuiper Airborne Observatory

Results of 40-160 micron broadband photometry of Comet Halley with apertures of 30-50 arcsecond diameter are reported. Measurements of the spatial distribution of the far-infrared radiation at several wavelengths are also discussed. A preliminary analysis of the data shows the following conclusions: the brightness of the comet varied by nearly a factor of two on two successive days in March 1986, during which the visible brightness also changed substantially and in the same sense. The far-infrared energy distribution varied significantly in this period in the sense that the decrease in flux with increasing wavelengths was significantly shallower on the day when the comet was fainter. The spectral slope on both days is considerably shallower than would be expected from an ensemble of grains most of which were much smaller than the observed wavelengths.

Harvey, P. M.↗

NASA NTRS1986

An MHD simulation of the interaction of the solar wind with the outflowing plasma from a comet

The interaction between the solar wind and the outflowing plasmas from a comet has been studied by using a two-dimensional time-dependent magnetohydrodynamic (MHD) simulation. The model reproduced several features of the comet-solar wind interaction predicted by earlier theories and observed on the recent cometary probes. These include the formation of the contact surface and the cometary magnetotail. For a constant interplanetary magnetic field (IMF) the cometary plasma captures field lines which drape over the comet to form an antiparallel magnetic field configuration in the tail and a thin plasma sheet. Eventually, tail magnetic reconnection begins to occur at several points. When the IMF orientation is reversed dayside magnetic reconnection occurs at the subsolar point and a large disturbance propagation down the tail.

Ogino, T.↗

NASA NTRS1986

The spacecraft encounters of Comet Halley

The characteristics of the Comet Halley spacecraft 'fleet' (VEGA 1 and VEGA 2, Giotto, Suisei, and Sakigake) are presented. The major aims of these missions were (1) to discover and characterize the nucleus, (2) to characterize the atmosphere and ionosphere, (3) to characterize the dust, and (4) to characterize the nature of the large-scale comet-solar wind interaction. While the VEGA and Giotto missions were designed to study all four areas, Suisei addressed the second and fourth. Sakigake was designed to study the solar wind conditions upstream of the comet. It is noted that NASA's Deep Space Network played an important role in spacecraft tracking.

Asoka Mendis, D.↗

NASA NTRS1986

The NASA infrared telescope facility Comet Halley monitoring program. I - Preperihelion results

The 1-20 microns IR monitoring program for Comet Halley whose results are presented covered January 1985 to January 1986, using a 7-arcsec aperture. It is noted that the comet's dust production was about 6 million g/s at a heliocentric distance of 1 AU; the albedo of the dust in the coma was 3-4 percent. These results are compared with those for previously observed comets.

Tokunaga, A. T.↗

NASA NTRS1986

Thermal-infrared and visual imaging of comet Giacobini-Zinner

Thermal-infrared and visual images obtained simultaneously of comet Giacobini-Zinner on UT 1985 August 4.4 are presented. The 10.8-micron map, which spans several arc minutes with 8-arcsec resolution, is the first ground-based infrared image of a comet. Considerable spatial structure is evident in the map, including a curved dust tail consisting primarily of dust grains larger than 100 microns. The width of this large-grain tail implies that these grains depart the nucleus with velocities less than 3 m/s. The distribution of grain albedo is determined for the central 1 arcmin. It is speculated that an observed increase in the albedo by a factor of 2 with distance from the nucleus and prolonged radius vector results from the confinement to the orbital plane of large fluffy grains previously considered for GZ by others; the lower albedo of this dust could result from multiple internal scattering increasing the probability of absorption of incident light. It is estimated that dust was departing the nucleus at the rate of about 8 x 10 to the 5th g/s, comparable to that derived for many other comets.

Telesco, C. M.↗

NASA NTRS1987

A Voyager-style tour of comets and asteroids 1994-2005

A low cost program that links a dual-comet flyby sample-return mission with a multicomet/asteroid tour is proposed. Two spacecraft are used to carry out this program: a three-axis stabilized Observer-class spacecraft and a smaller spin-stabilized sample-return probe. The Observer spacecraft uses earth-swingby and propulsive maneuvers to accomplish the small-body tour, which includes flybys of three comets (Tempel-1, Tempel-2, and Encke) and two asteroids (46-Hestia and 433-Eros) over a 12-year period. Two of these comets (Tempel-1 and Tempel-2) are also the shared targets, the Observer serves as a navigational aid for the probe, which scoops up dust particles as it flies through the cometary atmosphere. After collecting the cometary dust samples, the probe returns to a low earth orbit where it is recovered by the Space Shuttle.

Farquhar, Robert W.↗

NASA NTRS1986

Observations of the interactions of heavy ions from Comet P/Giacobini-Zinner with the solar wind

During the encounter between the ICE spacecraft and Comet Giacobini-Zinner, intense fluxes of energetic heavy ions were observed up to 4 x 10 to the 6th km from the comet. In this paper the authors correlated observations of heavy ions, the solar wind velocity and the magnetic field direction, and compare the observations of the ions with the theoretical predictions for their variations with distance from the comet, wih the solar wind velocity and with the magnetic field direction.

Sanderson, T. R.↗

NASA NTRS1986

Linearly polarized magnetic fluctuations at comet Giacobini-Zinner

The fully electromagnetic linear Vlasov dispersion equation for instabilities in a homogeneous, magnetized plasma is solved. Parameters appropriate to conditions observed during the ICE encounter with comet Giacobini-Zinner are used to study the polarization of electromagnetic instabilities driven by the relative streaming of two ion components. If the ion/ion relative drift is near the Alfven speed, the associated long-wavelength modes may be approximately linearly polarized at relatively small angles of propagation with respect to the magnetic field. The relatively inhomogeneous plasma background of the comet may impose such oblique propagation, so that this result provides an alternate explanation to the observations of linearly polarized waves near the comet.

Gary, S. Peter↗

NASA NTRS1987

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

The possibility that comets containing up to 100 tons of ice encounter the earth's atmosphere at a rate of one every 20 min is discussed. Cometary 'hail storms' were proposed to explain observed regular transient decreases in the atmospheric UV dayglow intensity. The decreases take the form of dark 'holes' up to 50 km across. The probability that clouds of objects assumed to be as dark as the nucleus of Comet Halley between the earth and moon would be detectable by ground-based electrooptical deep space telescopes is considered. Conflicting projections of the number of objects which would be detected per hour are examined. High correlations are noted between cometary passages (Comets Encke, Tuttle, Tempel) and intervals of meteor showers (Taurids, Leonids, Geminis, etc.). The holes, however, are not correlated or coincident with the showers. It is suggested that dedicated searches for the unclassified dark objects be carried out in November, when cometary fluxes are high.

Soter, Steven↗

NASA NTRS1987

The bare nucleus of comet Neujmin 1

Simultaneous visible and infrared observations of comet P/Neujmin 1 1984c are presented which show that the comet has a large (mean radius 10 km), dark (geometric albedo 2-3 percent) nucleus with a surface which is mostly inert material but which still shows a low level of gaseous activity. This is the first physical evidence that cometary nuclei can leave behind an inert body after the coma activity ceases. No asteroid or asteroid class has been found to match the reflectance and albedo of this comet except possibly some D asteroids.

Campins, Humberto↗

NASA NTRS1986

IRAS observations of comets

The moderate spatial resolution and high sensitivity of the Infrared Astronomical Satellite (IRAS), which surveyed the celestial sphere during 1983 at wavelengths of 12, 25, 60, and 100 microns, were particularly well suited to detecting extended thermal emission from cometary dust. Sources with infrared color temperatures characteristic of solar system bodies, and at the ephemerides position of known comets were selected for analysis by the IRAS Asteroid Data Analysis System (ADAS). The data base is now available for use by researchers. This paper describes the development of the data base, details its entries, and presents a statistical analysis of its contents. The IRAS survey contains multiple observations of many periodic comets. A brief description and analysis is given of the observed infrared and derived physical properties for several comets of special interest.

Walker, R. G.↗

NASA NTRS1987

Electron impact ionization in the vicinity of comets

The solar wind interacts very strongly with the extensive cometary coma, and the various interaction processes are initiated by the ionization of cometary neutrals. The main ionization mechanism far outside the cometary bow shock is photoionization by solar extreme ultraviolet radiation.Electron distributions measured in the vicinity of comets Halley and Giacobini-Zinner by instruments on the VEGA and ICE spacecraft, respectively, are used to calculate electron impact ionization frequencies. Ionization by electrons is of comparable importance to photoionization in the magnetosheaths of Comets Halley and Giacobini-Zinner. The ionization frequency in the inner part of the cometary plasma region of comet Halley is several times greater than the photoionization value. Tables of ionization frequencies as functions of electron temperature are presented for H2O, CO2, CO, O, N2, and H.

Cravens, T. E.↗

NASA NTRS1987

The solar wind interaction with comets: A post encounter view

The recent spacecraft encounters with comets Giacobini-Zinner and Halley have led to an enormous increase in our knowledge of comets, including their dust, neutral gas, plasma, and magnetic field environments. The latter has in turn led to better understanding of the nature of the solar wind interaction with the well developed atmosphere of a comet. The post-encounter understanding of this interaction is reviewed, underscoring the differences with pre-encounter reasoning. The problems outstanding in this area are emphasized.

Mendis, D. A.↗

NASA NTRS1987

Diversity and similarity of comets

The evolution of comets from an early stage where they were all similar, to the later diversity is reviewed. The elemental abundances of all pristine comets are likely to be primitive, that is in solar abundance ratios for all elements including C, N, O, S, but with the exception of H (and assumedly He and Ne) that are severely depleted. The solid phase was originally in very fine grains, typically 0.1 micron or less, eventually sintered into larger grain clusters. The volatile phase contains H, C, N, O, and S molecules frozen in the pores of the grain clusters; cosmic ray plus solar irradiation changes the volatile to refractory ratio of the crust. Differences in dust tails, in plasma tails, in photometry between young and old comets, and in the variable carbon depletion of the gas phase seem to be induced by the decay processes.

Delsemme, A. H.↗

NASA NTRS1987

Anisotropic emission from comets: Fans versus jets. 1: Concept and modeling

Jets and fans of dust ejected from discrete emission sources on the sunlit side of comet nuclei were studied. Computer-generated images of these features illustrate the diversity of patterns that can be explained by varying the position of the spin axis, rotation period, location and extent of the active area, and the ejecta's dynamical constants for a given Sun-Earth-comet geometry. It is shown that an emission fan is generated by rotation of a high-latitude source about the spin axis of the nucleus with a relatively high obliquity, but that the fan's formation is rather insensitive to the rotation period. It appears that at least for comets with perihelia well outside the Earth's orbit, the position angle of a fan's observed axis virtually coincides with the projected direction of the spin axis, thus constraining the spatial orientation of the latter.

Sekanina, Z.↗

NASA NTRS1987

Comet showers as a cause of mass extinction

Three independent pieces of evidence supporting a connection between comet showers and clustering in terrestrial cratering and mass extinctions are presented. The temporal profile of a comet shower triggered by a star passing through the Oort cloud is calculated. Four weak peaks are found in the age of distribution of impact craters over the past 100 Myr, as well as two compact clusters of ages of impact glass broadly coincident with crater-age peaks. Recent paleontological observations are reviewed that indicate a stepwise character for some well-documented mass extinctions in the past 100 Myr which roughly coincide with three of the four peaks in crater ages and which have a duration compatible with comet shower predictions.

Hut, Piet↗