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NASA NTRS1981

Faint comet searching

Comets are usually discovered to a magnitude limit of about 19 at best. This can be improved by one magnitude with a blink-search technique that is described here. To find a new comet, the required search area near opposition is about 600 square degrees at a magnitude limit of about 20.0. Three faint comets were found this way; they are inconspicuous and would not have been discovered in any other manner. It therefore appears that the presently known statistics must be incomplete at the faint end due to incompleteness of the discovery observations. Another future method to find such inconspicuous comets is with electronically scanning cameras and computerized reduction. If such a CCD scannerscope were to have an aperture of about 1.8 m, it could discover at least 40 comets per year.

Gehrels, T.↗
NASA NTRS1982

Visibility of comet nuclei

Photography of the nucleus of comet Halley is the goal of several planned space missions. The nucleus of a comet is surrounded by a cloud of dust particles. If this cloud is optically thick, it will prevent observation of the nuclear surface. Broadband photometry of nine comets has been analyzed to determine the visibility of their nuclei. Only in the case of comet West near perihelion was the dust dense enough to interfere with imaging. Comparison of the visual brightness of the well-observed comets with that of Halley in 1910 leads to the conclusion that the nucleus of Halley can be imaged without significant obscuration by the dust.

Ney, E. P.↗
NASA NTRS1981

Distribution and activity of discrete emission areas on the nucleus of periodic Comet Swift-Tuttle

A general model is proposed which accounts for the dynamical evolution of the observed jets, envelopes and tail bands of periodic Comet Swift-Tuttle, the parent comet of the Perseid meteor stream, in terms of dust ejection from discrete active regions on the rotating nucleus. High-resolution drawings and measurements of cometary jets made upon the comet's first appearance in 1862 are used to infer a nuclear rotation period of 2.77 days, obliquity of 80 deg, and spin axis orientation with respect to the solar direction of 60 deg. The observed jets are attributed to eight discrete active regions covering not more than 1% of the cometary surface and producing bursts of duration of about 0.1 day. Calculations show that waning dust jets develop into envelopes and that old envelopes in turn become the observed tail bands. No evidence of truly violent explosions is found, and effects of active region outgassing on cometary orbital motion are negligible. Potential applications of the model to periodic Comet Halley and other comets are noted.

Sekanina, Z.↗
NASA NTRS1982

The path and surviving tail of a comet that fell into the sun

A satisfactory orbital solution for Comet Howard-Koomen-Michels 1979 XI is found on the assumption that the comet's line of apsides coincided with that of the Kreutz sungrazing comet group. The derived perihelion distance then shows that this is the first known case of a comet falling into the sun. A dust tail that survived the comet is studied as a particle flow phenomenon controlled by no force other than solar gravity and solar radiation pressure. The tail's outline is interpreted in terms of an onset of dust production, a peak repulsive force on the particles, and a circumsolar dustfree zone due to particle sublimation. It is shown that the surviving debris consisted mostly of absorbing, submicron size particles in hyperbolic trajectories convex to the sun and curving toward the earth. The tail width may be a product of the interaction of charged dust in the tail with a complicated structure of the coronal magnetic field.

Sekanina, Z.↗
NASA NTRS1982

Infrared photometry of periodic comets Encke, Chernykh, Kearns-Kwee, Stephan-Oterma, and Tuttle

Nearly simultaneous photometry of the reflected and thermal infrared spectra of periodic comets Encke, Chernykh, Kearns-Kwee, Stephan-Oterma, and Tuttle are presented. The 10-micron silicate emission feature has been detected for the first time in periodic comets and was observed in three of these objects. The albedo of the dust particles in the comae of these comets is calculated and compared to that of Comet Kohoutek. The peculiar behavior of the dust in Comet Encke is discussed.

Campins, H.↗
NASA NTRS1982

Coronagraphic observations of two new sungrazing comets

The discovery has recently been reported of a comet (Howard-Koomen-Michels: 1979 XI) that apparently collided with the sun on 30 August 1979. A report is presented of observations of two additional sungrazers that encountered the sun on 27 January 1981 and 20 July 1981, respectively. Like comet 1979 XI, these two new comets seem to have been members of the Kreutz group of sungrazers, and like 1979 XI the new comets did not reappear after their encounters with the sun. The discovery of three previously unreported comets during the initial 2.3 yr of satellite coronal observations suggests that sungrazers are much more common than one might suppose from the list of only nine known sungrazers observed during the years 1668-1970.

Sheeley, N. R., Jr.↗
NASA NTRS1982

Visual and infrared observations of the distant Comets P/Stephan-Oterma /1980g/, Panther /1980u/, and Bowell /1980b/

The paper reports combined visual imagery and spectroscopy, near-infrared spectroscopy, and broadband infrared photometry of comets P/Stephan-Oterma (1980g), Bowell (1980b), and Panther (1980u) at intermediate heliocentric distances. The visual data indicate the existence of solid grains in extended halos around the nuclei of the three comets. Broadband near-infrared and thermal infrared measurements of Comet Panther suggest the presence of 2-4-micron-radius particles in the coma which most likely contain molecules incorporating the N-H bond, but which are more complex and less volatile than NH3. Such molecules can be produced in the grains by cosmic-ray reprocessing. Near infrared spectral features identical to those seen in comet Panther similary suggest the presence of a molecule incorporating the N-H bond in comet Bowell.

Jewitt, D. C.↗
NASA NTRS1982

Observations of the red auroral oxygen lines in nine comets

The digital sky-subtracted spectra of nine cometary nuclei are used to measure the 6300, 6363-A Auroral red doublet of the O I forbidden line, and in view of the confinement of the cometary oxygen lines to their nuclear source, a small but variable correction for NH2 blending in different comets is made. For seven of the comets measured, and under the assumption of H2O's exclusive role as the oxygen parent, the average Q(H2O)/Q(O I) ratio is about 20. This is in excellent accord with theory, and it is conjectured that the O I production rate can be used to scale the total cometary volatile loss in even faint, distant comets. The data obtained are consistent with heliocentric production rates that vary as power laws, with n values of 2.0 for 'gassy' comets and nearer to 5.0 for dusty ones. The comets with the greatest volatile mass-loss rates all were found to have dusty spectra.

Spinrad, H.↗
NASA NTRS1983

On photochemical heating of cometary comae - The cases of H2O and CO-rich comets

It is found that the effect of energy addition due to photochemical processes in the cometary coma on the heating and accelerating of the expanding neutral gas is limited by the kinematic property of the energy transfer from the energetic fragments to the bulk flow. The effective scale length of the photochemical zone for an H2O comet is estimated to be only 10% of the dimension of the collision region due to the large mass ratio of H atom and H2O molecule. When this physical property is taken into consideration, the H2O gas is found to be expanding at a highly supersonic speed. In addition, it is shown that the addition of a mixture of CO and/or CO2 molecules in a medium-bright comet with total production rate of 10 to the 29th molecules/s would not change this conclusion since the photodissiation length scales of these molecules are a factor of 10-30 larger than that of the H2O molecules. Thus, most of the CO and CO2 molecules will be dissociated outside of the collision zone without depositing the excess kinetic energy to the neutral atmosphere. However, if the gas production rate is controlled by CO or CO2, the dimension of the collision zone of a CO comet at 1 AU will be increased by a factor of seven. The photochemical heating due to CO dissociation would thus be more effective, causing the atmospheric flow to be weakly supersonic. It is concluded that the neutral gas outflows from H2O comets and CO comets could be substantially different.

Ip, W.-H.↗
NASA NTRS1984

Close encounters and collisions of comets with the earth

A computer search for earth-approaching comets among those listed in Marsden's (1983) updated orbit catalog has identified 36 cases at which minimum separation distance was less than 2500 earth radii. A strong representation of short period comets in the sample is noted, and the constant rate of the close approaching comets in the last 300 years is interpreted to suggest the lack of long-period comets intrinsically fainter than an absolute magnitude of about 11. A comet-earth collision rate derived from the statistics of these close encounters implies an average period of 33-64 million years between any two events. This rate is comparable with the frequency of geologically recent global catastrophes which appear to be associated with extraterrestrial object impacts, such as the Cretaceous-Tertiary extinction 65 million years ago and the late Eocene event 34 million years ago.

Sekanina, Z.↗
NASA NTRS1984

Comet Bowell 1980b

Optical filter photometry, and optical and ultraviolet spectrophotometry data collected between November 1980 and June 1982 of Comet Bowell are presented. It was determined that Comet Bowell began producing significant amounts of OH, though not other species, before perihelion at a heliocentric distance near 4.6 AU. As the comet approached perihelion at 3.4 AU, OH production decreased and CN and C2 species were detected at normal concentrations. An outburst in April 1982 was dominated by OH. The grains in 1982 near perihelion showed increases in albedo between 3150 and 4500 A and 1.2 and 1.6 microns. The presence of two grains populations was suggested, together with the conclusion that Comet Bowell was physically a typical new comet entering the inner solar system from the Oort cloud for the first time.

Feldman, P. D.↗
NASA NTRS1982

A worldwide photographic network for wide-field observations of Halley's Comet in 1985-1986

A global network of ground-based observatories for the study of Halley's Comet in 1985/1986 is discussed. Recommendations are made with respect to improving coordination between reporting observatories, in order to ensure detailed imaging of such fast-generating cometary phenomena as plasma-tail knots, helices, disconnected tails, rays and condensations. A method for calibrating telescopes is considered by which well-studied objects will be photographed to provide references for images of Halley's Comet. This procedure is expected to reduce errors to approximately 0.05 mag. A coordinated study of Halley's Comet will provide important data on the physical properties of the Comet. Examples of the topics of study related to the plasma physics of the Comet's tail include: magnetic reconnection, rippling and tearing modes, kink instability, Kelvin-Helmholtz instability, and the flute instability.

Niedner, M. B., Jr.↗
NASA NTRS1984

Temporal and spatial behavior of the ultraviolet emissions of comet IRAS-Araki-Alcock 1983d

The spatial distribution and temporal variation of the ultraviolet emissions from comet IRAS-Araki-Alcock 1983d, observed by IUE near closest approach of the comet to the earth, suggest a model for this comet of a rotating cometary nucleus with a nonuniform distribution of volatile ices. The short lifetime of S2 (about 500 s), a species whose presence was first identified in this comet and is presumed to be present generally in the cometary ice, makes S2 a valuable indicator of short-term nuclear activity. In the case of comet 1983d, the S2 production rate decreased by a factor of 10 over a period of 28 hr.

Feldman, P. D.↗
NASA NTRS1984

The Mariner Mark II Comet Rendezvous/Asteroid Flyby Mission

The Comet Rendezvous/Asteroid Flyby will be the first mission in the Mariner Mark II program. The July 1990 launch will result in a fast flyby of the asteroid Tanete in May 1991 and a rendezvous with comet Kopff in February 1994, 879 days before comet perihelion. The spacecraft will return detailed data on the comet nucleus and its environment for nearly 1000 days. Several important comet characteristics will not be precisely known until the spacecraft performs its first measurements, which means that operations strategies must be adaptable to a range of conditions. In addition, the operation of a spacecraft in a dusty environment near a low-mass body imposes unique constraints on trajectory and orbit design.

Stetson, D. S.↗
NASA NTRS1984

Astrometric needs for the ISEE-3/ICE mission to Comet Giocobini-Zinner

The Third International Sun-Earth Explorer satellite (ISEE-3) was rechristened Internation Cometary Explorer (ICE) when the spacecraft left the Earth-Moon system after a close Lunar flyby on 1983 December 22. On 1985 September 11, ICE will pass through the inner parts of the tail of Comet Giacobini-Zinner to obtain the first in-situ measurements of any comet. Since the spacecraft has no cameras, its trajectory will be determined only from radio tracking. Astrometric updates of Comet Giacobini-Zinner will be critical for the final targeting of the spacecraft to achieve a successful encounter. ICE's flyby of Comet Giacobini-Zinner will provide valuable experience for the astrometry needed to target other spacecraft to encounter Hally's Comet six months later.

Dunham, D. W.↗
NASA NTRS1984

Charge coupled device (CCD) spectroscopy of comets - Tuttle, Stephan-Oterma, Brooks 2, and Bowell

CCD technology allows the acquisition of high quality spectra from faint comets that provide simultaneous spatial and spectral resolution. Most emissions in the spectra thus obtained are due to CN and NH2. The most interesting results for the continuum were displayed at comet Bowell, whose coma is a factor of 10 more extended than the other comets studied. Calculated production rates are presented for NH2 and CN in the cases of the comet Tuttle and Stephan-Oterma, and first-order comparisons are conducted for measured production rates and those of the H2O molecule, which should be the dominant one in comet activity.

Johnson, J. R.↗
NASA NTRS1985

Comet nucleus gamma-ray spectrometer penetrator - An in situ nucleus composition experiment

An engineering design study of a comet nucleus penetrator system for in situ determinations of comet composition is presented. The system is designed for participation in the Comet Rendezvous/Asteroid Flyby mission to comet Kopff in 1996. The instrumentation payload of the penetrator system will include a passive gamma-ray spectrometer; a high frequency response accelerometer for measurements of the impact deceleration profile of the system as it penetrates to the nucleus of the comet; and several thermocouples mounted along the length of the penetrator to measure temperature and temperature gradients. The mechanical, electrical and thermal interfaces between the penetrator and the Mariner Mark II spacecraft are described. A series of line drawings is provided which illustrates the proposed configuration of the system.

Swenson, B. L.↗
NASA NTRS1984

The ultraviolet spectrum of periodic Comet Encke (1980 XI)

A comparison of the ultraviolet spectrum of periodic Comet Encke (1980 XI), recorded by the IUE between October 24, and November 5, 1980 with similar spectra, of short- and long-period comets, shows the gaseous composition of P/Encke to be nearly identical to that of the other comets observed by the IUE. If P/Encke is indeed the remains of a once-giant comet, this similarity implies a homogeneous radial structure for the cometary ice nucleus. The OH brightness distribution shows a spatial variation similar to the visible fan-shaped image of the comet, suggestive of a nonuniform distribution of volatile ices on the surface of the nucleus. The total derived water production rate appears to be a factor of 5 higher than that derived from HI Lyman-alpha observations made during the 1970 apparition, and shows a variation with heliocentric distance (r) as r to the -3.3 power over the range 0.81 to 1.02 AU.

Feldman, P. D.↗