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NASA NTRS1991

Photometry of the comet 2060 Chiron

The comet 2060 Chiron has proven to be an interesting and enigmatic object. Situated between the orbits of Saturn and Uranus, it was originally classified as the most distant asteroid. It began to show cometary behavior in 1987 by increasing a full magnitude in brightness and developing a coma; there is evidence also for similar earlier outbursts. A thorough study of Chiron is important for two reasons: (1) it is a transition object defining the relationship between comets, asteroids, and meteorites; and (2) a full description of its changes in brightness - particularly on time scale of hours - will provide an empirical foundation for understanding the physical mechanisms (including outgassing, sublimation of volatiles, and even significant mass ejections) driving the evolution of comets. Short term outbursts were observed in early 1989, and a rapid decrease in brightness of Chiron's coma was observed in 1990 in the V and R filters. Also, a rotational lightcurve was detected of the nucleus with an amplitude only 1/4 that observed in its quiescent state: this fact indicates the increased importance of the optically thin coma to the observed brightness.

Buratti, Bonnie J.↗
NASA NTRS1991

Infrared observations of comets

Selected comets are observed in the near infrared (1 to 2.2 micron) and thermal infrared (3.5 to 20 micron) with the NASA Infrared Telescope Facility (IRTF) and other telescopes as appropriate, in order to characterize the physical properties of the dust grains; their composition, size distribution, emissivity, and albedo. Systematic variations in these properties among comets are looked for, in order to understand the heterogeneity of comet nuclei. Spectrophotometry of the 10 micron silicate emission feature is particularly emphasized. The rate of dust production from the nucleus and its temporal variability are also determined. Knowledge of the dust environment is essential to S/C design and mission planning for NASA's CRAF mission.

Hanner, Martha S.↗
NASA NTRS1991

Physical processes in comets

Post-Halley comets are known to be irregular objects with most nucleus activity very localized and with the dust coma capable of fragmentation and apparently being a source of gas. Older, 1-D strategies which assume steady isotropic outflow of material can give poor time-and-space averaged results, at best. With 2-D data, images through interference filters, it is hoped that: dust structures can be seen that give evidence of the proper geometry for data reduction; gradients along the axes of symmetry and evidence of fragmentation can be studied; and that evidence for gas abundance gradients associated with the dust can be found. High quality data from brighter comets can then be used to suggest improved data reduction procedures for fainter ones. To obtain such data, large image-quality interference filters were procured for use with a charge coupled device (CCD) camera at Lick Observatory, where the scale of the 1 m Nickel reflector is ideal for brighter comets. Whenever possible, data is taken simultaneously with other telescopes and equipment, especially spectroscopy at the Lick 3 m or infrared photometry at the Infrared Telescope Facility (IRTF) on Mauna Kea.

Newburn, Ray L., Jr.↗
NASA NTRS1991

Spectroscopy of comets

Observations of NH2, OI, CH, CO(+), CO2(+), H2O(+), and N2(+) in optical spectra of comets represent ionization and dissociation product abundances of N2, NH3, H2O, CH4, CO2, and CO. The primary objectives are to determine: (1) accurate production rates for the observed species, and (2) accurate relative abundances of condensates in a sample of comet nuclei. The ultimate goal is to constrain models of comet formation and chemical processing in the outer primordial solar nebula.

Wyckoff, Susan↗
NASA NTRS1991

Physical observations of comets: Their composition, origin and evolution

Observations of Comet P/Schwassmann-Wachmann 1 (SW1) during one observing run each in 1989 and 1990 are discussed, and the new significant information that was obtained is presented. Also discussed are near-UV observations of comets. The near-UV is a mostly unexplored spectral region for comets since it is not visible to spacecraft such as IUE and most ground-based detectors and spectrographs are not sensitive in the near-UV.

Cochran, Anita L.↗
NASA NTRS1991

Laboratory simulation of the surface of Halley's Comet

In the past year we have developed a model for the dissociation of icy grains in comets. In general, this model does not predict the radical yield that is observed in the jets of comets. To reproduce these yields, the concentration of the radical's precursors will have to be too high to be consistent with the abundance that we expect in comets. We have also constructed a theoretical model for the formation of ions on icy cometary grains. That model shows that only a small number of the cometary ions could be produced as ions in icy grains if the particle has only positively charged ions. If it contains both positive and negative ions then it could have a higher concentration of ions without violating charge neutrality. When such a particle evaporates, it can release ions with both charges into the gas phase. These ions could produce radicals via dissociative recombination with electrons if they are positive ions or by photoattachment of electrons if they are negative electrons. Finally, an avalanche process involving the acceleration of electrons towards charged particles is evaluated. It is shown that this will not lead directly to gas phase ions, but it might lead to additional ionization and dissociation on the grain.

Jackson, William M.↗
NASA NTRS1991

Organic synthesis in the outer Solar System: Recent laboratory simulations for Titan, the Jovian planets, Triton and comets

We tabulate the most abundant gases and their radiation yields, for two experimental pressures: 0.24 mb, more relevant to upper atmosphere excitation, and 17 mb, more relevant to tropospheric, cosmic ray excitation. The yields computed in the 0.24 mb experiment combined with measured electronic fluxes and a simple, eddy diffusion model of Titan's atmosphere predict abundances of detected molecules in agreement with those found by Voyager and for heavier products, in somewhat better agreement with observation than photochemical absolute reaction rate kinetics models. All Voyager organics are accounted for and no detectable products are found that Voyager did not detect. A striking increase of products with multiple bonds is found with decreasing pressure. Hydrocarbon abundances decline slowly with increasing carbon number. Additionally, we list preliminary estimates for the yield of the heteropolymer, which seems to be produced in a quantity comparable (in moles of C+N consumed) to the total amount of gaseous product. The production rate required to sustain Titan's haze against sedimentation also indicates yields of this order. As can be seen from the table, over 10(exp 9) years substantial amounts of these products can accumulate on the surface -- ranging from cm thickness for the (C+N equals 4) species to a meter or more for HCN and C2H2; we also expect a meter or more of tholins. Similar analyses have been or are being done for the Jovian planets and Triton. Charged particle irradiation of hydrocarbon clathrates or mixed hydrocarbon/water ices produces a range of organic products, reddening and darkening of the ices and characteristic infrared spectra. From such spectra, the predicted emission by fine particles in cometary comae well-matches the observed 3.4 micron emission spectra of Comet Halley and other recent comets. Heliocentric evolution of organic emission features in comets is predicted. Organic products of such ice irradiation may account for colors and albedos on some of the satellites in the outer solar system, especially Triton and Pluto, where solid methane is known to exist.

Sagan, C.↗
NASA NTRS1991

Systems for COMET

The Centers for Commercial Development of Space (CCDS) have become the nation's primary focus for stimulating private sector investment in space. In establishing the COMmercial Experiment Transporter (COMET) program, the CCDS's have taken a major step toward the full scale commercial development of space. The COMET program will enable the CCDS community to turn the goal of commercial space into reality, and in the process, establish U.S. leadership in the global market for space systems and services. Some details are presented on the systems that make up the COMET missions, specifically as they describe the accommodation for potential users and mission parameters.

Andrews, Harry↗
NASA NTRS1992

Asteroid and comet orbits using radar data

For the 30 asteroids and 4 comets for which radar astrometric data were given by Ostro (1991), orbits have been computed using both the radar and the existing optical measurements. The techniques required to process radar data in orbit determination solutions are outlined, and future radar observation opportunities for asteroids and comets are identified. For asteroids and comets that have only short intervals of optical astrometric data, the additional use of only a few radar observations allows a far more accurate extrapolation of their future motions. The use of radar data can often ensure an object's successful recovery at future earth returns and greatly assist efforts in monitoring the motions of the rapidly growing population of known near-earth objects, including their future close-earth approaches.

Yeomans, D. K.↗
NASA NTRS1992

A self-consistent model for the particles and fields upstream of an outgassing comet. II - A time-dependent description

In a previous paper, the global variation of the magnetic field and solar wind flow parameters in the unshocked region upstream of an outgassing comet was determined using a kinetic treatment for the cometary ions. Two different assumptions were made concerning the cometary ion distribution function: the pickup cometary ions formed either a velocity space gyrotropic ring distribution or a velocity space isotropic shell distribution in the solar wind frame of reference. In the present paper the general case is considered wherein the newly picked up ions are elastically pitch angle scattered from the initial ring distribution to a shell with some characteristic time scale. Using theoretically determined parameters which reflect the conditions expected at Comet Kopff (a typical short-period comet and a possible target for the CRAF/Cassini mission) for various heliocentric distances, how the pitch angle scattering rate affects the global morphology is determined.

Flammer, K.↗
NASA NTRS1992

The dust coma of Comet P/Giacobini-Zinner in the infrared

The paper presents 1-20-micron photometry of P/Giacobini-Zinner obtained at the NASA Infrared Telescope Facility, during 1985 June-September (r = 1.57-1.03 AU). A broad, weak 10-micron silicate emission feature was detected on August 26.6; a similar weak emission feature could have been hidden in the broadband photometry on other dates. The total scattering and emitting cross section of dust in the inner coma was similar to that in other short-period comets, but a factor of 10 (r = 1.56 AU) to 100 (r = 1.03 AU) lower than the amount of dust in Comet Halley. The thermal emission continuum can be fit with models weighted toward either small or large absorbing grains. The dust production rate near perihelion was about 100,000 g/s (small-grain model) to about 1,000,000 g/s (large-grain model). The corresponding dust/gas mass ratio on August 26 was about 0.1-1. A silicate-rich heterogeneous grain model with an excess of large particles is compatible with the observed spectrum of Giacobini-Zinner on August 26. Thus, weak or absent silicate emission does not necessarily imply an absence of silicates in the dust, although the abundance of silicate particles not greater than 1 micron radius must have been lower than in Comet Halley.

Hanner, M. S.↗
NASA NTRS1992

Comet nucleus and asteroid sample return missions

Three Advanced Design Projects have been completed this academic year at Penn State. At the beginning of the fall semester the students were organized into eight groups and given their choice of either a comet nucleus or an asteroid sample return mission. Once a mission had been chosen, the students developed conceptual designs. These were evaluated at the end of the fall semester and combined into three separate mission plans, including a comet nucleus same return (CNSR), a single asteroid sample return (SASR), and a multiple asteroid sample return (MASR). To facilitate the work required for each mission, the class was reorganized in the spring semester by combining groups to form three mission teams. An integration team consisting of two members from each group was formed for each mission so that communication and information exchange would be easier among the groups. The types of projects designed by the students evolved from numerous discussions with Penn State faculty and mission planners at the Johnson Space Center Human/Robotic Spacecraft Office. Robotic sample return missions are widely considered valuable precursors to manned missions in that they can provide details about a site's environment and scientific value. For example, a sample return from an asteroid might reveal valuable resources that, once mined, could be utilized for propulsion. These missions are also more adaptable when considering the risk to humans visiting unknown and potentially dangerous locations, such as a comet nucleus.

Source record↗
NASA NTRS1992

Comets: Role and importance to exobiology

The transfer of organic compounds from interstellar space to the outskirts of a protoplanetary disk, their accretion into cometary objects, and the transport of the latter into the inner solar system by orbital diffusion throw a new light on the central problem of exobiology. It suggests the existence of a cosmic mechanism, working everywhere, that can supply prebiotic compounds to ubiquitous rocky planets, in search of the proper environment to start life in many places in the Universe. Under the heading of chemistry of the cometary nucleus, the following topics are covered: radial homogeneity of the nucleus; the dust-to-ice ratio; nature of the dust grains; origin of the dust in comets; nature of the volatile fraction; the CO distribution in comet Halley; dust contribution to the volatile fraction; elemental balance sheet of comet Halley; quantitative molecular analysis of the volatile fraction; and isotopic ratios. Under the heading of exogenous origin of carbon on terrestrial planets the following topics are covered: evidence for a high-temperature phase; from planetesimals to planets; a veneer of volatile and organic material; and cometary contribution.

Delsemme, Armand H.↗
NASA NTRS1992

IUE observations of periodic comets Tempel-2, Kopff, and Tempel-1

We summarize the results of observations made between 10 Jun. - 18 Dec. 1988 with the International Ultraviolet Explorer (IUS) of comet P/Tempel-2 during its 1988 appearance. The derived water production rate and relative gas/dust ratio are compared with those of P/Halley, observed with IUE in 1985-86, and other potential Comet Rendezvous/Asteroid Flyby (CRAF) target comets, P/Kopff and P/Tempel-1, both observed with IUE in 1983.

Feldman, Paul D.↗
NASA NTRS1992

Production rates for comet P/Temple 2 from long slit CCD spectroscopy

Since comet P/Temple-2 is one of the potential targets for the CRAF (Comet Rendezvous Asteroid Flyby) mission, we obtained long slit spectroscopic data with our CCD spectrograph during its 1988 apparition. As the same spectrograph was extensively used for observations of P/Halley, this allowed a direct spectroscopic comparison between the two objects. Furthermore we could choose a P/Halley spectrum which was taken at a heliocentric distance very close to that of P/Temple-2. Finally, we could adjust the integration windows along the slit to compensate for the different geocentric distances, so that roughly the same projected distance of the comets' comae was observed. The parameters for our observations are given.

Fink, Uwe↗
NASA NTRS1992

Spectroscopic observations of comet Austin (1989c)

Longslit CCD spectra (lambda = 5100-6400 A, delta(lambda) approximately 3 A) were obtained with the Michigan-Dartmouth-MIT 1.3 meter telescope in May 1990 (r = 0.74 AU, delta = 0.50 AU). The spectra were reduced with the Interactive data Reduction and Analysis Facility (IRAF). Spectral extractions offset sunward and tailward from the nucleus were analyzed. Species identified in the spectra include the following: C2, NH2(10-0), NH2(9-0), H2O(+), and CO(+). Spatial extractions of rotational line intensities in the NH2(10-0) band extend approximately 10(exp 4.5) km from the nucleus. A fit of the vectorial model to the NH2(10-0) spatial profile is consistent with an NH3 parent molecule. The NH2 production rate and an ammonia to water abundance ratio, NH3/H2O approximately 3 percent, were derived. The ammonia abundance obtained for comet Austin is consistent with that found for several other comets and is indicative of comet formation under very homogeneous conditions.

Heyd, Rodney↗
NASA NTRS1992

First images of a possible CO(+)-tail of comet P/Schwassmann-Wachmann 1 observed against the dust coma background

Comet P/Schwassmann-Wachmann 1 was observed with the 2m-Ritchey-Cretien Telescope of the Bulgarian National Observatory, Rozhen, Bulgaria, using the CCD-camera and focal reducer of the Max-Planck-Institute for Aeronomy. Images were taken in a red continuum window and in the 2-0 A(exp 2)Pi - X(exp 2)Sigma(+) band of CO(+) located in the blue part of the spectrum. The red images reveal an extended dust coma. From a comparison of the red and blue images a dust reddening of 13.2 percent per 1000 A is derived. At 642 nm the magnitude of the comet with a square diaphragm of 4.5 arcsec is 16.6. The blue images, taken in the CO(+) band, show a significantly different brightness distribution which is interpreted as presence of a CO(+) coma and tail superimposed on the continuum. A column density of several 10(exp 10) CO(+) molecules cm(exp -2) is derived. The tail thickness of 10(exp 5) km is unexpectedly small. We estimate the CO(+) production rate to about 6 x 10(exp 26) CO(+) particles s(exp -1). This value does not support the idea that the outbursts of this comet are caused by crystallization of amorphous water ice.

Jockers, Klaus↗
NASA NTRS1992

Comet nongravitational forces and meteoritic impacts

We have considered those comets whose original orbits have been determined to be hyperbolic when only planetary perturbations are accounted for. It is found that formally unbound incident trajectories correlate most confidently with orbits that have small perihelion distances and move in a retrograde sense relative to planetary motion. Arguments are presented that these results are not due to measurement error or to selection effects. We conclude that the phenomenon is attributable to enhanced volatility leading to abnormally large nongravitational forces. Since the effect is absent in the prograde small-perihelia population, increased insolation is not the sole explanation. It is suggested that the significance of the retrograde correlation is connected with a larger energy of relative motion between retrograde comets and a population of prograde ecliptic meteoroids which impact the comet mantle exposing the underlying volatiles. The subsequent enhanced outgassing is the cause of the larger nongravitational forces.

Matese, John J.↗