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NASA NTRS1992

Long slit spectroscopy of NH2 in comets Halley, Wilson, and Nishikawa-Takamizawa-Tago

Long-slit spectra of comets Halley, Wilson and Nishikawa-Takamizawa-Tago were obtained with the 3.9 meter Anglo-Australian Telescope. Spectra of comets Halley and Wilson were obtained with the IPCS at a spectral resolution of 0.5 A and a spatial resolution of 10(exp 3) km. Spectra of comets Wilson and Nishikawa-Takamizawa-Tago were obtained with a CCD at a spectral resolution of 1.5 A and a spatial resolution of approximately 3 x 10(exp 3) km. Surface brightness profiles for NH2 were extracted from the long-slit spectra of each comet. The observed surface brightness profiles extend along the slit to approximately 6 x 10(exp 4) km from the nucleus in both sunward and tailward directions. By comparing surface distribution calculated from an appropriate coma model with observed surface brightness distributions, the photodissociation timescale of the parent molecule of NH2 can be inferred. The observed NH2 surface brightness profiles in all three comets compares well with a surface brightness profile calculated using the vectorial model, an NH3 photodissociation timescale of 7 x 10(exp 3) seconds, and an NH2 photodissociation timescale of 34,000 seconds.

Rettig, Terrence W.↗
NASA NTRS1992

Comet impacts and chemical evolution on the bombarded earth

Amino acids yields for previously published shock tube experiments are used with minimum Cretaceous-Tertiary (K/T) impactor mass and comet composition to predict AIB amino acid K/T boundary sediment column density. The inferred initial concentration of all amino acids in the K/T sea and in similar primordial seas just after 10 km comet impacts would have been at least 10 exp -7 M. However, sinks for amino acids must also be considered in calculating amino acid concentrations after comet impacts and in assessing the contribution of comets to the origin of life. The changing concentration of cometary amino acids due to ultraviolet light is compared with the equilibrium concentration of amino acids produced in the sea from corona discharge in the atmosphere, deposition in water, and degradation by ultraviolet light. Comets could have been more important than endogenous agents for initial evolution of amino acids. Sites favorable for chemical evolution of amino acids are examined, and it is concluded that chemical evolution could have occurred at or above the surface even during periods of intense bombardment of earth before 3.8 billion years ago.

Oberbeck, Verne R.↗
NASA NTRS1992

8- to 13-micron spectroscopy of Comet Levy 1990 XX

The results are reported of IR spectroscopy of Comet Levy 1990 XX over a three-day period when the comet was about 1.54 AU from the sun roughly 70 days before perihelion. Comet Levy 1990 XX was bright, and for at least part of its inbound journey toward perihelion, active. At a distance of 1.54 AU from the sun it showed strong structured silicate emission with peaks or shoulders at 9.8 and 11.2 microns. These features resemble those of Comets P/Halley and Bradfield 1987 XXIX. The comet was variable in brightness. Specifically, the contrast of the silicate features changed by a factor of two relative to the continuum level and showed some evidence for a shape change as well.

Lynch, David K.↗
NASA NTRS1993

The enigmatic object 2201 Oljato - Is it an asteroid or an evolved comet?

The orbital properties of near-earth object 2201 have been associated with meteor showers, and its modeled orbital evolution is chaotic - a property which might indicate a history related to comets. Telescopic observations of its visible and near-infrared spectral reflectance, broad-band visible and near-infrared photometry, infrared radiometric measurements, and radar echoes are reported here from two apparitions, 1979 and 1983. This asteroid has a high radiometric albedo, a property not associated with comet nuclei. In certain wavelength regimes it is classified as an S-type asteroid, in others, an E-type, but its overall spectral reflectance is not typical of either taxonomic type, and neither type is thought of as cometlike. Unexpectedly high ultraviolet reflectance at the 1979 apparition was suggested to be the result of residual outgassing as in a comet. The UV photometric data are modeled as fluorescent emission from neutral species found in comets. The resulting calculations indicate a plausible value for OH and CN emission at 0.3085 and 0.38 micron relative to the observed range of active comets.

Mcfadden, Lucy A.↗
NASA NTRS1993

The solar wind interaction with Comet P/Grigg-Skjellerup

The Giotto spacecraft is scheduled to intercept Comet P/Grigg-Skjellerup on July 10, 1992. The observed outgassing rate of this comet is over an order of magnitude smaller than Comet Giacobini-Zinner and over two orders of magnitude smaller than that of Comet Halley. Consequently, the new data obtained during the upcoming encounter will strengthen our understanding of how the solar wind interaction with comets depends upon the neutral gas production rate. In this brief note, we make predictions of the location of the flow transition regions - i.e., the bow shock and the ionopause, and discuss the expected level of wave turbulence.

Flammer, K. R.↗
NASA NTRS1993

Orbital evolution of dust from comet Schwassmann-Wachmann 1: A case of one-to-one resonance trapping

In a recent study we have modeled the orbital evolution of dust particles released from comets and asteroids in the solar system. The source bodies were either asteroids inside Jupiter's orbit or comets from the Jupiter family of comets. However there are other dust producing parent bodies in the solar system of interest, one of these is comet P/Schwassman-Wachmann 1. Since comet Schwassman-Wachmann 1, which has an orbit outside of Jupiter's orbit, is an active dust producer and has low eccentricity, dust particle evolution from it is of interest. We report on a particular 2 micron radius particle that captured into a 1 to 1 mean motion resonance orbit with Saturn.

Jackson, A. A.↗
NASA NTRS1992

Observations of comets with the IUE

Comet Shoemaker-Levey was observed on several dates in June and July 1992. Comparison of the relative brightness of a given emission between IUE and the Hubble Space Telescope (HST) Faint Object Spectrograph apertures provides information about the spatial distribution of the emitting species from which the excitation mechanisms can be inferred. The comparison between the HST and IUE spectra was abetted by the fact that comet Shoemaker-Levy did not show any indication of short-term temporal variability. Comet Swift-Tuttle is a moderately bright periodic comet that shows relatively strong temporal variations on a scale of hours and days. IUE observations of this comet were made in November 1992 and the detected variability was reported on IAU circular 5663. Coordinated ground-based observations led to the first detection of the CI(lambda)9850 transition that complements CI(lambda)1931 observed by the IUE short wavelength spectrograph and provides a means of indirectly determining the CO abundance.

Feldman, Paul D.↗
NASA NTRS1992

Comet nucleus and asteroid sample return missions

During the 1991-92 academic year, the Pennsylvania State University has developed three sample return missions: one to the nucleus of comet Wild 2, one to the asteroid Eros, and one to three asteroids located in the Main Belt. The primary objective of the comet nucleus sample return mission is to rendezvous with a short period comet and acquire a 10 kg sample for return to Earth. Upon rendezvous with the comet, a tethered coring and sampler drill will contact the surface and extract a two-meter core sample from the target site. Before the spacecraft returns to Earth, a monitoring penetrator containing scientific instruments will be deployed for gathering long-term data about the comet. A single asteroid sample return mission to the asteroid 433 Eros (chosen for proximity and launch opportunities) will extract a sample from the asteroid surface for return to Earth. To limit overall mission cost, most of the mission design uses current technologies, except the sampler drill design. The multiple asteroid sample return mission could best be characterized through its use of future technology including an optical communications system, a nuclear power reactor, and a low-thrust propulsion system. A low-thrust trajectory optimization code (QuickTop 2) obtained from the NASA LeRC helped in planning the size of major subsystem components, as well as the trajectory between targets.

Melton, Robert G.↗
NASA NTRS1993

Thermal history of comets during residence in the Oort cloud - Effect of radiogenic heating in combination with the very low thermal conductivity of amorphous ice

The thermal history of long-period comets initially composed of amorphous ice is studied. It is shown that such comets with a small nucleus thermal conductivity (kappa) experience a runaway increase in the internal temperature during residence in the Oort cloud. The temperature increase is a result of rapid release of the latent heat at crystallization triggered by gradual heating due to decay of radioactive nuclides. The time of the runaway temperature increase is about ten to a hundred million years after the formation of the nucleus depending on the fraction of refractory grains which contain radioactive nuclides. Most of the amorphous ice in the nuclides except just beneath the surface transforms into crystalline ice due to the runaway temperature increase. This implies that the ice in short-period comets is crystalline from the initial time when the long-period comet becomes a short-period one. In comets with large kappa the temperature does not rise much compared to the small kappa case and the initial amorphous ice is preserved. A criterion for the crystallization of the nucleus ice is derived.

Haruyama, Jun'ichi↗
NASA NTRS1995

Comets in other planetary systems?

Comets in our solar system appear to have provided a bridge between the cold, volatile-rich outer solar system, and the warm, but volatile-poor inner solar system. Excluding tidal and possible extinct radionuclide heating sources, only in the inner solar system are temperatures high enough for liquid water, and therefore life as we know it, to exist for times comparable to the age of the solar system. Comets may have been crucial for providing biogenic volatiles and perhaps organic molecules to this warm environment. It is therefore interesting from an exobiological point of view to ask if comets exist in other planetary systems. Most attempts to detect comets around other stars or in interstellar space have failed. However, there is growing spectroscopic evidence for comet-like bodies orbiting the star Beta Pictoris.

Chyba, C. F.↗
NASA NTRS1994

A new derivation of the tensile strength of cometary nuclei: Application to comet Shoemaker-Levy 9

The splitting of comets as exemplified by comet Shoemaker-Levy 9, when it passed near Jupiter, is a common phenomenon. Multiple splitting is also not an uncommon occurrence. It is clear that the comet nucleus is fragile, i.e., its tensile strength is small compared with that of solid materials. We show that aggregates of sub-micron interstellar dust particles presumed to consist of a silicate core, an inner mantle of complex organic refractory molecules, and an outer mantle dominated by H2O ice (Greenberg, 1982) provide the basis for a quantitative derivation of the tensile strength of comet SL9 using molecular interactions at the contact interfaces. In fact, using a mean particle size representing interstellar dust as it would appear in its final presolar state one derives a tensile strength which describes remarkably well the multiple splitting phenomenon. This derivation of the tensile strength of a particle aggregate resulting from molecular interactions is quite general and can be applied to physical situations involving any sorts of aggregates as well as those representing comet nuclei.

Greenberg, J. Mayo↗
NASA NTRS1994

Tidal disruption and the appearance of periodic comet Shoemaker-Levy 9

A unified model is formulated to interpret quantitatively the observed characteristics of the nuclear train, the two dust trails, and the tail region of P/Shoemaker-Levy 9 in terms of a collisionally modified rotation velocity distribution of the comet's debris. Since there is no evidence for the comet's out-gassing, the model does not assume activity. The discruption of the parent comet was due primarily to tidal stesses during its extremely close approach to Juptier in July 1992. The original nucleus is found to have been most probably approximately 5 km in radius or, equivalently, approximately 10(exp 17) g in mass. The dynamical separation of the debris occurred apparently approximately 2.2 hr after the perijove passage, even though the actual fragmentation of the original mass is likely to have begun before closest approach. Physical breakp was accompanied by ubiquitous low-velocity collisions among the particulates, resulting in a rearrangement of the initial rotaional velocities into a rapidly 'thermalized' distribution, characterized by a long tail of relatively high velocities for the debris that populates the dust trails far from the nuclear train. Compelling evidence is presented for secondary fragmentation events, indicative of the comet's continuing disintegration. Secondary fragmentation may be caused by rotational bursting of the massive fragments that had been cracked but unbroken during the tidal disruption. Impact conditions and possible interactions of the comet's debris with the jovian system during the encounter in July 1994 are described.

Sekanina, Z.↗
NASA NTRS1994

Methanol and hydrogen sulfide in comet P/Halley

The Neutral Mass Spectrometer on the Giotto spacecraft measured the gas and ion composition in the coma of comet P/Halley. A detailed model of the ion chemistry inside the contact surface located at 4660 km is used to interpret the measured ion desnity profiles in the 32 to 35 amu/e mass range. The masses 33 and 35 amu/e are dominated by the protonated methanol and hydrogen sulfide ions CH3OH2(+) and H3S(+). Both profiles are essentially compatible with CH3OH and H2S originating from the nucleus only. The production rates relative to water are Y(CH3OH) = Q(CH3OH)/Q(H2O) = 1.7% and Y(H2S) = 0.41%. Our Y(CH3OH) agrees well with a determination from IR spectra obtained about 6 weeks after the Giotto encounter with P/Halley. In 7 other comets IR and microwave observations give Y(CH3OH) values between about 0.7 and 6%, indicating that the methanol abundance shows a strong variability from comet to comet. In three other comets Y(H2S) values between 0.2 and 0.5% have been reported. In addition to H2S(+), only ions containing minor isotopes of H, C, O and S contribute to mass 34 amu/e (e.g. (34)S(+), (13)CH3OH2(+), CH4DO(+)). These contributions can be calculated from the measured densities of the ions containing the major isotopes and the H2S(+) contribution from the measured H3S(+) density. From mass 34 amu/e we can also derive an upper limit of 1% for the abundance of deuterated methanol. This limit is at most marginally compatible with a direct interstellar origin of the CH3OH in P/Halley as the measured interstellar abundance of deuterated methanol is 1 to 6%.

Eberhardt, P.↗
NASA NTRS1991

Upper Limits of SO AND SO2 in Comets

We have calculated synthetic spectra of SO and compared them with spectra of Comets I-A-A and P/Halley, in which Wallis and Krishna-Swamy claimed a tentative identification of SO. We found no evidence for the presence of So in these spectra. The derived upper limits on the production rates of SO and S02 in Comet I-A-A were found to be at least 50 times less than the S2 production rate. We also found that the SO and S02 upper limits in Comets P/Halley, P/Encke, and Bradfield (1979X) were less than the S2 abundance, reported in Comet IRAS-Araki-Alcock. Our result, contradict recent irradiation experiments, which produced abundant SO compared with S2 under most circumstances. This may suggest totally different models of the S2 producing mechanism in comets.

Kim, Sang J.↗
NASA NTRS1997

Long-Period Comets and the Oort Cloud

The long-period comets pose a unique problem for the impact hazard problem. Because of their very long orbital] periods and generally large distances from the Sun, they cannot be surveyed and catalogued in the same manner as the near-Earth asteroids and short-period comets. They appear at random, uniformly distributed on the celestial sphere. Current technologies can detect long-period comets at distances of approx. 5 AU, giving somewhat less than a one year warning time for potential Earth impactors. The mean impact probability for a long-period comet crossing the Earth's orbit is 2.2 to 2.5 x 10(exp -9) per perihelion passage. The mean impact velocity is approximately 52 km sec-1 but the most probable impact energy is characterized by a velocity of 56 to 58 km/sec. The estimated current impact rate for cometary nuclei large enough to create 10 km diameter (or larger) craters on the Earth is between 5 x 10(exp -7) and 2.8 x 10(exp -6) per year, with a bed estimated value of 1.0 x 10(exp -6) per year. Nuclei large enough to initiate global climatic disturbances strike the Earth on average every 16 Myr. The impact frequency may be increased substantially for brief periods of time during cometary showers, initiated by major perturbations of the Oort cloud. Improved technologies are needed to detect approaching long-period comets at large heliocentric distances so as to increase the warning time for potential impactors.

Weissman, Paul R.↗
NASA NTRS1999

Asteroids and Comets Outreach Compilation

Contents include various different animations in the area of Asteroids and Comets. Titles of the short animated clips are: STARDUST Mission; Asteroid Castallia Impact Simulation; Castallia, Toutatis and the Earth; Simulation Asteroid Encounter with Earth; Nanorover Technology Task; Near Earth Asteroid Tracking; Champollian Anchor Tests; Early Views of Comets; Exploration of Small Bodies; Ulysses Resource Material from ESA; Ulysses Cometary Plasma Tail Animation; and various discussions on the Hale-Bopp Comet. Animation of the following are seen: the Stardust aerogel collector grid collecting cometary dust particles, comet and interstellar dust analyzer, Wiper-shield and dust flux monitor, a navigation camera, and the return of the sample to Earth; a comparison of the rotation of the Earth to the Castallia and Tautatis Asteroids; an animated land on Tautatis and the view of the motion of the sky from its surface; an Asteroid collision with the Earth; the USAF Station in Hawaii; close-up views of asteroids; automatic drilling of the Moon; exploding Cosmic Particles; and the dropping off of the plasma tail of a comet as it travels near the sun.

Source record↗
NASA NTRS1992

Book Review: The Origins of Comets

In The Origins of Comets, Bailey, Clube, and Napier propose that the answer to whether the ancient heavens were more interesting is a resounding "yes." The sky, in fact, has changed and is still changing. The authors trace the study of comets back to ancient Babylonian times with a focus on theories of the origins of these enigmatic visitors. The book is really of three distinct parts: the first six chapters provide an excellent and delightfully readable historical account of comet studies up to this century. The next few chapters give a rather detailed treatment of current models for comet origins. The last section treats the authors' own theories about the relationship between giant comets and extinctions on Earth.

McKay, Christopher P.↗
NASA NTRS2001

An Overview of the Comet Nucleus TOUR Discovery Mission and a Description of Neutral Gas and Ion Measurements Planned

The CONTOUR (Comet Nucleus TOUR) Mission led by its Principal Investigator Professor Joseph Veverka of Cornell is presently under development at the Johns Hopkins Applied Physics Laboratory for launch in July of 2002 with a flyby of Comet Encke scheduled for November 3, 2003 at a solar distance of 1.07 au. A robust Whipple dust shield is designed to allow a close nucleus approach distance (less than 150 km). The 2nd nominal CONTOUR target is Comet Schwassmann-Wachmann 3, although the spacecraft can alternately be directed to a new comet if such an interesting target is discovered. CONTOUR contains 4 instruments: an imaging spectrometer (CRISP) developed at APL that will obtain both high resolution nucleus images through 8 filters and IR spectra (800 to 2550 nm) of the nucleus, a narrow field of view forward imager (CFI) to locate the target days before the encounter, a dust composition time of flight mass spectrometer (CIDA) provided by Dr. J. Kissel and von Hoemer & Sulger, GmbH, and a mass spectrometer (NGIMS) provided by Goddard Space Flight Center to measure neutral gas and ambient ions. Laboratory calibration of the NGIMS has now been completed. NGIMS also includes an in-flight calibration system that we plan to exercise before and after each comet encounter. We will provide an overview of the CONTOUR Mission and discuss more specifically the NGIMS measurement goals for this mission.

Mahaffy, Paul↗