2 results
Photometric Properties of Vesta
- Jian-Yang Li, L. Jorda, H. U. Keller, N. Mastrodemos, S. Mottola, A. Nathues, C. Pieters, V. Reddy, C. A. Raymond, T. Roatsch, C. T. Russell, B. J. Buratti, S. E. Schroder, M. V. Sykes, T. Titus, F. Capaccioni, M. T. Capria, L. Le Corre, B. W. Denevi, M. De Sanctis, M. Hoffmann, M. D. Hicks
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- Journal:
- Proceedings of the International Astronomical Union / Volume 10 / Issue H16 / August 2012
- Published online by Cambridge University Press:
- 05 March 2015, p. 179
- Print publication:
- August 2012
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- Article
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The Dawn spacecraft orbited Asteroid (4) Vesta for a year, and returned disk-resolved images and spectra covering visible and near-infrared wavelengths at scales as high as 20 m/pix. The visible geometric albedo of Vesta is ~ 0.36. The disk-integrated phase function of Vesta in the visible wavelengths derived from Dawn approach data, previous ground-based observations, and Rosetta OSIRIS observations is consistent with an IAU H-G phase law with H=3.2 mag and G=0.28. Hapke's modeling yields a disk-averaged single-scattering albedo of 0.50, an asymmetry factor of -0.25, and a roughness parameter of ~20 deg at 700 nm wavelength. Vesta's surface displays the largest albedo variations observed so far on asteroids, ranging from ~0.10 to ~0.76 in geometric albedo in the visible wavelengths. The phase function of Vesta displays obvious systematic variations with respect to wavelength, with steeper slopes within the 1- and 2-micron pyroxene bands, consistent with previous ground-based observations and laboratory measurement of HED meteorites showing deeper bands at higher phase angles. The relatively high albedo of Vesta suggests significant contribution of multiple scattering. The non-linear effect of multiple scattering and the possible systematic variations of phase function with albedo across the surface of Vesta may invalidate the traditional algorithm of applying photometric correction on airless planetary surfaces.
The Deep Space 1 Encounter With Comet 19P/Borrelly
- Diane H. Wooden, L. A. Soderblom, D. T. Britt, R. H. Brown, B. R. Sandel, R. V. Yelle, B. J. Buratti, M. D. Hicks, R. M. Nelson, M. D. Rayman, J. Oberst, N. Thomas
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- Journal:
- International Astronomical Union Colloquium/ Volume 186 /
- pp. 301-324
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NASA's Deep Space 1 (DSl) spacecraft successfully encountered comet 19P/Borrelly near perihelion and the Miniature Integrated Camera and Spectrometer (MICAS) imaging system onboard DS1 returned the first high-resolution images of a Jupiter-family comet nucleus and surrounding environment. The images span solar phase angles from 88° to 52°, providing stereoscopic coverage of the dust coma and nucleus. Numerous surface features are revealed on the 8-km long nucleus in the highest resolution images (47-58 m/pixel). A smooth, broad basin containing brighter regions and mesa-like structures is present in the central part of the nucleus that seems to be the source of jet-like dust features seen in the coma. High ridges seen along the jagged terminator lead to rugged terrain on both ends of the nucleus containing dark patches and smaller series of parallel grooves. No evidence of impact craters with diameters larger than about 200-m are present, indicating a young and active surface. The nucleus is very dark with albedo variations from 0.007 to 0.035. Short-wavelength, infrared spectra from 1.3 to 2.6 μm revealed a hot, dry surface consistent with less than about 10% actively sublimating. Two types of dust features are seen: broad fans and highly collimated “jets” in the sunward hemisphere that can be traced to the surface. The source region of the main jet feature, which resolved into at least three smaller “jets” near the surface, is consistent with an area around the rotation pole that is constantly illuminated by the sun during the encounter. Within a few nuclear radii, entrained dust is rapidly accelerated and fragmented and geometrical effects caused from extended source regions are present, as evidenced in radial intensity profiles centered on the jet features that show an increase in source strength with increasing cometocentric distance. Asymmetries in the dust from dayside to nightside are pronounced and may show evidence of lateral flow transporting dust to structures observed in the nightside coma. A summary of the initial results of the Deep Space 1 Mission is provided, highlighting the new knowledge that has been gained thus far.