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NASA NTRS1992

Rotation of split cometary nuclei

A simple model for the rotational motion of split cometary nuclei is studied. A large-amplitude precession is easily excited due to the change of the moments of inertia even if the perturbation is small at the splitting. The damping timescale of the excited precession is widely ranged because of the uncertainty of the physical parameter of cometary nuclei. Another possibility for clarifying the evolution of the short period comets by studying the split cometary nuclei is also discussed.

Watanabe, Jun-Ichi↗

NASA NTRS1991

Cometary orbital dynamics and astrometry

An overview of research on cometary orbital dynamics and astrometry is presented, with emphasis on the nongravitational acceleration model which provides accurate orbits and ephemerides, and on new asymmetric models that represent the cometary outgassing phenomena more closely. Using a combination of long-focus telescopes, CCD detectors, microdensitometer reductions, and modern star catalogs, cometary astrometric data that are accurate to the subarcsecond level can be generated.

Yeomans, D. K.↗

NASA NTRS1991

Cometary activity, discrete outgassing areas, and dust-jet formation

Conceptual models for various types of features observed in cometary comae (jets, spirals, halos, fans, etc.), their computer simulation, and the hydrodynamic models for jet formation are critically reviewed, and evidence for anisotropic, strongly collimated flows of ejecta emanating from discrete active regions (vents) on the rotating cometary nuclei is presented. Techniques employed to generate synthetic comet images that simulate the features observed are described, and their relevance to the primary objects of coma-morphology studies is discussed. Modeling of temporal variations in the water emission from discrete active regions suggests that production curves asymmetric with respect to perihelion should be commonplace. Critical comparisons with the activity profiles of Enke's comet and with light curves of disappearing comets and comets that undergo outbursts are presented. Recent developments in the understanding of the processes that cause the nongravitational perturbations of cometary motions are reviewed, and the observed discontinuities are identified with the birth of new sources and/or deactivation of old vents.

Sekanina, Z.↗

NASA NTRS1993

Cometary globules in the southeast quadrant of the Rosette nebula

We present a study of newly identified cometary globules in the southeast quadrant of the Rosette nebula using the J = 1-0 transition of carbon monoxide. The globules are found to be blueshifted by about 6 km/s with respect to the adjacent Rosette molecular cloud. The masses of the globules vary from 50 to 300 solar masses, and their sizes are between 1 and 3 pc. Two of the globules have cometary morphology and show velocity gradients of about 1.5 km/s/pc along their symmetry axes. These globules are associated with the IRAS sources 06314+0421, X0632+043, 06322+0427, and 06327+0423 which coincide with local maxima in the (C-13)O emission. The derived physical parameters of the globules are found to be consistent with those predicted by recent theoretical models of photoevaporating cometary clouds. We suggest that star formation induced by radiation driven implosion has occurred.

Patel, Nimesh A.↗

NASA NTRS1993

HST imaging of hydrogen-poor ejecta in Abell 30 and Abell 78 - Wind-blown cometary structures

HST Faint Object Camera images of hydrogen-poor gas in the planetary nebulae A30 and A78 have revealed remarkable 'cometary' structures in the O III 5007-A forbidden line. Most of these cometary structures, consisting of compact (0.15-0.5 arcsec) knots with radial tails several arcsec in length, are located in an equatorial plane in both nebulae. In addition, two bright, compact (0.3 arcsec) polar knots are present in A30, one of them forming a bow shock. Corresponding polar features in A78 are more diffuse. The central stars of both nebulae have energetic winds which are most likely responsible for the 'cometary' knot morphology. We interpret this morphology in terms of dense (several thousand electrons per cu cm) H-poor condensations whose outer expanding layers are swept outward by stellar winds. Photoionization modeling indicates that while dense knot cores are mostly heated by atomic photoionization, expanding tenuous gas is heated by photoelectrons ejected from abundant dust grains. Our models predict steep temperature gradients for which there is observational evidence and possible abrupt phase transitions in the expanding gas.

Borkowski, Kazimierz J.↗

NASA NTRS1993

Collecting cometary soil samples? Development of the ROSETTA sample acquisition system

In the reference scenario of the ROSETTA CNRS mission, the Sample Acquisition System is mounted on the Comet Lander. Its tasks are to acquire three kinds of cometary samples and to transfer them to the Earth Return Capsule. Operations are to be performed in vacuum and microgravity, on a probably rough and dusty surface, in a largely unknown material, at temperatures in the order of 100 K. The concept and operation of the Sample Acquisition System are presented. The design of the prototype corer and surface sampling tool, and of the equipment for testing them at cryogenic temperatures in ambient conditions and in vacuum in various materials representing cometary soil, are described. Results of recent preliminary tests performed in low temperature thermal vacuum in a cometary analog ice-dust mixture are provided.

Coste, P. A.↗

NASA NTRS1997

Physical Processing of Cometary Nuclei

Cometary nuclei preserve a cosmo-chemical record of conditions and processes in the primordial solar nebula, and possibly even the interstellar medium. However, that record is not perfectly preserved over the age of the solar system due to a variety of physical processes which act to modify cometary surfaces and interiors. Possible structural and/or internal processes include: collisional accretion, disruption, and reassembly during formation; internal heating by long and short-lived radionuclides; amorphous to crystalline phase transitions, and thermal stresses. Identified surface modification processes include: irradiation by galactic cosmic rays, solar protons, UV photons, and the Sun's T Tauri stage mass outflow; heating by passing stars and nearby supernovae; gardening by debris impacts; the accretion of interstellar dust and gas and accompanying erosion by hypervelocity dust impacts and sputtering; and solar heating with accompanying crust formation. These modification processes must be taken into account in both the planning and the interpretation of the results of a Comet Nucleus Sample Return Mission. Sampling of nuclei should be done at as great a depth below the surface crust as technically feasible, and at vents or fissures leading to exposed volatiles at depth. Samples of the expected cometary crust and near-surface layers also need to be returned for analysis to achieve a better understanding of the effects of these physical processes. We stress that comets are still likely less modified dm any other solar system bodies, but the degree of modification can vary greatly from one comet to the next.

Weissman, Paul R.↗

NASA NTRS1998

"CHON" particles: The interstellar component of cometary dust

Interstellar dust is characterized by strong absorption in the ultraviolet and the mid-IR. Current models of interstellar dust are based on three chemically distinct components: a form of carbon (usually graphite), a silicate, and a blend of polycyclic aromatic hydrocarbons or other carbonaceous material. Previous work using effective medium theories to understand the optical properties of cometary dust suggested that an amalgam of materials could reproduce the observed interstellar and cometary dust features. Recently, Lawler and Brownlee (1992) re-analyzed the PIA and PUMA-1 data sets from the Giotto flyby of P/Halley and discovered that the so-called "CHON" particles were actually composed of a blend of carbon-bearing and silicon-bearing materials. Based on effective medium theories, the absorption spectrum of such a material would display the spectral features of each of the components - strong UV absorption from the carbonaceous component and strong absorption in the IR from the silicate component. To test this idea, vapor-deposited samples were created using two different deposition techniques: sputtering with an argon RF magnetron and deposition from an argon plasma torch. Two different compositions were tested: a blend of graphite and silica in a 7:1 ratio and an amalgam of materials whose approximate composition matches the "CHON"-silicate abundances for the uncompressed PIA data set of Lawler and Brownlee: graphite, iron oxide, magnesium oxide, ammonium sulfate, calcium carbonate, and silica in mass ratios of 6:4.3:4:2.2:1:9. The samples were finely ground and pressed into 2" diameter disks using a 40 ton press. In all, four different experiments were performed: one with each of the compositions (C:SiO and "CHON") in both the RF magnetron and the plasma torch chambers. The RF magnetron created a uniform dark thin film on the substrate surface, and the plasma torch created a coating of small (<100 micron) diameter grey particles. The spectra of all four samples show a strong, broad absorption feature at around 220 nm as well as a strong but narrower absorption peak near 10 microns. The RF magnetron sputtered samples showed some sub-structure in the UV, and the peak of the absorption was shifted toward longer wavelengths. The UV absorption in the plasma torch deposited samples have no sub-structure, and the peak absorption is very near 220 nm. Strong absorption near 9 microns is seen in the spectra from both sample preparation techniques, and is consistent with the IR spectra of some terrestrial silicates. Other features, particularly at 6.2 and 8.6 microns, are seen in the interstellar medium. A strong feature near 2 microns is due to absorbed water in the sample. Based on the results of these experiments, there is evidence that a material with a composition similar to that detected in "CHON" particles in the coma of P/Halley have a spectral signature which reproduces the main absorption features of interstellar dust. This suggests that the "CHON" particles could be the interstellar component of cometary dust.

Lien, David J.↗

NASA NTRS2001

Odyssey Comet Nucleus Orbiter: The Next Step in Cometary Exploration

Cometary nuclei are the most primitive bodies in the solar system, containing a cosmo-chemical record of the primordial solar nebula. Flyby missions to comets, such as those that encountered Comet Halley in 1986, provide a glimpse at this record. However, to study a cometary nucleus in detail requires a rendezvous mission, i.e., a nucleus orbiter. Only an orbiter provides the ability to map the entire nucleus surface at high resolution, to study the complex chemistry in the cometary coma and its variation with time, and to determine the mass and bulk density of the nucleus, key parameters in understanding how small bodies first formed in the solar nebula. A nucleus orbiter also provides the opportunity to sense the nucleus surface in preparation for more ambitious landing and sample return missions in the future. Additional information is contained in the original extended abstract.

Weissman, P. R.↗

NASA NTRS2002

Cometary Science After Hale-Bopp

Comets and the chondritic porous interplanetary dust particles (CP IDPs) that they shed in their comae are reservoirs of primitive solar nebula materials. The high porosity and fragility of cometary grains and CP IDPs, and anomalously high deuterium contents of pyroxene-rich CP IDPs imply these aggregate particles contain significant interstellar grain components. Spectrophotometry of comets at thermal IR wavelengths (3-40 microns) reveal the presence of a warm (near-IR) featureless emission modeled by amorphous carbon grains, and mid-IR and far-IR broad and narrow resonances modeled by chondritic (50% Fe and 50% Mg) amorphous and Mg-rich crystalline silicate minerals, respectively. Cometary amorphous silicate resonances are well matched by IR spectra of CP IDPs dominated by 0.1 micron spherules of Glass with Embedded Metal and Sulfides (GEMS) that are thought to be the interstellar Fe-bearing amorphous silicates produced in the cooling outflows of Asymptotic Giant Branch (AGB) stars. Acid-etched microtomed CP IDP samples, however, show that both the carbon phase (aliphatic) and the amorphous silicate phase (Mg-rich) are not optically absorbing while the embedded Fe nanoparticles make the IDPs dark. The CP IDPs suggest either significant processing has occurred in the ISM or that the AGB amorphous silicates have Mg-rich stoichiometry and possibly grew on Fe particle condensates. Cometary crystalline silicate resonances are well matched by IR spectra of laboratory submicron Mg-rich olivine crystals, [Mg(sub y),Fe(sub 1-y)]2SiO4 with y/ge0.85, and in the case of Hale-Bopp at r(sub h) less than or equal to approx. 1.5 AU, by Mg-rich pyroxene crystals, [Mg(sub x),Fe(sub 1-x)]SiO3 with x/ge0.85. While a fraction of AGB stardust (less than or equal to 15%) are Mg-rich crystals, this interstellar star dust component is insufficient to account for the deduced abundance of crystalline minerals in comet dust. An insufficient source of ISM Mg-rich crystals leads to the inference that Mg-rich crystals in comets may be hot, early solar nebula condensates that traveled large radial distances out to the comet-forming zone.

Wooden, Diane H.↗

NASA NTRS2003

The Size Distribution of Jupiter-Family Cometary Nuclei

Introduction: We are continuing our program to determine the size distribution of cometary nuclei. We have compiled a catalog of 105 measurements of 57 cometary nuclei, drawn from the general literature, from our own program of CCD photometry of distant cometary nuclei (Lowry and Weissman), and from unpublished observations by colleagues. We model the cumulative size distribution of the nuclei as a power law. Previous determinations of the size distribution slope do not agree. Fernandez et al. found a slope of alpha = 2.65+/-0.25 whereas Lowry et al. and Weissman and Lowry each found a slope of alpha = 1.60+/-0.10.

Weissman, Paul R.↗

NASA NTRS2004

Insight into Primordial Solar System Oxygen Reservoirs from Returned Cometary Samples

The recent successful rendezvous of the Stardust spacecraft with comet Wild-2 will be followed by its return of cometary dust to Earth in January 2006. Results from two separate dust impact detectors suggest that the spacecraft collected approximately the nominal fluence of at least 1,000 particles larger than 15 micrometers in size. While constituting only about one microgram total, these samples will be sufficient to answer many outstanding questions about the nature of cometary materials. More than two decades of laboratory studies of stratospherically collected interplanetary dust particles (IDPs) of similar size have established the necessary microparticle handling and analytical techniques necessary to study them. It is likely that some IDPs are in fact derived from comets, although complex orbital histories of individual particles have made these assignments difficult to prove. Analysis of bona fide cometary samples will be essential for answering some fundamental outstanding questions in cosmochemistry, such as (1) the proportion of interstellar and processed materials that comprise comets and (2) whether the Solar System had a O-16-rich reservoir. Abundant silicate stardust grains have recently been discovered in anhydrous IDPs, in far greater abundances (200 5,500 ppm) than those in meteorites (25 ppm). Insight into the more subtle O isotopic variations among chondrites and refractory phases will require significantly higher precision isotopic measurements on micrometer-sized samples than are currently available.

Brownlee, D. E.↗

NASA NTRS2005

Gas Dynamics and Kinetics in the Cometary Coma: Theory and Observations

Our ability to describe the physical state of the expanding coma affects fundamental areas of cometary study both directly and indirectly. In order to convert measured abundances of gas species in the coma to gas production rates, models for the distribution and kinematics of gas species in the coma are required. Conversely, many different types of observations, together with laboratory data and theory, are still required to determine coma model attributes and parameters. Accurate relative and absolute gas production rates and their variations with time and from comet to comet are crucial to our basic understanding of the composition and structure of cometary nuclei and their place in the solar system. We review the gas dynamics and kinetics of cometary comae from both theoretical and observational perspectives, which are important for understanding the wide variety of physical conditions that are encountered.

Combi, Michael R.↗

NASA NTRS2004

On The Effect of Electron Collisions in the Excitation of Cometary HCN

The electron-HCN collision rate for the excitation of rotational transitions of the HCN molecule is evaluated in comets C/1995 01 (Hale-Bopp) and C/1996 B2 (Hyakutake). Based on theoretical models of the cometary atmosphere, we show that collisions with electrons can provide a significant excitation mechanism for rotational transitions in the HCN molecule. Computed values of the cross section sigma(sub e-HCN) can be as high as 1.3 x cm2, more than 2 orders of magnitude greater than the commonly assumed HCN-H2O cross section. For the ground rotational transitions of HCN, the electron-HCN collision rate is found to exceed the HCN-H2O collision rate at distances greater than 3000 km from the cometary nucleus of Hale-Bopp and 1000 km from that of Hyakutake. Collisional excitation processes dominate over radiative excitation processes up to a distance of 160,000 km from the cometary nucleus of Hale-Bopp and 50,000 km from that of Hyakutake. Excitation models that neglect electron collisions can underestimate the HCN gas production rates by as much as a factor of 2.

Lovell, Amy J.↗

NASA NTRS2007

Characteristics of Cometary Dust Tracks in Stardust Aerogel and Laboratory Calibrations

The cometary tray of the NASA Stardust spacecraft s aerogel collector has been examined to study the dust that was captured during the 2004 fly by of comet 81P/Wild-2. An optical scan of the entire collector surface revealed 256 impact features in the aerogel (width > 100 microns). 20 aerogel blocks (out of a total of 132) were removed from the collector tray for a higher resolution optical scan and 186 tracks were observed (track length > 50 microns and width > 8 microns). The impact features were classified into three types based on their morphology. Laboratory calibrations were conducted which reproduce all three types. This work suggests that the cometary dust consisted of some cohesive, relatively strong particles as well as particles with a more friable or low cohesion matrix containing smaller strong grains. The calibrations also permitted a particle size distribution to be estimated for the cometary dust. We estimate that approximately 1200 particles bigger than 1 micron struck the aerogel. The cumulative size distribution of the captured particles was obtained and compared with observations made by active dust detectors during the encounter. At large sizes (>20 microns) all measures of the dust are compatible, but at micrometer scales and smaller discrepancies exist between the various measurement systems which may reflect structure in the dust flux (streams, clusters etc.) along with some possible instrument effects.

Burchell, M. J.↗

NASA NTRS2007

Estimation of Cometary Rotation Parameters Based on Camera Images

The purpose of the Rosetta mission is the in situ analysis of a cometary nucleus using both remote sensing equipment and scientific instruments delivered to the comet surface by a lander and transmitting measurement data to the comet-orbiting probe. Following a tour of planets including one Mars swing-by and three Earth swing-bys, the Rosetta probe is scheduled to rendezvous with comet 67P/Churyumov-Gerasimenko in May 2014. The mission poses various flight dynamics challenges, both in terms of parameter estimation and maneuver planning. Along with spacecraft parameters, the comet's position, velocity, attitude, angular velocity, inertia tensor and gravitatonal field need to be estimated. The measurements on which the estimation process is based are ground-based measurements (range and Doppler) yielding information on the heliocentric spacecraft state and images taken by an on-board camera yielding informaton on the comet state relative to the spacecraft. The image-based navigation depends on te identification of cometary landmarks (whose body coordinates also need to be estimated in the process). The paper will describe the estimation process involved, focusing on the phase when, after orbit insertion, the task arises to estimate the cometary rotational motion from camera images on which individual landmarks begin to become identifiable.

Spindler, Karlheinz↗

NASA NTRS2009

Cometary Amino Acids from the STARDUST Mission

NASA's Stardust spacecraft returned samples from comet 81 P/WiId 2 to Earth in January 2006. Examinations of the organic compounds in cometary samples can reveal information about the prebiotic organic inventory present on the early Earth and within the early Solar System, which may have contributed to the origin of life. Preliminary studies of Stardust material revealed the presence of a suite of organic compounds including several amines and amino acids, but the origin of these compounds (cometary vs. terrestrial contamination) could not be identified. We have recently measured the carbon isotopic ratios of these amino acids to determine their origin, leading to the first detection of a cometary amino acid.

Cook, Jamie Elsila↗

NASA NTRS2010

Cometary Glycine Detected in Stardust-Returned Samples

In January 2006, NASA's Stardust spacecraft returned samples from comet 81P/Wild 2 to Earth. The Stardust cometary collector consisted of aerogel cells lined with aluminum foils designed to capture impacting particles and facilitate removal of the aerogel. Preliminary examinations of these comet-exposed materials revealed a suite of organic compounds, including several amines and amino acids which were later examined in more detail. Methylamine (NH2CH3) and ethylamine (NH2C2H5) were detected in the exposed aerogel at concentrations greatly exceeding those found in control samples, while the amino acid glycine (NH2CH2COOH) was detected in several foil samples as well as in the comet-exposed aerogel. None of these three compounds had been previously detected in comets, although methylamine had been observed in the interstellar medium. Although comparison with control samples suggested that the detected glycine was cometary. the previous work was not able to conclusively identify its origin. Here, we present the results of compound-specific carbon isotopic analysis of glycine in Stardust cometary collector foils. Several foils from the interstellar side of the Stardust collector were also analyzed for amino acid abundance, but concentrations were too low to perform isotopic ana!ysis.

Elsila, Jamie E.↗