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The soft X rays from Comet Hyakutake observed by Lisseet al.(1996,Science274, 205-209) can reasonably be attributed to electron-neutral and electron-dust collisions. The neutrals and dust come from the comet and the electrons from the solar wind and from photoionization of neutrals. We assume the electron energy spectrum shape to be the same as seen by Vega 2 at Halley's comet, but otherwise use cometary...
In this paper we report first optical reflectance spectra in the wavelength range 3800-9000 Å for seven members of the Veritas asteroid family, as determined by V. Zappala, A. Cellino, P. Farinella, and A. Milani (1994,Astron. J.116, 291-314). The observed asteroids are 490 Veritas, 844 Leontina, 1086 Nata, 2428 Kamenyar, 2934 Aristophanes, 5592 Oshima, and 1985 TQ 1 . In addition, we also...
We present numerical orbital integrations of thousands of massless particles as they evolve from Neptune-encountering orbits in the Kuiper belt for up to 1 Gyr or until they either impact a massive body or are ejected from the Solar System. The median dynamical lifetime of these objects is 4.5 x 10 7 years. We found that about 30% of these objects became visible comets. (We refer to a comet...
The dynamical structure of the 2/3 resonance at small inclination is explored using numerical integrations of test particles to compute the evolution of proper elements with time. The basic features are related to the analytically computed geography of secular resonances. This paper focuses on the existence of slowly diffusing chaotic orbits, which escape from the 2/3 resonance after billions of years...
Spectra from 1.7 to 3.3 μm acquired at the NASA Kuiper Airborne Observatory include two of Saturn's near-IR atmospheric transmission windows that are at least partially obscured by telluric H 2 O and CO 2 absorptions at ground-based telescopes. This entire spectral region was fitted to a model that included gaseous absorption by H 2 , CH 4 , NH 3 , and PH ...
We present the results of two scans through the dust trail of Comet P/Kopff made with ISOCAM on 26 March 1996. Positions behind the comet at mean anomalies of 0.5 o and 1 o less than the nucleus were imaged using the LW10 (IRAS Band 1, 12 μm) filter. The trail is clearly detected and appears to have varied since it was observed with the IRAS satellite in 1983.
Spectra of 14 asteroids of varying classes were acquired using a moderate resolution spectrograph (dispersion 3.5 Å/pixel) at the McDonald Observatory 2.7-m telescope as random targets of opportunity over the past 6 yr. The utility of high-quality measurements of asteroid reflectance in the 3200-5500 Å wavelength range was demonstrated by the presence of subtle absorption features and the confirmation...
We have extended our previous calculations of planetary accretion rates (Y. Greenzweig and J. J. Lissauer 1990,Icarus87, 40-77, and 1992,Icarus100, 440-463) to the case of a planet on an eccentric orbit. We have found that the dependence of accretion rates on planetary and planetesimal eccentricities can be reduced to a single parameter and that the extent of a planet's single-pass feeding zones depends...
We present spectra of Comet Hale-Bopp (C/1995 O1) covering the range 1.4-2.5 μm that were recorded when the comet was 7 AU from the Sun. These spectra show broad absorption features at 1.5 and 2.05 μm. While we recognize that much of the light from the comet is scattered from the coma, we show that some, but not all, of the absorption features can be matched by an intimate mixture of water ice and...
The structure and morphological properties of polymers produced photochemically from the UV irradiation of cyanoacetylene and cyanoacetylene mixtures have been examined to evaluate their possible contribution to the haze layers found on Titan. A structural analysis of these polymers may contribute to our understanding of the data returned from the Huygens probe of the Cassini mission that will pass...
In March 1994, we used the newly refurbished Hubble Space Telescope (HST) to obtain global imaging of Io at five wavelengths between 0.34 and 1.02 μm, with a spatial resolution of 160 km. The images provided the clearest view of Io since Voyager and the first systematic observations in the wavelength range 0.7-1.0 μm. We have produced absolutely calibrated global mosaics of Io's reflectance in all...
New measurements of Titan's hemispheric brightness asymmetry from HST images from 260 to 1040 nm show that the contrast is strongest near 450 nm (blue) and, with the opposite sign, at 889 nm (methane band). Comparison with a full Titan year of disk-integrated albedo data indicates that the seasonal cycle in asymmetry is smooth, but is insufficient to explain the variation in albedo, and a twice-per-year...
We predict the orbital distribution of Earth-approaching asteroids with diameter,d, ranging from 10 m to 10 km, assuming they originate as the fragments of main-belt asteroids. The calculation uses Opik theory to account for the effects of close encounters with the planets and analytic theory to account for the effects of the ν 6 secular resonance and the 3:1 mean motion resonance with Jupiter...
High precision spectrophotometry of 4 Vesta, the third largest asteroid, was used to establish the surface composition of this body and to investigate mineralogical variations across its surface. The average surface of Vesta is analogous to howardite and/or polymict eucrite assemblages, regolith-derived members of the HED meteorite suite which consist of a eucrite matrix containing differing amounts...
We report on the physical properties of the particles produced after the collision of fragments E, G, and H of Comet Shoemaker-Levy 9 with Jupiter on July 1994. CCD images were acquired at the 4.2-m William Herschel Telescope at La Palma (Spain) through three filters centered at 3600, 8920, and 9480 Å. Radiative transfer models in combination with a Mie scattering code were used to retrieve the possible...
Data from satellite impact experiments and the scanning of recovered spacecraft surfaces and solar cells offer an extended time base for examining, using a consistent methodology, microparticle fluxes at low Earth orbital (LEO) altitudes. Revised estimates of the ratio of natural micrometeorites to space debris at micrometer dimensions at ~500 km altitude show that the debris population is not as...
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