Observations of comets and asteroids

All available ground based observational techniques were used to study the chemical and physical properties of the small bodies of the solar system, primarily comets and next asteroids. The ultimate goal is to use these bodies to understand the formation and evolution of the solar system. Progress and accomplishments, and publications are given

Observations of comets and asteroids

All available ground-based observational techniques are used to study the chemical and physical properties of the small bodies of the solar system, primarily comets and secondarily asteroids. The ultimate goal is to use these bodies to understand the formation and evolution of the solar system. The analysis of charge coupled device (CCD) images of comet P/Halley continued. This included a mojor revision of a paper on the periodicities of CN jets in Halley showing that their periodicity is 7.3 days, not 2.2 days. It also included an analysis of the continuum images which showed that the dust in jets is much redder than in the ambient coma. Calculations with Mie theory suggest that the particles that show the strongest effects of radiation pressure. Much effort was expended devising methods to estimate the level of sky background of the many images in which comet extends to the edge of the chip. Examined images of comet Wilson were taken over many months in 1987. Wilson, a dynamically new comet, shows none of the short-term variability or jet structure (either dusty jets or jets of radicals seen in Halley. This is presumably due to the lack of a mantle or the nucleus. Median-imaging of sky was as a technique for removing residual flat-fielding errors in CCD images. A Monte Carlo model and a convolution model were developed for studying temporal variability of gaseous daughter products

Comets

The IUE was used to study comets including the first dynamically new comet to approach closer than 3 AU. Differences between old and new comets are studied. Results relevant to the nature of cometary nuclei are discussed. Identification of species in the spectra; relative abundances; variability of comets; and comet mass are considered

Theoretical spectroscopy of comets

Theoretical spectra of various emitting species in cometary comae were calculated both to investigate physical parameters that are measureable with cometary spectra and to provide fluorescence efficiencies for the derivation of abundances from fluxes. NH was found to be much further from nucleus than proviously thought and, because previous fluorescence efficiencies were much too high, NH is 5 times more abundant than previously thought. The spectrum of S sub 2 in comet I-A-A was analyzed. Despite its short lifetime, S sub 2 reaches fluorescent equlibrium. The equilibrium spectrum matches the relative band strengths in IUE spectra much better than does single cycle fluorescence. Equilibrium (but not single-cylce) flurescence also predicts strong optical emission bands which are seen in ground-based spectra of comet I-A-A. Equilibrium fluorescence efficiencies are much higher than single-cycle fluorescence efficiencies implying S sub 2 is only half as abundant as previously estimated. Fluorescence spectrum of SO was calculated and compared with several IUE spectra including those in which Wallis and Krishna Swamy claim SO to be present. Relative intensities and shapes of bands are inconsistent with any real identification

Theoretical spectroscopy of comets

Theoretical spectra were calculated of various emitting species in cometary comae both to study physical parameters that are measurable with cometary spectra and to provide fluorescence efficiencies for the derivation of abundances from fluxes

The gas production rate of periodic comet d'Arrest

Comet P/d'Arrest is a potential target for a rendezvous mission to a short period comet. Its light curve is rather peculiar, the comet being active only after perihelion passage. One apparition out of two is easy to observe from the ground. The 1995 apparition of the comet will offer a unique opportunity to characterize the outgassing properties of its nucleus

Chiron: Evidence for historic cometary activity

The non-asteroidal brightening of (2060) Chiron, first noted by Tholen in 1988 is now ascribed to cometary activity. Photometry since 1988 has revealed a broad surge in brightness that peaked in 1989 about 1.0 mag above the brightness in the mid-1980s. The surge is evidently due to sporatic formation of dust coma, which is itself driven by the presence of extremely volatile ices at or near the surface. CN emission was recently reported. Since Chiron is now nearing perihelion, there is interest in determining whether it has exhibited anomalous brightening in the past, particularly at greater heliocentric distances. Photographic plates dating back to 1895 are known to contain images of Chiron. Using some of these archival material, the initial results are presented for a project to determine Chiron's brightness history over orbital timescales. A particularly homogeneous and high-quality set of plates taken prior to and around the time of Chiron's discovery in Oct. 1977 at the 1.2 m Oschin Schmidt telescope at Mt. Palomar Observatory were examined. Images of Chiron were identified and digitized using a PDS microdensitometer, and images of field stars around Chiron were both similarly digitized and photometrically calibrated using recently acquired B and V band CCD frames. As a result of the present work, eleven new data, including estimated errors, were added between 1969 and 1977. The implications that Chiron can be active at any heliocentric distance in its present orbit suggest that the active volatile is either N2, CH4, or CO, and that a substantial degree of mantling may have developed. Further historical data is presented, the error bars discussed, and possible mechanisms suggested for the observed activity

g-Factors of the SH (0-0) Band and SH Upper Limit in Comet P/Brorsen-Metcalf (1980o)

Since H2S was detected in Comets Austin (1989cl) and Levy (1990c) in the microwave range, there has been increasing interest in searching for SH, which is the prime dissociative product of H2S. We present g-factors for the A-X (0-0) band of SH as a function of heliocentric velocity at r = 1.0 AU. We derive an upper limit production rate, Q(SH)/Q(H2O) less than 0.017, for Comet Brorsen-Metcalf (1989o) and calculate a dissociative lifetime of 105 sec at a heliocentric distance, r = 1.0 AU, and at a heliocentric velocity, v(r) = -28.5 km/sec

The 15 years of comet photometry: A comparative analysis of 80 comets

In 1976, a program of narrowband photometry of comets was initiated that has encompassed well over 400 nights of observations. To date, the program has provided detailed information on 80 comets, 11 of which were observed during multiple apparitions. The filters (initially isolating CN, C2, and continuum and later including C3, OH, and NH) as well as the detectors used for the observations were changed over time, and the parameters adopted in the reduction and modeling of the data have likewise evolved. Accordingly, we have re-reduced the entire database and have derived production rates using current values for scalelengths and fluorescence efficiencies. Having completed this task, the results for different comets can now be meaningfully compared. The general characteristics that are discussed include ranges in composition (molecular production rate ratios) and dustiness (gas production compared with Af(rho)). Additionally an analysis of trends on how the production rates vary with heliocentric distance and on pre- and post-perihelion asymmetries in the production rates of individual comets. Possible taxonomic groupings are also described

Secular variation of activity in comets 2P/Encke and 9P/Tempel 1

We compare production rates of H20 derived from International Ultraviolet Explorer (IUE) spectra from multiple apparitions of 2 comets, 2P/Encke and 9P/Tempel 1, whose orbits are in near-resonance with that of the Earth. Since model-induced errors are primarily a function of observing geometry, the close geometrical matches afforded by the resonance condition results in the cancellation of such errors when taking ratios of production rates. Giving careful attention to the variation of model parameters with solar activity, we find marginal evidence of change in 2P/Encke: a 1-sigma pre-perihelion decrease averaging 4%/revolution over 4 apparitions from 1980-1994, and a 1-sigma post-perihelion increase of 16%/revolution for 2 successive apparitions in 1984 and 1987. We find for 9P/Tempel 1, however, a 7-sigma decrease of 29%/revolution over 3 apparitions from 1983-1994, even after correcting for a tracking problem which made the fluxes systematically low. We speculate on a possible association of the character of long-term brightness variations with physical properties of the nucleus, and discuss implications for future research