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At least 1,297 records · Page 72
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NASA NTRS1986

IUE observations of Comet Halley: Evolution of the UV spectrum between September 1985 and July 1986

The ultraviolet spectrum of comet P/Halley was monitored with the IUE between 12 September 1985 and 8 July 1986 (r <2.6 AU pre and post-perihelion) at regular time intervals except for a two-month period around the time of perihelion. A complete characterization of the UV spectrum of the comet was obtained to derive coma abundances and to study the light emission mechanisms of the observed species. The Fine Error Sensor (FES) camera of the IUE was used to photometrically investigate the coma brightness variation on time scales of the order of hours. Spectroscopic observations as well as FES measurements show that the activity of the nucleus is highly variable, particularly at the end of December 1985 and during March and April 1986. The production rates of OH, CS and dust are derived for the entire period of the observations. The total water loss rate for this period is estimated to be 150 million metric tons.

Feldman, P. D.↗
NASA NTRS1986

Detection of OH from Comet Halley in the far-infrared

The 2 Pi 3/2 (J = 5/2 yields 3/2 + yields - parity) transition of OH was detected in comet Halley at 119.44 microns. The upper limit to the line intensity of the - yields + parity transition is 119.23 microns. The ratio of these lines is consistent with radiative pumping of the rotational levels through absorption of near ultraviolet solar photons which excite the low lying vibrational levels of OH. The far-infrared lines probe the inner regions of the coma where OH is produced through photodissociation of a parent molecule (presumably H2O). Hence, these lines complement the 18 cm radio measurements which are sensitive to the outer regions of the comet coma. The OH production rate is highly model dependent but is of the order 2 to 4 times 10 to the 29th power molecules/sec.

Stacey, Gordon J.↗
NASA NTRS1986

Spectra of the plasma tail of Comet P/Giacobini-Zinner at the time of the ICE counter

Ground-based CCD spectra of comet P/Giacobini-Zinner were obtained at the time of the ICE encounter. A spectral extraction gives the spectrum of the comet at the position and time of the spacecraft at midencounter. Column densities are given for neutral and ion molecules observed. The surface brightness profile for H2O(+) from 1000 km sunward to 34,600 km tailward obtained at the time of the encounter is also presented.

Wyckoff, S.↗
NASA NTRS1986

A comparison of the high resolution IUE observations of CS emission in Comets Halley and Giacobini-Zinner

The high resolution spectra of comets Giacobini-Zinner and Halley were measured in the region of the CS emission. Both spectra have essentially the same shape, but the Halley spectrum has a better signal to noise ratio. The rotational distribution of the radical in these comets was derived by using an inversion program and spectral simulation. Both procedures suggest that the rotational distribution is dominated by CS radical with J" is less than or = 5. Despite this it appears that another distribution with higher J''' is needed to explain the shape of the rotational distribution. It is suggested that this second distribution is due to a balance between collisions and radiation in the inner coma.

Jackson, William M.↗
NASA NTRS1986

High precision low resolution spectrophotometry of Comets Giacobini-Zinner and Halley

Optical window (300 to 1000 nm) observations were made of Comet Halley during 12 to 14 Mar. 1986 by a scanning spectrometer. The results provide accurate spectrophotometric measurements of the principal molecular bands from the 308 nm OH through the 918 nm CN emissions. The observations can be used to tie space derived ultraviolet and groundbased infrared observations to the optical observations and to one another. Additional observations were made of comet Giacobini-Zinner on 30 September 1985 of the same OH band and the 388 nm band of CN. The Swan band sequence ratios are used to test the best models of C2, with different ratios favoring different models, calling into question the assumptions of these very complete models.

Odell, C. R.↗
NASA NTRS1986

Thermal infrared imaging of Comet Halley

Thermal infrared images (10.8, 12.8, and 19.2 microns) of comet Halley were obtained on 1986 Mar. 28.7, 29.7 and 30.7 UT using the NASA-MSFC 20-pixel bolometer array at the NASA Infrared Telescope Facility. Analysis of the observations shows: the presence of spatial structure indicative of dust ejection occurring mainly on the sunward side of the nucleus; a dust coma deficient in large particles (radius greater than 100 microns) compared with that of Comet P/Giacobini-Zinner, temporal variability consistent with an extrapolation of a quasi-sinusoidal light curve observed by IUE on 1986 March 23, 24, and 25; and a maximum at the nuclear condensation of the color temperature and the strength of the silicate emission feature.

Campins, H.↗
NASA NTRS1986

Airborne photometry of Comet Halley from 40 to 160 microns

Far infrared photometry of comet Halley was obtained from NASA's Kuiper Airborne Observatory. The brightness of the comet decreases by nearly a factor of two on the second day in phase with the variability observed in blue light by IUE during the same time period. The energy distribution also varies significantly with a shallower (cooler) spectrum on the second day indicating fewer small particles on that day. Observations 30 and 60 arcsec away from the nuclear condensation show lower 50 to 100 microns color temperatures on both dates. This is consistent with observations by Giotto and Vega which indicate a higher proportion of large particles away from the nucleus.

Campins, H.↗
NASA NTRS1986

Sunward spike and equatorial plane of Halley's Comet

Photographs from late April until at least early June 1986 of comet Halley show a narrow spike in the Sun's direction. It extends to distances of at least 700,000 km from the nucleus in projection onto the sky on computer processed photographs. The spike is composed of dust and its enormous sunward extent (compared to other dust features) suggests an anomalously high ratio of particle ejection velocity to solar radiation pressure. The implied grains are either dielectric or slightly absorbing and much less than 1 micron in size, which are undetectable optically, unless Earth is located in or very near a plane of their concentration. The only plane to which these grains can possibly be confined for long is the equatorial plane of the nucleus, especially when the obliquity is small. Accordingly, the spike's position as a function of time contains information on the comet's spin axis.

Sekanina, Z.↗
NASA NTRS1986

Dust distribution of Comet Halley from the Giotto radio science experiment

Measurements from the Giotto radio science experiment representing the Doppler frequency shift and the intensity level of the X-band downlink signal of the Giotto spacecraft during its encounter with comet Halley are shown. Continuous data reception was maintained throughout the encounter. The Doppler shift measured within a time interval of 100 sec is due to drag effects in the cometary atmosphere causing a deceleration of the spacecraft. The total change of velocity of Giotto is 23.2 cm/sec resulting in a best estimate of 0.32 g for the total cometary mass impacting Giotto during the Halley flyby. A dust jet structure for the inner coma of comet Halley is derived. Several sharply confined dust jets are distinguishable; for the most prominent one, characteristic properties are deduced. Features of this jet structure are shown to correlate with measurements from on-board Giotto experiments and with Earth-based observations.

Edenhofer, P.↗
NASA NTRS1986

A three-dimensional model of the nucleus of Comet Halley

The nucleus of comet Halley was observed by the Halley Multicolor Camera (HMC) during the last minutes of the flyby sequence before reaching the point of closest approach. The phase angle change during the observational period was less than 17 deg, so HMC data can only define a two-dimensional contour of the nucleus at a certain time. The overall geometry during the HMC observations is outlined and the findings are related to information available from other sources at different times. The nucleus of the comet is described as a triaxial ellipsoid with major axes of 16, 10, and 9 km. The rotation axis is directed towards delta 1950 = -(40 + or - 5) deg and alpha 1950 = (50 + or - 10) deg. The rotation with a period of P = (54 + or - 1) is in a prograde sense with respect to the orbital motion.

Wilhelm, K.↗
NASA NTRS1986

Ground-based detection of water in Comet Halley

Gaseous water was detected in the coma of comet Halley. Emission was observed in the upsilon 1 + upsilon 3 band at 1.4 microns, and the upsilon 2 + upsilon 3 band at 1.9 microns. Based on the upsilon 1 + upsilon 3 band intensity, the mass loss rate is 2 times 10 to the 29th power molecules/sec (with an uncertainty of a factor of 2) on 29 Oct. 1985, marginally consistent with other detections. The method is applicable to observations of H2O in other fast-moving comets.

Knacke, R. F.↗
NASA NTRS1986

Systematics of the CHON and other light-element particle populations in Comet Halley

Based on chemical signatures measured by the PIA experiment during the Giotto flyby of comet Halley, particle classifications were designated. In addition to silicate-like grains and particles of mixed (cosmic) composition, there appear to be several light-element rich populations, including the CHON, (H,C), (H,C,O), and (H,C,N) particle types. These compositional classes are further distinguished by differences in mass distributions, a density indicator, and variations in relative abundance within the coma. These particle populations are evidence for chemical heterogeneity in the surface of the cometary nucleus. Particles found mainly in the inner coma may be volatile icy grains. Most of the N of the comet may be found in up to three different populations of grains; one or more of these may be responsible for the observation of cyanojets.

Clark, Benton↗
NASA NTRS1986

Photoelectric photometry of Comet P/Halley from Mauna Kea Observatory

A program of photoelectric photometry of comet Halley yielded 60 nights of usable photometry. The uncertainty in the photometry in all filters except OH is better than 2%. Throughout the apparition, the comet is brightest in the filters that isolated the neutral gas emission features of OH, CN, C2, and C3. The fluxes in the filters that isolated the ionic emission features of CO(+) and H2O(+) are comparable to the continuum when centered upon the optocenter, thereby making the ionic emission weak to undetectable above the continuum. The continuum colors do not appear to be a function of phase angle; therefore, the colors are probably indicative of dust grain composition and are not dominated by scattering geometry effects.

Piscitelli, J. R.↗
NASA NTRS1986

Post-perihelion brightening of Halley's Comet: A case of nuclear summer

Increased brightness and gas production rates for Halley's comet after perihelion are explained as a result of seasonal effects on an obliquely rotating nucleus. The highly eccentric cometary orbit causes a rapid change in solar declination as the comet rounds perihelion, resulting in drastic changes in the insolation reaching the Northern and Southern Hemispheres of the nucleus. The rapid heating of the Northern Hemisphere post-perihelion likely results in substantial cracking of the non-volatile surface crust due to thermal stresses, exposing areas of fresh volatile ices. The orientation of the triaxial ellipsoid nucleus may also play a role in exposing more surface area to continuous sunlight and sublimation after perihelion. Post-perihelion brightening models based on heat flow and storage in sub-surface layers of the nucleus pre-perihelion are likely not viable because of the low thermal conductivities of porous, low density cometary surface materials.

Weissman, Paul R.↗
NASA NTRS1986

Atlas of Comet Halley 1910 II

With the impending return of Halley's Comet in 1986, a major effort began to collect the material obtained at its last appearance in 1910. This material displays the evolving coma and tail phenomena, and is useful for comparison with the present quantitative studies of spectroscopic and structural phenomena. Images in the atlas are arranged in chronological order by day. Days that have multiple images with varying scale are arranged in two sequences. Photographs showing tail phenomena are first, followed by photographs obtained with longer focus instruments showing the head or near-nuclear region. Drawings of Comet Halley, made from visual observations in 1835 and 1910, also are included.

Bertram Donn↗
NASA NTRS1986

Thermal emission from the dust coma of Comet Bowell and a model for the grains

An analysis of the physical characteristics of Comet Bowell is presented based on 10 and 20 microns emissions observed with the NASA 3 m IR telescope during 1981-82. Numerical models are defined for quantifying the number of dust particles at a given radius and at a given angular distance from the nucleus, the contribution of icy grains to the 20 microns flux and the average albedo, and the dust distribution in the coma. Large, dirty ice grains are judged to be responsible for production of OH at a rate of about 10 to the 29th mol/sec during the spring of 1981. The total surface area of the dirty ice grains is projected to be 10 to the 15th sq cm. The coma in 1982 would therefore have consisted of nonvolatile grains, i.e., old dirty ice grains. Further observations of other, similar comets are needed to identify the composition of the emitted grains and the heliocentric distance at which high water production is initiated.

Hanner, M. S.↗
NASA NTRS1986

Probing the inner regions of the coma by UV observations of CS in comets

New vibrational bands of CS have been identified in several comets. The relative intensities of these bands are in agreement with a simple model, based upon most of the CS radicals being in the lowest vibrational level. The present observational accuracy is not sufficient to decide between this simple model and an earlier radiative equilibrium model. The observed intensity ratios appear to be similar in different comets, which means that they cannot depend strongly upon the total gas production rates.

Butterworth, P. S.↗
NASA NTRS1986

Diatoms in comets

The fossil record of the microscopic algae classified as diatoms suggests they were injected to earth at the Cretaceous boundary. Not only could diatoms remain viable in the cometary environment, but also many species might replicate in illuminated surface layers or early interior layers of cometary ice. Presumably they reached the solar system on an interstellar comet as an already-evolved assemblage of organisms. Diatoms might cause color changes to comet nuclei while their outgassing decays and revives around highly elliptical orbits. Just as for interstellar absorption, high-resolution IR observations are capable of distinguishing whether the 10-micron feature arises from siliceous diatom material or mineral silicates. The 10-30-micron band and the UV 220-nm region can also provide evidence of biological material.

Hoover, R.↗